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

In a bubbler (dip tube) level measuring system, for a bubble to be emitted at the open end of the purge pipe, the air pressure at that point is _______ the pressure exerted by the liquid at that point. 

A. twice
B. equal to
C. less than 
D. three times

CCST Answer

The correct answer is B, “equal to.” In order for air to be discharged from the end of a bubbler purge tube, the air pressure in the tube must be equal to (or higher than) the pressure exerted by the liquid head in the tank.
As the tank level is decreased, the liquid head pressure at the tip of the purge tube decreases, and more bubbles per unit of time can escape. The corresponding reduction in pressure in the purge tube is proportional to the level in the tank.
Therefore, the point at which the liquid head pressure and the purge tube pressure are equal is the highest level (URV = 100%) that the device will measure.


Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.


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