In this post, we compare leading thermal mass flow meter manufacturers’ in-situ calibration methods revealing there is only one convenient and accurate process.
Thermal mass flow meters are factory-calibrated to correlate the operating power to mass flow. Since the calibration requires an NIST traceable flow instrument, it is impossible for the end-user to calibrate a thermal flow meter in the field. For this reason, the meter must be returned to the manufacturer for periodic recalibration. This process can be time-consuming, expensive, and even require the purchase of a second flow meter to swap out the meter being re-calibrated.
Regulatory Influence for On-Site Calibration
Many companies have internal laboratories to maintain their equipment including calibrating instruments on the premises. This is partly driven by regulatory mandates that require users to calibrate their flow meters periodically when measuring and reporting of greenhouse gas emissions. There is more information available in our white paper, “Greenhouse Gas Emissions Monitoring Using Thermal Mass Flow Meters.”
Sage Metering compares thermal mass flow meter manufacturers’ calibration verification methods revealing there is only one convenient, accurate, and in-situ choice.
Available In-Situ Calibration Methods
Thermal mass flow meter manufacturers have responded to the industry’s need for a method to verify, while in the field, that their flow meters are calibrated and accurate. In a perfect world, the manufacturer’s verification method would check both the sensor and transmitter while the meter is still in the pipe. Currently, the following methods are employed by thermal mass flow meter manufacturers:
Sage Metering Option
In the Sage Metering In-Situ Calibration Verification, the sensor is withdrawn from the pipe into an isolated chamber where a no-flow condition exists. The user compares the zero-flow mW signal on the flow meter’s display with the original calibration data printed on the flow meter’s nameplate.
Advantages – There is no special equipment required, and the in-situ test can be performed while the process is in operation. The method examines both the sensor and the electronics. It verifies that the operating zero-flow data matches the calibration curve; any offset at no-flow will be reflected over the entire curve. If the zero-flow data matches the original calibration data, the user can be assured that the performance has not shifted.
Disadvantages – None
One manufacturer approaches the challenge by removing the sensor from the pipe and positioning it within a specialized compartment which isolates the sensor from the flow. Then, a known amount of a reference gas from an external bottled source is flowed past the sensor. This step is repeated multiple times and the data is compared to the initial factory calibration data.
Advantages – This method checks both the sensor and transmitter against a known reference.
Disadvantages – The process requires the purchase of expensive hardware, and it is unable to add the equipment at a later date. It is also cumbersome and requires initial factory calibration data on both the process and reference gases.
Another manufacturer’s method requires that the unit be taken out of service though it stays in the pipe. The flow signal from the sensor is isolated, and an external signal is simulated. The sensor elements signal (power) is varied, and the transmitter evaluates the electrical output which will genenate a pass or fail message.
Advantages – This method requires no extra hardware, and it can be performed on site.
Disadvantages – Unfortunately it only tests the electrical signal and electronics. It does not check the sensor’s operation, or if the sensor is clean. One of the common difficulties for flowmeters in industrial environments is the buildup on the sensor.
In a third manufacturer’s test, the meter is removed from the pipe, the sensor is isolated, and the transmitter is connected to a computer. The computer sends the flow meter a known signal and confirms that the transmitter output matches the expected value. Of course, this requires a computer and the purchase of specialized software.
Advantages – There are no clear benefits to this method.
Disadvantages – This procedure only tests the electronics verifying that the output matches the calibration curve. It does not check the sensor operation or if the sensor has drifted.
Competitor # 4
In another manufacturer’s method, the meter is removed from the pipe, and the sensor is tested under no-flow and a simulated high-flow condition using a water bath. The user then compares the results to the original calibration data.
Advantages – In this procedure both the sensor and electronics are tested at known values.
Disadvantages – The downside here is that the thermal mass flow meter must be removed from the pipe.
If the objective of an on-site calibration verification is to validate that the meter is accurate, and the sensor and transmitter are performing correctly, only Competitor 1, 4 and the Sage Metering Option meet the criteria. If we add the stipulation that the meter does not have to be removed from the pipe and no extra hardware has to be purchased, only the Sage Metering Option fulfills the requirements.
Out of the methods presented, the Sage In-Situ Calibration Verification is the only procedure where the meter does not have to be removed from the pipe, is easy, quick, and verifies the performance of both the sensor and transmitter.
Sage Metering, Inc. 8 Harris Court, Bldg. D1. (n.d.). Retrieved from http://www.sagemetering.com/pdf/Understanding_the_Sage_Metering_Calibration_Verification.pdf_br
New Video on In-Situ Calibration
Watch and listen as I explain the Unique Sage Metering In-Situ Calibration method.