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The '''pound per square inch''' (psi) is a unit of pressure: one pound of force on one square inch. The letters by themselves do not say where zero is. '''psia''' counts from a vacuum. '''psig''' counts from the air around the gauge. A number in psi is not interchangeable with the same number in psia or psig until that zero is named.

== The unit ==
One psi is 6,894.757 pascals.<ref name="nist">NIST, [https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e2008.pdf ''Guide for the Use of the International System of Units (SI)''], Special Publication 811, 2008 edition, Appendix B. The pound-force per square inch converts to the pascal by the exact relation of the inch (0.0254 m) and the pound-force.</ref> Air at sea level is about 14.7 psi. One standard atmosphere is 14.696 psi.

== Absolute pressure ==
psia means pounds per square inch absolute. Zero is a perfect vacuum, so the number is the pressure the gas is actually at.

Sea-level air is about 14.7 psia. A cabin held at 5.0 psia is about a third of that. On Apollo the command-module cabin in flight was near 5.0 psia of oxygen, and suit pressure on a moonwalk was a little under 4 psia. Those figures are absolute. They are not a difference from the air outside the spacecraft.

== Gauge pressure ==
psig means pounds per square inch gauge. Zero is whatever pressure is on the outside of the gauge, not a vacuum. On the ground at sea level that outside pressure is about 14.7 psi, so a tank that reads 900 psig is at about 915 psia. The gauge has already subtracted the air around it.

The outside port decides the zero. If the port sees sea-level air, zero is about 14.7 psia. If the port sees the vacuum outside a spacecraft, gauge and absolute are the same number. If the port sees the cabin, zero is cabin pressure, and the gauge reads only how far the measured gas sits above the cabin.

The Apollo handbooks label the service-module oxygen supply 900 psig. The 1972 flight report labels that same supply 900 psia, within 35 psi. The 15 psi that separates a sea-level gauge reading from an absolute reading sits inside that 35 psi band. The two labels are one oxygen supply. They are not two different tank pressures. The wording, the regulators, and the photographs are on [[Apollo command module oxygen]].

== Inches of water column ==
An inch of water column is a pressure. It is the pressure at the bottom of a column of water one inch tall. It is not a depth of water on the floor, and it is not water standing in the cabin.

About 27.7 inches of water column equal 1 psi, so the unit is for differences too small to bother stating in psi. Two inches of water column is about 0.07 psi. Three inches is about 0.11 psi. Nine inches is about 0.33 psi.<ref name="nist" />

The command-module suit regulator was specified this way because suit pressure and cabin pressure differed by a fraction of a psi. The suits were held a few inches of water column below cabin pressure so the cloth stayed soft. A relief valve opened if suit pressure rose between 2 and 9 inches of water column above cabin pressure.

== See also ==
* [[Apollo oxygen]]
* [[Apollo command module oxygen]]

== References ==
<references />

[[Category:Apollo evidence]]
[[Category:Spaceflight]]