Comment

December 2009

ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

A tank level is measured using a differential pressure transmitter (DPT-1) and a bubbler tube. The tank is vented to atmosphere. The bubbler tube is 1 foot from the bottom of the tank, and the tank wall is 20 feet high. A 0-10 psig differential pressure gauge (DPI-1), accurate to .25 per cent of full scale, is connected to the bubbler tube connection at the high side of the transmitter. The low pressure side is connected to the tank top. With the tank containing liquid with a specific gravity of 1.2 and the level in the tank at 14 feet, the reading on DPI-1 in pounds per square inch (psi) is most nearly equal to:

 A.  5.6 psi
 B.  6.1 psi
 C.  6.8 psi
 D.  7.3 psi

CCST answer

The correct answer is C, 6.8 psi.

Fluid at a specific gravity of 1.0 exerts .43 psi per foot of level.  The fluid in this tank, at a specific gravity of 1.2, will exert .52 psi per foot of level.  The bubbler tube has been offset 1 foot off of the bottom of the tank, therefore, it is only measuring 13 of the overall 14 feet.  13 ft × .52 psi/ft = 6.8 psi

Reference: Donald R. Gillum; Industrial Pressure, Level, and Density Measurement,Second Edition, ISA Press, 2009.