Life support

Atmosphere

Cabin pressure USLAB000058 · in mmHg

The station is held at sea-level pressure — 14.7 psi with 21 % oxygen, the same 760 mmHg you are breathing — so daily life needs no acclimatisation. Spacewalks still do: the suit runs at 4.3 psi, and that drop has to be paid for with a prebreathe protocol before the hatch opens.

Cabin temperature USLAB000059 · in °C

Measured at the inlet of Destiny’s port air conditioner, so this is the air coming back from the cabin rather than the chilled air leaving it. The station is kept at ordinary room temperature; the air conditioners hold both temperature and humidity, and the water they wring out of the air ends up in the drinking supply.

Destiny ppO₂ USLAB000053 · in mmHg

Partial pressure of oxygen: this, not the percentage, decides whether air is breathable — about 160 mmHg at sea level, and held between 146 and 178 aboard. The atmosphere sensors report rarely, so read the age beside the value before comparing two of them.

Destiny ppN₂ USLAB000054 · in mmHg

Nitrogen does nothing for breathing. It is the diluent: NASA-STD-3001 requires at least 30 % of the cabin gas to be inert, to hold down the fire risk and the oxygen the lungs absorb over a long stay, and aboard the mix is kept close to sea-level air. It is also the gas a spacewalker has to flush out of the blood before an EVA, which is what the prebreathe is for.

Destiny ppCO₂ USLAB000055 · in mmHg

Carbon dioxide builds up in a sealed volume, and the threshold that matters is lower than most people expect. NASA-STD-3001 caps the 1-hour average at 3 mmHg, down from an earlier 3.8–7.5 mmHg range: crews reported headaches from 2.8 mmHg upward, and 19 of 49 astronauts studied had them. Below 2.5 mmHg the risk falls under 1 %.

Tranquility ppO₂ NODE3000001 · in mmHg

The second oxygen analyser, in the module that houses the oxygen generator. It once read 17.7 mmHg apart from Destiny’s, which turned out to be eight days between the two measurements rather than a difference between modules — both sensors have been re-sending old readings for weeks.

Tranquility ppN₂ NODE3000002 · in mmHg

Tranquility’s reading of the diluent. Ventilation moves cabin air between the modules, so the two nitrogen readings should agree once their timestamps do; both partial-pressure analysers report rarely and have stalled for weeks, so check the age before reading a gap between them as a gradient.

Tranquility ppCO₂ NODE3000003 · in mmHg

The reading from the module where the scrubbing is done: Tranquility houses the carbon dioxide removal assembly, along with the oxygen generator, the water recovery racks, the toilet and the treadmill. The 3 mmHg one-hour limit applies here as it does in Destiny; this sensor, like the others, has re-sent the same value for weeks.

Water and oxygen

Potable water tank NODE3000009 · in %

Filled by recycling: distilled urine and condensate from the cabin air become drinking water again.

Urine tank NODE3000005 · in %

The toilet in Tranquility’s hygiene compartment collects urine into this tank, and the urine processor draws it down by vacuum distillation. What distillation cannot recover is left as brine, and the brine processor now takes water out of that too — the step that lifted total recovery from 93–94 % to the 98 % NASA reported in 2023.

Waste water tank NODE3000008 · in %

Not sewage: this is the water processor’s feed tank, holding the distillate from the urine processor and the humidity the air conditioners condense out of the crew’s breath and sweat. From here the water goes through filters and a catalytic reactor, gets a dose of iodine against microbes, and fills the potable tank.

Urine processor state NODE3000004 · a state

NORMAL is the working state, and the usual one: the assembly distils urine under vacuum and leaves a brine behind. STANDBY and IDLE are the machine waiting, MAINTENANCE and SHUTDOWN are it being serviced or brought down, and SYSTEM INITIALIZED is the state just after power-up, before it has been told what to do.

Water processor state NODE3000006 · a state

PROCESS means wastewater is being turned into drinking water: a series of filters, then a catalytic reactor that breaks down trace contaminants, then iodine to stop anything growing in it. STANDBY is the pause between runs; HOT SERVICE and FLUSH are servicing states rather than faults, and STOP, SHUTDOWN and WARM SHUTDOWN are the halts.

Water processor step NODE3000007 · a state

The finer phase within the state above: a run does not begin at FLOW, the machine vents, heats up and purges first, and the TEST steps are self-checks. NONE is what it reports whenever no sequence is running. Enumerations like this change rarely, and an old timestamp here dates the last transition, not a dead sensor.

Oxygen generator state NODE3000010 · a state

Electrolysis splits the water molecule: oxygen goes into the cabin, hydrogen is vented or recombined with CO₂.

Oxygen production rate NODE3000011 · in lb/day

The sensor stopped reporting weeks ago — the stream keeps re-sending its last value. Its magnitude does not match what the generator can produce either.

Air conditioning

Destiny port air conditioner USLAB000064 · a state

ON is the working state: cabin air is drawn through a heat exchanger chilled by the low-temperature water loop, which cools it and condenses the humidity out, and that condensate goes to the water processor. DRAIN and DRYOUT are what is done to the wet heat exchanger when the unit comes out of service. Destiny has two, port and starboard; the cabin temperature is measured at this one’s inlet.

Destiny starboard air conditioner USLAB000065 · a state

Destiny’s second air conditioner, in the starboard rack bay opposite the port unit. The same states: ON is cooling and dehumidifying the lab’s air over a heat exchanger fed by the low-temperature loop, DRAIN and DRYOUT are the servicing of that heat exchanger.

Harmony air conditioner NODE2000003 · a state

Harmony has an air conditioner of its own. Besides being the junction the laboratories hang off, the module holds four of the crew’s sleeping quarters, so this is the unit that conditions the air the crew sleep in. ON is cooling and drying; DRAIN and DRYOUT are servicing states.

Tranquility air conditioner NODE3000018 · a state

Tranquility’s unit serves the module that holds the toilet, the treadmill and the weight-lifting device, and its condensate has the shortest journey of any: the water recovery racks that turn it back into drinking water are in the same module. ON is the working state; DRAIN and DRYOUT are servicing of the heat exchanger.

Destiny air coolant temp USLAB000061 · in °C

In weightlessness warm air does not rise: without forced ventilation, a bubble of carbon dioxide would form around each astronaut.

Destiny avionics coolant temp USLAB000060 · in °C

The temperature of Destiny’s moderate-temperature water loop, which nominally runs at 17 °C and takes most of the systems heat — the avionics and the payload racks. Water rather than ammonia, because a leak inside a habitable module must be harmless. This sensor is slow rather than dead: it sat still through a one-minute capture and resumed changing over five.

Harmony air coolant temp NODE2000006 · in °C

Harmony’s low-temperature water loop, designed like Destiny’s to run at 4 °C — cold enough that the air conditioner’s heat exchanger, which this loop feeds, condenses humidity out of the cabin air. Destiny, Harmony and Tranquility each have such a pair of loops, and a pair can be joined and run as one to spare a pump or ride out a failure.

Harmony avionics coolant temp NODE2000007 · in °C

Harmony’s moderate-temperature loop, the one Destiny runs at 17 °C. Harmony’s job includes converting and handing on power and cooling from the truss to the elements attached to it, and this loop’s heat goes the same way — through an interface heat exchanger into the external ammonia, and out through the radiators.

Tranquility air coolant temp NODE3000013 · in °C

Tranquility’s low-temperature loop, the cold one — designed for 4 °C in Destiny — so that the air conditioner it feeds can condense water out of the air. In Tranquility that condensate has only to cross the module to reach the water processor that turns it back into drinking water.

Tranquility avionics coolant temp NODE3000012 · in °C

Tranquility’s moderate-temperature loop, the kind that nominally runs at 17 °C in Destiny. Two loops at two temperatures rather than one: NASA’s thermal overview gives the reasons as keeping the heat loads apart, simpler management, and redundancy when a pump fails — in which case the pair can be run as a single loop.

Airlock

Airlock pressure AIRLOCK000054 · in mmHg

Quest has two compartments: the equipment lock, where the suits are stored and serviced, and the crew lock, which is the one actually emptied to vacuum.

Crewlock pressure AIRLOCK000049 · in mmHg

This is the one to watch during a spacewalk: it falls from cabin pressure towards vacuum, pausing at 5 psi for a five-minute leak check before the hatch can open.

High-pressure O₂ valve AIRLOCK000050 · a state

Quest carries the station’s high-pressure oxygen and nitrogen in tanks mounted on its outside, and this valve is on the high-pressure oxygen line. The tanks are refilled from visiting cargo vehicles through the Nitrogen/Oxygen Recharge System. IN-TRANSIT is the valve caught between positions; FAILED means it did not reach the one commanded.

Low-pressure O₂ valve AIRLOCK000051 · a state

The second oxygen line out of Quest’s tanks, at a lower pressure than the first. Oxygen and nitrogen are the two gases the life-support system meters into the cabin to hold the partial pressures and the total pressure where they belong — O₂/N₂ control is one of its listed functions. The same four positions as the other valves.

Nitrogen supply valve AIRLOCK000052 · a state

Oxygen the station makes from water; nitrogen it cannot make, so the diluent comes up from Earth, is stored in the tanks on Quest’s hull and topped up from cargo vehicles through the recharge system. This valve is open when nitrogen is being let into the cabin to hold up the total pressure.

Airlock air conditioner AIRLOCK000053 · a state

Quest has an air conditioner of its own because it is a habitable module in its own right: the equipment lock stays at cabin conditions while the crew lock beyond the hatch is pumped down. The same states as the other units — ON is cooling and drying the air, DRAIN and DRYOUT are servicing of the heat exchanger.

Where it lives on the station

• Destiny U.S. Laboratory

  The American laboratory and nerve centre of the US segment: the station is commanded from here, and most experiments run here.

• Harmony Node 2

  The forward node: it connects the European and Japanese laboratories and receives vehicles docking at the front.

• Tranquility Node 3

  The node housing the life-support systems: water recycling, oxygen generation, toilet and exercise equipment.

• Quest Joint Airlock

  The American airlock: astronauts spend the night inside at reduced pressure before a spacewalk.

Each link opens the 3D twin on that part, with its live readings beside it.