Figure (click image to enlarge). Wall flowmeter. The words read "Air Flowmeter. Pressure Compensated. Calibrated at 414 kPa & 21oC. USE NO OIL"
Which of the following is not compatible with the above type of flowmeter?
A. Back pressure applied distally, e.g. due to kinked tube, has no effect on its performance
B. The flow indication may be lower than the actual delivered flow if the connecting tube is kinked or having other obstruction
C. The needle valve for flow control is distal to the glass-housed indicator
D. The pressure in the tube is equal to the working pressure when connected to a gas source
Answer: B
The figure shows a compensated Thorpe tube flowmeter. Point B is the characteristic of an uncompensated Thorpe tube flowmeter.
A compensated Thorpe tube flowmeter means that it is back-pressure compensated, and it has its needle valve positioned distal to the Thorpe tube. In the presence of back pressure, the flow indicated by the position will still read accurately and reflect the actual delivered flow. The pressure in the tube is equal to the working pressure when connected to a gas source.
In an uncompensated Thorpe tube, the needle valve is proximal to the Thorpe tube, and the flow indication may not necessarily be the actual delivered flow in case of kinking or other obstruction; it may be lower than the actual delivered flow. The flowmeter is calibrated with the pressure inside of the tube equal to ambient pressure.
See the following reference for details.
Reference