The position of the control valve in flowmeters can be crucial when measuring gas flow, regardless of whether the valves are built into the device or separate. If a valve is used to reduce the flow at the inlet of the measuring device, the pressure in the measuring tube behind the valve is reduced. Because gases are compressible, this change in operating pressure results in a significant measurement error.
For example, a pressure reduction from 5 to 4 bars of absolute pressure in the measuring tube of a variable area flowmeter results in a measurement error of almost 12%. Therefore, more or less gas flows unnoticed through the device than is displayed. However, when the valve is positioned at the outlet, the drop in pressure does not occur until after the measuring tube and thus does not affect measurement. For this reason, Krohne offers the ‘control valve at the outlet’ version for its DK series. The DK series consists of a range of low volume flowmeters with glass or metal cones used in many different applications ranging from inlet pipes to gas chromatography and gas inertisation measurements in chemistry to the measurement of gases used for anaesthesia in hospitals.
In some instances, the control valve can be in front of the measuring device, for example when a gas is measured against a constant pressure (eg, atmospheric pressure). In this case, the measuring device is not calibrated based on the primary pressure but instead based on the constant output pressure prevailing in the measuring tube. The position of the valve is not crucial when measuring liquids, as the compressibility of liquids is negligible even at high pressures.
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