E-Zine October 2014
Click here to read "Introduction"
Click here to read "Flowmeter Technologies"
Calibrating flowmeters is complicated by the dynamic nature of steady-state flow. In comparison, consider that a level transmitter can typically be calibrated by adjusting the transmitter with known levels in the tank. Pressure transmitters can be calibrated in the field by developing a known pressure using a dead-weight tester that distributes a weight over an area. Temperature transmitters can be calibrated in the field at 0°C and 100°C by using ice water and boiling water respectively. These statements may seem somewhat simplistic because there is quite a bit of detail involved to perform a proper calibration, but the basic concept is that of static measurements.
Flowmeter calibration is much more involved. It is not possible to take a liter per minute (lpm) into the field and throw it through a flowmeter to check performance. However, some flowmeters can be removed and brought into a flow calibration laboratory for calibration. As compared with the apparatus used to calibrate the other process variables mentioned, these laboratories are generally more complex because the weight, density, temperature and composition of the laboratory fluid and process fluid (and other items) should be taken into consideration. Field calibration can also be performed using provers when the piping is appropriately designed.
Due to expense and complexity, flow laboratories and provers are typically only available when flow measurements are extremely important, such as in custody transfer applications. Therefore, most industrial facilities have no way to wet calibrate flowmeters. As a surrogate, the technician checks for proper calibration based upon differential pressures, millivolts, frequencies, weights, volumes and the like. These procedures will not detect a plugged impulse tube, a deformed orifice plate, a damaged magnetic flowmeter liner, and other potentially serious problems. As such, these commonly performed transmitter “calibrations” are better than doing nothing, but they do not check the calibration of the flowmeter. Calibration can be performed by flowing a known amount of fluid through the flowmeter under test, and comparing this amount with the amount of flow measured by the flowmeter.
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From Flow Control (August 2002)