Washington University ChE 433 Digital Process Control Laboratory Pressure and Level Control Lecture

Pressure Units P = FA, Force * Area psia = pounds/in^ 2 absolute 1Atm = 14.7 psia psig = pounds/in^ 2 gauge or psi above atmospheric Example: 50 psig = 64.7 psia psid = pounds/in^ 2 differential, i.e. A pressure drop or loss across anything in the pipe line i.e. A control valve. psi units are usually assumed to be psig. (See convert.exe on web site for other conversions.)

Other Useful Units Inches of water 27.67” H2O = 1 60 DegF 1 ft^3 of water weighs 62.4 pounds 1 gallon of water weighs 8.33 pounds

Level Measurement Measure the pressure at the bottom of the tank. Pressure = height * density; inches*pounds/in^3 or pounds/in^2

Our Lab Level Measurements use D/P, differential pressure

differential pressure The difference The bottom pressure minus the top pressure. Then If we know the fluid density, we know the level. Be careful of the density change vs. temperature

Level Transmitters What other ways can we measure level? And How do these things work?

Instruments Radar, Ultrasonic and RF Level Transmitters.pdf Pressure Transmitters.pdf

Level and Pressure Control Level Control.pdf Pressure Control.pdf