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How do you check if a room behind a door aboard a spaceship has an atmosphere and pressure? This is so that you would not accidentally open a door to a room which is open to space.

I imagine there would be some sort of sensor on the door. However, this would probably require electric power. What if the ship has lost its power? How would a survivor check whether the other room has an atmosphere?

Doors could have a window and there could be a cage with birds, so one might see whether the birds inside the room are alive. But this is not a very scientific way to go about this.

What would be a more modern way to do this? Maybe some sort of chemical sticker (reacting with oxygen) on the window?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Oct 27 '17 at 3:24
  • $\begingroup$ If two doors are the same one within the pressurized cabin, and one adjacent to a possibly open-to-space room. Knock on the "within" door, notice the sound it makes, and knock on the "possibly open-to-space" door. If it's open to space it ought to be quieter. It's like an astronaut's equivalent to knocking on wood to find a stud. $\endgroup$ – user42036 Oct 28 '17 at 21:57

23 Answers 23

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There's no need to start coming up with new science function stuff for this - real world designs are quite sufficient. Here is a door from the ISS:

enter image description here

If you zoom in on the image, you can probably make out the opening and closing instructions.

Space stations doors are generally designed mechanically to not open if there's any significant pressure differential in either direction between the compartments. If the door would be designed to open even with a large pressure differential, opening a door against lower pressure would be impossible due to the force required to open the door against pressure (assuming door opens "inwards" as is customary). On the other hand, opening such a door against a higher pressure would be possibly catastrophic, due to the door swinging open with huge force.

Instead, there's a manual pressure equalization valve on the door. You know, an adjustable hole, nothing fancier than that. (Black handle bottom right on the image.) Since life support is regenerative, simply opening a small hole to the next compartment is enough to slowly fill it - and when the pressure differential is small enough, the door can be opened. If the compartment has a leak in it, or is open to space - tough luck, there's no possibility of opening the door, short of cutting or exploding it open.

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    $\begingroup$ Do tourists on a spaceship read instructions? $\endgroup$ – Justin Thyme Oct 23 '17 at 15:18
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    $\begingroup$ Unless the mechanism for operating bolts provides very large ratio (read: door is specifically designed to be opened under pressure), opening the door outside would be equally impossible. The same force that kept the door shut in inside example would press bolts and increase friction on them to the point that retracting the bolts would be impossible for a bare-handed human. Constructor could design small notches on the bolts that would require pulling the door back to unlock. Manual and the valve are there to teach how to work around physics. Doors in space don't open easily, no need to care. $\endgroup$ – Agent_L Oct 23 '17 at 17:11
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    $\begingroup$ If there is a leak in the other compartment, wouldn't you only notice this after some time, when you realize that the pressure never equalizes? And doesn't this mean that you already lost half your oxygen/air/whatever into outer space? $\endgroup$ – pipe Oct 25 '17 at 13:34
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    $\begingroup$ @pipe If the point is to see if there is a vacuum on the other side, acknowledging that you're not in a vacuum and the door absolutely will not open is enough. $\endgroup$ – person27 Oct 26 '17 at 0:53
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    $\begingroup$ @pipe Yes and yes. Unless you had other equipment available (usually there always is), simply opening the manual pressure equalization valve doesn't give any feedback if the other compartment is actually increasing in pressure, or just staying at vacuum. There is usually a way to see or check the pressure through the door though, in which case you'd just wait until you can see a pressure increase, then close the valve and see if the pressure drops back to vacuum. $\endgroup$ – Nakedible Oct 26 '17 at 21:22
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Take a leaf from early submarines. Specifically the inner torpedo tube doors which you do NOT want to open if there is pressure on the other side (A boat sinking flood sort of mistake)...

There is all kinds of interlocking, but that requires power, the ultimate safety check is a small valve which you open before even thinking about undoing the dogs on the tube, if water comes out with considerable force behind it you probably think twice.

A simple valve with a whistle arrangement would probably do it and needs no power (Interlock the valve so it has to be opened before you can spin the hatch wheel), but there are many simple pneumatic things you could run off a bar of pressure differential.

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    $\begingroup$ A whistle! that would be so cheery! "Toot, toot! Everyone in here is dead!" $\endgroup$ – Neal Oct 23 '17 at 18:18
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    $\begingroup$ Lee Corey (G. Harry Stein) used this in The Space Doctor. It was part of newbee training. Before opening any door, check the valve for a whistle. $\endgroup$ – ShadoCat Oct 23 '17 at 18:31
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    $\begingroup$ @Neal Reminds me of the "space welcome wagon" by Three Dead Trolls in a Baggie. "<peaceful harp music>Psssst! Your airlocks have depressurized." $\endgroup$ – Wayne Werner Oct 23 '17 at 19:11
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    $\begingroup$ @Neal they should hook up an Aztec Death Whistle youtube.com/watch?v=I9QuO09z-SI $\endgroup$ – user151841 Oct 23 '17 at 19:51
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    $\begingroup$ I had imagined it more like Thomas the Tank Engine. : ) $\endgroup$ – Neal Oct 23 '17 at 20:27
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A passive system is really quite simple. Whether or not it becomes widespread is another issue. Put a thick flexible but strong membrane in the door. The membrane would bow into the suspect room if the pressure was lower. The degree that it bows in is the measure of the pressure difference. Like the lids on vacuum-sealed jars are pulled into (pushed into?) the jar as long as it is sealed tight.

You also have to consider the direction the door swings. If it swings towards the room of high pressure and away from the vacuum, you would never be able to open it. If it swings in the direction of low pressure, it would spring open once the latch is released. If it slides, it would depend on how frictionless the door tracks were.

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    $\begingroup$ Bingo. Have it open into the corridor and the pressure difference will keep it shut if there's a leak on the other side $\endgroup$ – nzaman Oct 22 '17 at 14:57
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    $\begingroup$ @nzaman: That’s fine, so long as you know in advance on which side of the door you will be, and on which side (hopefully the other!) that the leak will be... $\endgroup$ – eggyal Oct 23 '17 at 1:22
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    $\begingroup$ You could just build all doors in two separate "layers" which open away from each other. That way, depressurization on either side will cause one layer to be stuck, as long as you can somehow disengage the latches (or don't have latches). $\endgroup$ – David Z Oct 23 '17 at 1:28
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    $\begingroup$ @DavidZ There's an even easier way, based on how Airbus aircraft keep their doors shut. The door has large crossbeams with rollers that sit in a track that's shaped vertically like a 7, where the left is low pressure and right is high pressure. To open the door, it has to be pulled back into the cabin while being lifted up. If you're not sure which side has pressure, make the track into a Z. This means the door will have to be free from pressure on both sides before it can be opened. Completely passive! $\endgroup$ – user71659 Oct 23 '17 at 2:56
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    $\begingroup$ If there's a pressure difference, the door would not "spring" open, it would explode open and exit the spaceship at high velocity, probably killing you and everyone else in its path. A small pressure differential times a huge door area makes for a surprisingly large force. $\endgroup$ – Mark Oct 23 '17 at 5:10
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What would be more modern way to do this? Maybe some sort of chemical sticker (reacting with oxygen) on the window?

A valve included in door (or wall) design

The simplest way to do this would be to have a cylindrical hole drilled through the door or through the wall next to the door, with a valve in it: in section:

 |           +----------------------+         |
 |           |        +-----+       |         |
 +-----------+        |     |       +---------+
+---------------------+     +------------------+
|                                              |
+---------------------+     +------------------+
 +-----------+        |     |       +---------+
 |           |        +--X--+       |         |
 |           +---Z-------X-----Z----+         |
 |       interlock: when the two X's are      |
 |       aligned, the door can be opened      |

You can have a spring mounted inside the chamber. If the pressure is equal on both sides, even in the dark, even with shipsuit gloves, even in microgravity, you'll still be able to move the valve end in and out slightly and feel it moving. If there's vacuum or overpressure on the other side, the valve will disappear inside or remain stuck outside, and chances are you won't be able to have it budge easily; again, this will be immediately apparent.

Additionally, the valve ending could be shaped as a whistle, so that forcing it open would cause a piercing whistle to be heard. And the valve might be made large enough that forcing it open on purpose (which would, at that point, require considerable strength) would exhaust the atmosphere in a reasonable time. This is because you might happen to be trapped in a pressurised room, with your suit on, the power off, and unable to exit because you can't open the door against the pressure. Opening the valve would depressurise the room, freeing the door and allowing you to reach the escape pod or whatever (in a similar way, if you ever find yourself in a submerged car, you won't be able to open the doors against the pressure until the car is full of water, which might require opening the windows).

You could also have, instead, a transparent window with two aneroid barometers at both ends, or devices such as a small sphere full of helium gas inside a pierced cylinder - it will float upwards in atmosphere, stay down in vacuum. But this kind of check looks more complicated and error-prone to me; in the case of the helium plunger, as Lenne observed, its operation also depends on there being gravity.

One advantage of the mechanical nipple sticking out or being drawn in is that it can be easily used to both electrically and mechanically override the door lock, preventing it from opening unless pressure is equalized. Then you'd need an override to the override, but that's engineering for you.

Fancier but less robust design

Same as above, but now the plunger (which will probably need to be larger) is also connected to a moving arm or gear, rotating a circular sign disk inside an armored glass window - sort of those toilette signs saying "OCCUPIED". The sign would be divided in three equal slices saying 'PRESSURE', 'NORMAL' and 'VACUUM' on both sides, with the two faces mirrors of one another and aligned on 'NORMAL'. Only one third of the circle would be visible through the window, and normally it would read NORMAL on both sides when the plunger is in the middle (you get the idea).

Emergency tests (for doors with no vacuum check valve)

So you're left in front of this door which you want to open to go on, but - what if there's no air on the other side? The door is not equipped with the valve described above. What can one do?

Door structure: check that the door is not bulging inwards or outwards. If it is not, chances are that it's too thick and rigid for Heikki Mäenpää's "knock test" to be conclusive, but -- try knocking all the same.

Noises: hisses would be strong indicators of a significant pressure differential (and an imperfect seal, which is bad in its own way). Also, if there's significant background noise, perhaps it is possible to check whether there are any of those background noises coming from the other side of the door. If there are, there must be air to transmit them.

Window observation: check on the other side with a strong light. Sharp shadows from a point source are indications of possible vacuum (warning: the Sun or an illuminated room are definitely not a point source). Dust motes are an almost sure indicator of atmosphere, but there are phenomenons (see Hal Clement's Dust Rag) which might mimic that. Moving shreds of papers etc. would also be telltales of there being an atmosphere. Evidence of explosive decompression would likely indicate there is no atmosphere anymore.

Temperature: if the room beyond the door was full of air and this got out, the lowering pressure should have led to quite a detectable drop in temperature. Then, vacuum being a very good insulator, things would have remained cold. This can be seen in the infrared, or maybe by touching the door and walls until they get heated from the pressurised side. Also, heating one side of the door would give different results if the other side is in vacuum or not.

A likely popular rule of thumb would be if you're not certain that there is an atmosphere, treat it as if it was a vacuum.

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    $\begingroup$ +1 for the valve and rod indicator. This is used on tire pressure sensors to indicate correct inflation. Your idea just puts one on each end. $\endgroup$ – Justin Thyme Oct 22 '17 at 18:31
  • $\begingroup$ This spring-mounted valve seems like the best answer so far. Anything like a chemical reaction or other evidence of past depressurization is useless as the room may have been repressurized. The membrane seems too prone to failure - need something solid like this valve that won't fail catastrophically after 'x' changes in pressure. Open door to test - or even open a valve (whistle test) to test risks losing too much air from the good side. And for those who say you'd already be in a suit - not necessarily, you might be one of the few survivors from the central core of the ship and not have one. $\endgroup$ – manassehkatz Oct 22 '17 at 23:26
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    $\begingroup$ Nothing would stop you from designing a whistle inside the valve. You shouldn't try and budge the valve, but if you do and succeed, a piercing whistle will warn of vacuum on the other side without wasting too much air, and it'll only whistle as long as you force the valve open. $\endgroup$ – LSerni Oct 23 '17 at 8:43
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    $\begingroup$ " a small sphere full of helium gas inside a pierced cylinder - it will float upwards in atmosphere, stay down in vacuum" Which way is up in zero-gravity? $\endgroup$ – Lenne Oct 24 '17 at 6:30
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    $\begingroup$ Also, if there's significant background noise, perhaps it is possible to check whether there are any of those background noises coming from the other side of the door. If there are, there must be air to transmit them. Solids do transmit sound, too. In fact, they are better at it than air. You would not hear a sound source that is floating in the room if there is no air in it, but you could hear it if it is fixed to the walls without a vibration dampener. $\endgroup$ – SJuan76 Oct 25 '17 at 23:01
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Knock on the door. If someone answers it, it's probably not open to space If it sounds hollow, there's atmosphere to resonate inside the compartment. If it sounds solid, the compartment is open to space. I happened to have a small, thick-walled plastic bottle at hand to test this hypothesis, and I could sort of hear the difference.

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  • $\begingroup$ I like this solution, since it requires no safety mechanisms that could pose as a source of failure. $\endgroup$ – 16807 Oct 23 '17 at 15:38
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    $\begingroup$ You could only "sort of" hear the difference with a thick-walled plastic bottle. I don't think that equates to knocking on a much thicker, much denser, and much larger object like the doors in question. That "could sort of" would easily become "can't" imo. But I dunno $\endgroup$ – user41674 Oct 23 '17 at 21:33
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    $\begingroup$ And if it knocks back, have your weapons ready? $\endgroup$ – Kevin Rubin Oct 24 '17 at 12:35
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A simple pressure gauge between the two rooms would be fairly easy to arrange and would not need to rely on electricity. There are many types of mechanical gauge for example: http://www.madehow.com/Volume-1/Pressure-Gauge.html

But other sorts are possible such as a mercury manometer such as this:

manometer

The red band is Mercury. If one end of the u tube was connected to one compartment and the other end was connected to the adjacent compartment then the pressure difference could be easily seen by looking at the tube. It would also be possible to include micro fine porous materials to allow the passage of air but prevent Mercury vapour from escaping into either compartment.

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    $\begingroup$ Actually, if you make the indicator out of something non-toxic, you can have one end closed, providing a "reference" pressure. Empty indicator means compartment open to space. If you want it visually unobstrusive when not needed, you could put a plastic plug in it that covers the text "open to space" when the pressure is nominal. $\endgroup$ – HAEM Oct 22 '17 at 14:22
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    $\begingroup$ @Slarty You are aware that liquid barometers rely on presence of gravitational pull? Gravitation exerting force on liquid creates pressure which counterbalances atmospheric pressure (you can make barometer with literally any liquid, but water barometer would be about 10m high IIRC). To make them work in freefall you have to add a lot of fancy contraptions with springs and membranes, creating a strictly worse aneroid barometer. Your (XVI level) suggestion literally doesn't work without anti-gravity and even if there is one, something that essential should never rely on it. $\endgroup$ – M i ech Oct 22 '17 at 17:23
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    $\begingroup$ -1. In the absence of gravity, all of the liquid will just shoot out of the vacuum end. Since spaceships often rotate along multiple axes even while in a gravity well, this is obviously not a good idea. $\endgroup$ – March Ho Oct 23 '17 at 0:26
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    $\begingroup$ @Slarty The question specified a spaceship. Spaceships are often in space, in zero-gravity. $\endgroup$ – Lenne Oct 24 '17 at 6:34
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    $\begingroup$ @Lenne "Spaceships are often in space" - [citation needed] ;) $\endgroup$ – brichins Oct 24 '17 at 18:42
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Pressure gauges have already been mentioned, along with several simple mechanisms to achieve this. If you want a real-world example (I work in a facility where rooms are kept under negative pressure) you would use something like a Magnehelic. This is based on the rubber diaphragm concept mentioned in another answer, but linked via a leaf spring to a dial for easy reading. The mechanism is purely mechanical, has few moving parts, is cheap and doesn't depend on gravity. The site also mentions that it works with a vacuum, although we use it for smaller pressure differentials.

this is not a very scientific way to go about this.

I work at a national research centre and this is the solution used in a recently-constructed facility, so can confirm this is a modern solution used by actual scientists ;)

Magnehelic (front) Magnehelic (cross-section)

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    $\begingroup$ +1 for an instrument that is isolated on one side of the door (aside from the blowout plug in the diagram, which could be eliminated for this use case). My problem with many of the other answers is having a hole all the way through the door, and (implicitly) relying on a delicate instrument of some sort to maintain air-tight resistance to hard vacuum. My only question here is: how do you read the dial from the other side of the door? Adaptations to make it visible through a window/portal (e.g. a viewing periscope or just sticking it straight on the window) could interrupt normal usage. $\endgroup$ – brichins Oct 24 '17 at 19:09
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    $\begingroup$ @brichins You worry too much. One atmosphere is 101.3kPa or about 10t for a hatch with an area of a square metre. But you need only a square millimetre to measure pressure, and the force would be 10g. If the gauge cracked you could seal it with chewing gum. $\endgroup$ – Peter Wone Oct 27 '17 at 10:10
  • $\begingroup$ @PeterWone Yes, it's not hard to seal a pinhole, but it's an unnecessary point of potential failure that should be eliminated. Constant force and a large temperature differential will eventually lead to failure, requiring periodic inspection and maintenance if you want to keep using the ship after a few depressurization cycles. $\endgroup$ – brichins Nov 2 '17 at 18:47
  • $\begingroup$ @brichins - ongoing maintenance is required in all environments for any machine more complex than a brick. Aircraft engines get a full overhaul every 10,000 hours here in Australia, and there is an inspection and maintenance schedule for every component of the aircraft. Why should spacecraft be different? $\endgroup$ – Peter Wone Nov 3 '17 at 0:53
  • $\begingroup$ @PeterWone it shouldn't be any different - which is why you shouldn't introduce more parts that require maintenance unless you have to. Maintenance is a necessary side effect of having a complex tool needed to achieve a goal - and having to do that maintenance in space has a far higher cost. Astronauts (especially in the near future) have limited time and far better things to do that are actually useful to the mission. $\endgroup$ – brichins Nov 3 '17 at 21:07
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Real world example. In medical contexts you often want low pressure rooms (often called negative pressure) for patients with infectious airborne diseases like flu or tuberculosis so that air will flow into the room and not escape with the infectious particles. Not perfect, but just one part of an infection control plan.

Anyways, these are hard to maintain and often break down so they have a clear plastic cylinder poking through the wall with a red ball inside. The cylinder is on its side with a slight incline down towards the higher pressure side. There's a small vent hole at each end. The negative pressure side has to be low pressure enough to suck the ball up the slope and into that side of the room. The ball then covers the vent hole. If you see the red ball on your side it means the other side is at close to equal or higher pressure.

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  • $\begingroup$ Solution much better than any barometer. At least if it's broken, you see it's broken. $\endgroup$ – Mołot Oct 23 '17 at 6:48
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    $\begingroup$ Still requires gravity, often absent in spaceships. Especially malfunctioning spaceships. $\endgroup$ – Lenne Oct 24 '17 at 6:36
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    $\begingroup$ Any centering force would do, including a spring. I.e. put the ball on a small metal lever which will bend towards the vacuum side. In the absence of a pressure difference, there's no force to bend the lever and the ball will be in the middle state. Cost: a few cents. $\endgroup$ – MSalters Oct 24 '17 at 11:36
  • $\begingroup$ @Lenne Doesn't need gravity if the ball fits the tube well enough. Atmospheric pressure, especially vacuum versus normal pressure, should overcome microgravity or even a moderate amount of spin against the tube. The ball can be hollow, extremely light, so almost any air flow in one direction or another will move it. $\endgroup$ – Jason K Oct 29 '17 at 2:23
  • $\begingroup$ @JasonK I referred to "The cylinder is on its side with a slight incline down towards the higher pressure side". No gravity means no "down". But if the ball is just resting at the "far" side, how do you know if it moved there due to pressure or just the capt'n made a sharp turn, so it got moved there by inertia? $\endgroup$ – Lenne Oct 30 '17 at 12:06
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The door just won’t open

If you've ever reopened a refrigerator right after you closed it, you know a tiny pressure differential makes a big difference on opening a door. Like the fridge, pulling a door toward you against a lower pressure outside will be simply impossible. That direction is easy. How about the other direction?

If you've ever done rope/cable/chain rigging with couplings, you know that it's quite hard to unlook a line that is under load. Most couplings either require that you unload them entirely and slack the lines (like a carabiner), or the mechanism binds very hard so as to make it impracticable without "taking a wrench to it".

Apply the same design principles to a door under pressure. Include an over-center mechanism or other arrangement. And then tune the length of levers and the ratio of gearing so it's very hard to open against atmo; consider this "good UI design".

Of course you must also help people distinguish between atmo on the other side of that door vs the door binding from damage. That is easy since atmo is bouncy and binding is not.

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As a combination of two previous ideas, and addressing potential issues with each one (but possibly creating new issues), consider two clear tubes, one twice the diameter of the other, and connected to form a loop which passes through the door or a nearby wall

Each tube contains a ball fitting snugly but able to move without requiring significant force. The smaller ball is easily identified by size, and may be coloured or made fluorescent to aid in telling it apart from its partner. It serves as the warning for vacuum on the other side.

They are connected by a cable or cord forming a loop, so that regardless of the direction, pulling or pushing one ball will cause the other to move at the same time and by the same distance. The balls are not permitted to quite reach the changeover point between tube sizes.

Small vents are placed across the changeover point on both sides of the door/wall. When one side is vacuum and the other pressured, the greater area of the large ball is experiencing four times the force of the small ball (radius-squared of a circular cross-section) and a net force of three times the force on the small ball pushing the large ball into the side with vacuum. The cord pulls the small ball into sight on the side with atmosphere, showing vacuum on the other.

A simple additional elastic band in the larger tube pulls the large ball into the middle when the two sides are equivalent pressure.

The design is somewhat roughly shown below.

a hand-drawn schematic of the vacuum indicator described above

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    $\begingroup$ Interesting low-tech approach, but the force imbalance it's based on is incorrect (force != pressure) for a passive device; also not sure about a cable holding up well against vacuum while frozen near 0K. I would propose this instead be a single tube with a ball firmly (but not permanently) fixed at either end, in full view (both sides have a 'green light'), and a ball valve seal in the middle. When one side loses pressure, the ball on the safe side is blown out of view and closes the seal in the middle of the door. $\endgroup$ – brichins Oct 24 '17 at 20:05
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    $\begingroup$ Pressure times area is force. For identical pressure, more area is more force. Similar principle to hanging two chunks of metal on a string over a pulley, mass times gravity is weight force, more mass is more force, so the one with more weight (or more area) gets moved in the direction of the force, and the other dragged the opposite way. $\endgroup$ – Nij Oct 25 '17 at 8:31
  • $\begingroup$ But the forces are distributed over areas of a fluid (air), not acting on point loads - equal pressure. By your reasoning, the water in a kettle would spontaneously squirt up the spout because its cross section is smaller than that of the pot. If you read the link on hydraulic lifts, which use the principle you're proposing, you'll see they rely on using a smaller force but equal pressures. Also, the smaller force (on the little ball) has to act through a larger distance to accomplish the same amount of work - your cable would have to stretch, unsuitable for the proposed use. $\endgroup$ – brichins Oct 25 '17 at 16:03
  • $\begingroup$ Ah, I see someone else has already suggested the single-ball indicator, and included a real-world example. $\endgroup$ – brichins Oct 25 '17 at 22:46
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It would be very easy to have an indicator in the window or door that raises a flag any time the pressure on one side changes. Some sort of membrane perhaps, that would move away from your room if the other room was depressurised.

I'm pretty certain something like this is used on aeroplane doors... It may be physical or electronic.

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  • $\begingroup$ Would this indicate well on partial evacuations? $\endgroup$ – Ash Oct 22 '17 at 14:59
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    $\begingroup$ Of course - in fact the amount the membrane moves would be a good indication of the pressure differential $\endgroup$ – Rory Alsop Oct 22 '17 at 15:02
  • $\begingroup$ Could you get a colour shift too? I don't know but to me colour has always been a big part of warnings in the modern world and I don't see that changing. $\endgroup$ – Ash Oct 22 '17 at 15:06
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    $\begingroup$ Colours in warnings are always an issue - think of colour blind folks. You want a physical flag of some type that moves. $\endgroup$ – Rory Alsop Oct 22 '17 at 15:09
  • $\begingroup$ True, maybe a symbolic warning that is only visible when the diaphragm inflates. $\endgroup$ – Ash Oct 22 '17 at 16:29
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Have the door open out

As in, you have to pull the door to open.

If there is no pressure on the other side, you won't be able to / it will be very difficult to open the door, or you'll be able to open it a little before it's quickly sucked closed again.

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    $\begingroup$ That causes problems, for the door, if you're in vacuum and the room has pressure, that's a lot of weight to hold for the fabric of the door. $\endgroup$ – Ash Oct 23 '17 at 12:27
  • $\begingroup$ True but the question specified you being on the side with the pressure... $\endgroup$ – colmde Oct 23 '17 at 22:12
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    $\begingroup$ It actually doesn't specify pressure on either side of the door, it just asks how to check whether there is any or not. $\endgroup$ – Ash Oct 24 '17 at 10:10
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    $\begingroup$ I came here to post this. Even a tiny pressure differential will make the door very difficult/impossible to open. $\endgroup$ – Dave Oct 25 '17 at 13:23
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People have mentioned pressure sensors already, and I want to reiterate that with a real-world example of using such a device, off-the-shelf in a spacecraft. Skydivers frequently wear mechanical ANEROID BAROMETERS on their hands; they read pressure out with a rotating dial indicator, like a clock-hand. It would be simple for anyone making a space door to change their output gearing and indicator face to rescale to a different range. (Felix Baumgartner took one to over 127,000 ft altitude, so I assume they are compatible with vacuum.) One model, the "Altimaster Galaxy", is $169 retail. The door could contain a pocket or cell in the door in which one of these could sit, open to the outside and having a transparent viewport to the inside. The pocket or cell could use a sufficiently small commutation tube to protect it from abrupt changes in pressure, as well.

Here you can see a wrist-mount aneroid barometer used as a cockpit instrument, mounted in the upper left corner of this picture inside the White Knight aircraft, whose cockpit was designed to match SpaceShipOne's cockpit for training reasons.

WhiteKnight1 cockpit

A similar device (this time on the upper right of the dash) is visible in this picture of SpaceShipOne in microgravity flight at the edge of space; judging from the pilot having no gloves covering his hands, notes that SpaceShipOne was intended to be sea-level-pressurized (and White Knight the same - to unlimited altitude), and the assumption that unpressurized craft cannot go into space with people aboard wearing no gloves, I assume they cannot be using this as an air-pressure altimeter and are instead using it to verify correct/ongoing cabin pressurization. Visually, it does not appear they changed the gearing or face of the aneroid barometer / pressure altimeter at all before using it. (And you can see the empty wrist-mount strap slots on it.) SpaceShipOne during low-gee flight

So, one of these altimeters (aneroid barometers) mounted on each side of the door you want to check for pressurized atmosphere and visible through the door would do the trick, without any other design impact on the door, its opening mechanism, power requirements, etc..

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As I said in the comments, odds are that if you've survived depressurisation, you're probably in a survival suit and aren't too worried about the atmospheric status of any given room. Be that as it may, my suggestion for visible room monitoring would be a dye capsule of some sort that bursts when the room is exposed to vacuum. This capsule would be next to (or even in) the door, in a pressure safe vestibule exposed to the room behind the door, but not the corridor beyond (there would be another in the other side of the door to indicate the status of the corridor for those in the room), this capsule would burst and stain the indicator window when exposed to vacuum giving a quick warning. Dye colour would of course depend on the species and their particular safety protocols and colour sensitivities.

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    $\begingroup$ Painting windows looks like a bad, bad idea to me. Especially in emergency situation, when you need all the help you can get, making it impossible to look into a room isn't nice. Unless, of course, this is only an additional, small window with only purpose to be painted. Sadly, then it stops to be obvious mark. $\endgroup$ – Mołot Oct 22 '17 at 16:11
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    $\begingroup$ @Mołot Yes sorry it's just an indicator window not the window into the room, which you probably wouldn't have anyway, privacy being hard enough to get on most starship designs I've seen. $\endgroup$ – Ash Oct 22 '17 at 16:14
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    $\begingroup$ you will have windows to some compartments you may need to look into before you enter. No need for privacy in a lab, for example. $\endgroup$ – Mołot Oct 22 '17 at 16:25
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    $\begingroup$ At some point you are going to want to take off your pressurized suit; at that time the presence or absence of air in your room will become of interest to you :) $\endgroup$ – Jeremy Friesner Oct 24 '17 at 0:54
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    $\begingroup$ @brichins Hmm death by safety equipment, I like it. $\endgroup$ – Ash Oct 25 '17 at 9:09
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In the first season of Dark Matter, they had a situation just like this. Here, one1 of the main characters simply felt the temperature and based on the fact that it felt cold, concluded that there was space on the other side of the door.

To me, it seems rather plausible that a bulkhead or a door functioning as a bulkhead, does the bare minimum to keep the insides warm enough, while the hull is built for comfort as well. As such, a difference in temperature seems like a good indicator to me.

1: As an added bonus, this wasn't just one of the main characters. It was One of the main characters. It was One. Yep, a guy named One.

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    $\begingroup$ I like my space doors insulated. ;) $\endgroup$ – X Goodrich Oct 24 '17 at 17:17
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    $\begingroup$ This seems unlikely - things don't cool down very quickly away from conductive matter, so if there is vacuum on the other side of the door it will not cool down very fast. See, for example, a vacuum flask, which uses vacuum to stop things from cooling down (or heating up). $\endgroup$ – Logan Pickup Oct 24 '17 at 22:53
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Temperature gauges on the doors. In vacuum no particles means no heat transfer. If windows all the better. I would think bulk heads and airlocks have a door locking mechanism that locks shut toward depressurization like an inner tube stop leak gel, it has a ledge inside the door jam that gets pulled toward the vacuum and seals the doors. I would also imagine you could check the doors with your sophisticated temperature gauge known as your hand. If you feel a difference from the door material temp to a different wall and it is of the same material it should tell you a lot about the atmosphere of the next room.

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  • $\begingroup$ A thermometer is a good idea, perhaps something that is invisible at normal temps but a color change reveals a giant warning message. As for checking with my fingers, I'm not taking my hand out of my pressure suit anytime soon after a decompression event. $\endgroup$ – brichins Oct 25 '17 at 22:52
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(The only solution that does not require the door to already contain a special device)

So you are the only survivor, there is no electricity, the normal safety devices are out of order, and the door is a Soviet era 100% aluminum door with no fancy features whatsoever.

It looks like you have no way to find out whether the other side of the door is outer space, or on the opposite still contains the oxygen you need.

... until you find that battery-powered thermal imaging camera, brought by the Korean team to study insulation in space.

Just point the camera at the door, and check the color of the door's outer frame:

  • Red (or similar to other parts of the room): you can open.
  • Blue: Only open if you want to get a taste of outer space's −270.45 °C temperature.

enter image description here enter image description here

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"Everything should be made as simple as possible, but not simpler." - Albert Einstein

Uh Oh! Reading back through the posts, it seems that Jasper had the same idea and posted it before me. DENIED!!!

Try placing the uncovered palm of one hand against the bulkhead near the door or across the aisle. Place the uncovered palm of the other hand against the door itself. If there is a significant difference in temperature, don't open it as the odds are high that it's both depressurized and atmosphere challenged. It's like checking for a fire behind a closed door, only colder.

If the temperatures are close, then it's probably okay to open it as it probably has both atmosphere and pressure since you wouldn't keep such a room heated to match it's surroundings.

If that fails, call either Spock or Scotty -- one of them will know the answer.

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Each side of the door has its own tube which initially is parallel to the floor, penetrating almost halfway into the door, then bending upwards and widening slightly. The vertical portions of each tube are visible via glass plates on the opposing side of the door. The plates are not looking at the same tube - each side has an opposing tube.

In the vertical portion, a ball either floats or not. if the other side of the door is vacuum, the ball is not floating and is probably not visible through the plate.

If the other side has pressure, then it is that pressure that floats the ball, making it visible.

Purely mechanical, requires no power and has no sensitive, fragile, interconnected moving parts. Just a tube with a ball in it times 2.

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  • $\begingroup$ How would air pressure make a ball "float"? (Especially in the absence of gravity.) $\endgroup$ – duskwuff Oct 25 '17 at 19:38
  • $\begingroup$ Assuming you mean a J-shaped tube, with the lower leg perpendicular to the window. This would rely on a sealed tube end with vacuum at the top of the J, and a ball/cylinder that was airtight with the tube while remaining very low friction. Failure of any seals in this device would display a false warning, which is nearly as bad as no warning after it 'cries wolf' a few times and wastes hours of diagnostics and repair. $\endgroup$ – brichins Oct 25 '17 at 22:57
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Light diffraction

No atmosphere means no particles to diffract light in the depressurized chamber. Sharp shadows could be an indicator.

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  • $\begingroup$ Interesting. But hard to spot, especially in emergency situations with bad lighting conditions. $\endgroup$ – rluks Oct 27 '17 at 13:55
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A simple solution is a diaphragm that keeps the door latched if it's bent 'too much' either way. (And also displays this fact somehow.

You have to be careful, though, as the diaphragm itself can be a source of leaks. Ditto, what happens if the mechanism jams?

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I would reccomend jamming a screwdriver in the door.

If you hear the distinct sound of wind, followed by your ears exploding, there is likely a vacuum on the other side of the door relative to you. Remove the screwdriver and act accordingly

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I would recommend something primitive. Such as a bellows, where the pressure difference depresses a sensor effect (or just closes a switch). That then powers red LEDs that say vacuum.

Pluses to this system: It doesn't need to be manually operated. It is red. I like red.

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    $\begingroup$ Please, take care and edit it. Proper uppercase at the beginning of sentences, for example. Spelling. Then there is content. Be precise. Avoid writing things like "or other". Last but not least, what you personally like is irrelevant as an answer to technical question. This site is about facts, not opinions. $\endgroup$ – Mołot Oct 23 '17 at 19:00

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