Advanced Diagnostics of Thermal Mass Flow Meters

Thermal mass flow meters are gaining wide acceptance throughout process industries to provide reliable mass measurement for air and gas flow applications, while ensuring a strong signal at low flows and low pressures with high turndown. Verifying a flow meter’s performance over the unit’s lifespan is essential to confirm it is meeting its specifications.

As a result, manufacturers of thermal mass flow meters design in diagnostics tests that can be run in the field, in the pipe (in-situ) or in the instrumentation shop. The Magnetrol® Thermatel® Model TA2 thermal mass flow meter, for example, features several convenient tests that an end-user can perform to verify proper operation of the device. The tests include verifying that the heater current is correct, the temperature sensors are reading correctly and the heat transfer characteristics of the 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.

The calibration verification diagnostic procedure provides cost savings by reducing process downtime and factory recalibrations.

Calibration Verification Procedure
In the past, flow meters had to be returned to the manufacturer in order to verify calibration had not changed. Thermal mass flow meter manufacturers have now provided procedures to perform this verification on-site (unless required to send back by regulation or internal requirement).

The THERMATEL TA2 thermal mass flow meter provides a two-step procedure for field verification of calibration. By performing the high flow validate and the low flow validate tests, a user can verify that the device’s response to flow, near the two extremes of the calibration curve, has not changed since the original calibration.

thermal_mass_flow_meters_1The 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 an initial value taken by MAGNETROL during flow meter calibration. The values should be within 1.5°C. Some variation is allowed to account for different test methods and temperatures. The user can store the newly obtained value or keep the initial, as the initial value will remain on the calibration certificate (“Hi Cal”) that is sent with every THERMATEL TA2.

The similar 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.

More Information
MAGNETROL offers a white paper that describes the advanced diagnostics of thermal mass flow meters that have made the technology an increasingly preferred alternative to other flow meter types. In addition to the calibration procedure, testing of the heater setting, zero power testing and configuration verification are reviewed.

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