The calibration is done to the instruments to improve the accuracy of the instrument. So we would get a value by the measurement of the instrument or the device, and this measurement can be compared to our standard value. The difference between the standard value and the measured value can be reduced with the help of calibration. Sometimes different parts of a system are calibrated by different methods well in the case of the flow meter with a transmitter and an indicator the flowmeter will be wet calibrated and the transmitter and the indicator will be dry calibrated.
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What are the types of calibration methods for the flow-meters?
The flow measurement devices are calibrated by three methods and they are
- Wet calibration – This would use the actual fluid flow
- Dry calibration – in this type flow simulation will be used and it could be electronic or mechanical
- Measurement check – In this the measurement check of the physical dimensions and use of empirical tables relating flow rate to these dimensions
This type of calibration would use actual fluid flow, this type of calibration would provide high accuracy for the flow meter. This type of calibration is done if accuracy is the main concern. Most of the precision flowmeters are wet calibrated while it is manufactured. The wet calibration is done in flow meters by using water, air, or hydrocarbon fuels. The wet calibration is done by using the actual fluid flow and this would give better accuracy for the flow-meter.
For the calibration that is done outside the facility, we must specify the flow range for the calibration, the number of points that will be taken, and the viscosity of the fluid. If the flow-meter which is to be calibrated is used for billing or other accounting purposes, then we might want to obtain an as-received and after adjustment calibration. The as-received results would allow us to determine the magnitude of error that was in the meter before calibration. If the error is high then we must do additional actions according to the need. If we need to get better accuracy from the flowmeter then we can do system calibration in which all the elements of the flow measurement systems are calibrated together.
In this type of calibration the flow is started very quickly while the starting of the test, after that it would be held constant, and then shut off at the end of the test. The total flow reading from the flowmeter is compared with the volume that is collected and thus the flowmeter performance can be determined. This type of calibration method can be used for the flow meters that have minimum sensitivity to the flow.
In this type of calibration, the flow is kept constant before starting the test. At the beginning of the test, the reading from the flow meter and the initial volume are read together and after the desired collection period the test is stopped. This type of calibration is affected by the meter’s speed of response.
This is the major type of wet calibration method and in this type, the output of the meter is compared with other flowmeters which have high accuracy, these flow meters are called master flow metes. While selecting the master flowmeter we must consider the desired range of the test stand, accuracy required, fluid properties, and also the allowable head loss.
This type of calibration is done in a flow-meter without the presence of fluid, the input signal could be Hz or mV. This is more uncertain than the wet calibration, the accuracy of the flow transmitter is inferred because the flow transducer is bypassed. We must provide the input signal for the dry calibration according to the measurement standard.