Pressure Measurements in Skydrol Hydraulic Systems

Skydrol is a phosphate ester hydraulic fluid used in aircraft.  Hydraulic systems in aircraft are most often used to position control surfaces and actuate the landing gear.  Broken hydraulic lines in and around hot engine parts were causing fires in the early years of air travel, so in the 1940s Douglas Aircraft  and Monsanto jointly developed Skydrol, a fire-resistant hydraulic fluid specifically formulated for use in aircraft systems.  Skydrol has since undergone several refinements, but it remains the standard aircraft hydraulic fluid.

The Validyne P55 pressure transducer is often used to measure of hydraulic pressures during flight testing.  The P55 is available in full scale ranges that can cover hydraulic pump pressures to 3200 psig.  The P55 can also measure low full scale differential pressures, and this is often used to monitor the pressure drop across valves or filters – even where small differential pressures occur at up to 3200 psig pump pressures.  Model P365, a variant of the P55, is available in ranges up to 10,000 psig and can be used for differentials as low as 8 psid.

The 410 steel sensor body combined with optional Viton O-Rings make both the P55 and P365 compatible with Skydrol hydraulic fluid.  The optional +/-5 Vdc or 4-20 mA analog outputs make data logging of hydraulic pressures easy, and the optional -65F to +250F temperature range of Validyne transducers are perfect for aircraft test environments.  The small, vibration-resistant P55 package has been used successfully in drone and propeller-driven aircraft flight test applications.