Flow MeasurementInstrumentation

Flow Measurement Errors


While measuring flow, errors can be obtained some of the errors are due to flow transmitters that can be nullified by calibration there are common areas where the flow readings can be inaccurate or invalid.


Particulate, suspended solids or debris in the piping will not only plug up the sensing lines, it will erode the sensing device. The orifice, by its design with a thin, sharp edge is most affected, but the flow nozzle and even venturi can also be damaged. As the material wears away, the differential pressure between the high and low sides of the sensor will drop and the flow reading will decrease

Over ranging Damage to the D/P Cell

 The system pressures are usually much greater than the differential pressure and three valve manifolds must be correctly used.otherwise the D/P cell in the transmitter will be damaged. To know more about manifold valves click the link below.Three way Manifold valve.

Vapour Formation in the Throat

D/P flow sensors operate on the relation between velocity and pressure. As gas requires less pressure to compress, there is a greater pressure differential across the D/P cell when the gas expands on the LP side of the sensor. The flow sensor will indicate a higher flow rate than there actually is. The turbulence created at the LP side of the sensor will also make the reading somewhat unstable. A small amount of gas or vapour will make a large difference in the indicated flow rate.

The opposite can occur if the vapour forms in the HP side of the sensor due to cavitation or gas pockets when the fluid approaches the boiling point. In such an instance there will be a fluctuating pressure drop across the D/P cell that will give an erroneously low (or even negative) D/P reading.

Clogging of Throat

Particulate or suspended solids can damage the flow sensor by the high velocities wearing at the flow sensor surfaces. Also, the build-up of material in the throat of the sensor increases the differential pressure across the cell.

The error in flow measurement will increase as the flow increases.

Plugged or Leaking Sensing Lines

Periodic maintenance and bleeding ofthe sensing lines is a must. The instrument error will depend on where the

If the plug/leak is:

1.On the HP side a plugged or leaking sensing line will cause a lower reading. The reading will become irrational if the LP pressure equals or exceeds the HP sensing pressure.

2.On the LP side a plugged or leaking sensing line will cause a higher reading.

Calibration procedure of DPT transmitter

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