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In space shuttle's operators manual, Page(4.1-4), it is written as:

The H2O quantity is determined by a PVT calculation based on H2O tank pressure and temperature transducer readings as well as GN2 tank pressure and temperature transducer readings. The calculated quantity is output to the HYD/APU display and to the SM SYS SUMM 2 and SPEC 86 APU/HYD displays enter image description here

Why was it necessary to measure water quantity? First and foremost, why was water necessary in the Space Shuttle? I am pretty sure that water was not for drinking purposes. Perhaps for the purpose of cooling but not the engine.

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Water was used for many purposes in the shuttle.

The particular application you are asking about is the Water Spray Boiler (WSB), part of the Auxiliary Power Unit / Hydraulic (APU/Hyd) system. This device provided cooling to the lubrication oil in the APU and to the hydraulic fluid as required. Each of the three APU/Hyd systems had a dedicated WSB.

This cartoon shows the three operating modes of the WSB.

enter image description here

Here is an overall schematic of the WSB, from the document linked in the question.

enter image description here

For the spray mode, water was stored in a nitrogen-pressurized tank, shown at the upper left of the schematic. This is the specific water you are asking about. Knowing the quantity of water was necessary to manage the system.

Other uses of water in the shuttle:

  • water cooling loops transferred heat from cabin equipment to Freon cooling loops
  • injector cooling water for the APU
  • Water was created as a byproduct of electrical power production - the O2 / H2 reaction in the fuel cells. This water was consumed by the crew and used for cooling the Freon loops.

Source: former Systems instructor

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  • $\begingroup$ Besides which, I would be surprised if there were any systems-level on-board resource that wasn't tracked somewhere. Perhaps not all mission resources (remaining paper towels - if such were on board) but anything at all related to the orbiter itself. $\endgroup$ Jan 18 at 18:05
  • $\begingroup$ @SoronelHaetir agreed, all the uses of other water on the orbiter that I mention all had their quantities tracked, as did hundreds of other quantities. This answer was written to focus specifically on what was asked. $\endgroup$ Jan 18 at 18:51

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