Quiz Corner: Positive displacement flowmeters

Which of the following statements are true about positive displacement flowmeters in liquid service?

Positive displacement flowmeters:

1. Have an output that is linear with the flow rate
2. Measure the volume of liquid passing through the flowmeter
3. Measure the velocity of the liquid passing through the flowmeter
4. Are not affected by the viscosity of the liquid
5. Measure lower (as a percentage of flow rate) than the actual flow at low flow rates

The output from positive displacement flowmeters is linear with the flow rate, so Answer A is true.

Positive displacement flowmeters continuously entrap liquid, so they measure the volume of the liquid passing through the flowmeter. Answer B is true, and Answer C is false.

Positive displacement flowmeters are designed to operate accurately independent of viscosity above a minimum viscosity limit. However, as the viscosity decreases below the limit, increasingly more liquid (as a percentage of the actual flow rate) slips through the flowmeter without being measured. Answer D could be true or false, depending upon the operating viscosity.

Decreasing flow increases the amount of liquid passing through positive displacement flowmeters without being measured (as a percentage of the actual flow rate), so Answer E is true. 

Additional complicating factors

The overall effect of liquid passing through positive displacement flowmeters without being measured (slippage) becomes more pronounced when low viscosity liquids are measured at low flow rates.