Verifying flowmeter performance is an essential step in ensuring your flowmeter is meeting its specifications. Today, manufacturers design-in diagnostic tests that can be run in the field, in the pipe (in-situ), or in the instrumentation shop to assist with meter verification.
For thermal mass flowmeters, diagnostic tests include verifying that the heater current is correct, the temperature sensors are reading correctly, and the heat-transfer characteristics of the flowmeter’s probe tips have not changed. These tests can be performed using the software display, HART or PACTware. Therefore, no external equipment must be purchased to run the procedures.
Testing the Heater Setting
Thermal mass flowmeters commonly include resistance temperature detectors (RTDs) and "heaters" in the small pins at the end of the probe. The probes utilize a heated pin and an unheated pin. The unheated pin operates at the process temperature. The other pin is heated with a variable power to maintain a constant temperature difference between the two pins.
The transmitter measures the amount of current being applied to the heated pin. This current is converted to power (mW), which infers mass flow rate based on the calibration. The relationship between power and mass flow rate is shown in the curve below. Having the right amount of current being measured by the transmitter is essential in ensuring correct flow measurement.
A common concern in combustible gas applications is the amount of heat being added to the process. It is important that the flow meter is not adding excessive heat into the system.
Zero Power Test
During calibration, the thermal flowmeter probe is placed into a water bath to calibrate the RTDs. This is an important step by the manufacturer to verify that the RTDs measure the same in order to maintain the correct constant temperature difference between them. Some manufacturers refer to the temperature difference as the set point.
The value should be less than 0.5 C to verify that drift in the RTDs has not occurred.
Calibration Verification Procedure
In the past, flowmeters had to be returned to the manufacturer in order to verify calibration has not changed. Thermal mass flowmeter manufacturers have now provided procedures to perform this verification on-site (unless required to send back by regulation or internal requirement). This capability saves process downtime and the cost of recalibrations.
The primary procedure is the High Flow Validate. It involves placing the probe into a water bath, making sure the tip is completely covered in water. The transmitter automatically sets the heater current to a relatively high fixed value and displays the temperature difference between the two sensor tips. Once the device has determined that the temperature difference has stabilized, the final value is displayed as well as the initial value during calibration. The values should be within 1.5 C. Some variation is allowed to account for different test methods and temperatures.
Similarly, a Low Flow Validate test is performed under a low-flow (no flow) condition. The no flow type test is more common amongst manufacturers. In this case, the sensor tip is covered and the transmitter sets the heater current to a relatively low fixed value. The same procedure applies as the High Flow Validate in comparing to the initial stored values.
Testing that the RTDs measure the same temperature difference at two different points verifies the calibration. It is not simply configuration verification, but a true test of heat transfer. The High Flow Validate can be easier to reproduce on-site and is acceptable to perform on its own. It is recommended that either test be conducted at room temperature.
One of the first troubleshooting tips for thermal mass flowmeters is verifying the configuration has not changed since it left the factory. Many times, "configuration" and "calibration" are terms that are used interchangeably. Configuration checks are not the same as the aforementioned Calibration Verification Procedure; they are only used to verify that the flowmeter is calculating expected values based on the original calibration data. This ensures that parameters have not been altered.
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