When calibrating a flowmeter, it is essential to ensure reliable in-situ calibration results. This will help to avoid false positives. Below we have listed five methods for calibration validation that will help to assure your flowmeter is functioning accurate and precisely.
CALIBRATION VALIDATION: RESISTANCE
A simple method of calibration validation involves the measurement of resistance across a velocity sensor. Most often a velocity sensor is a platinum resistance temperature detector, which is used to measure resistance that is directly related to the temperature of the sensor. When using this device, the temperature should be equal to the space surrounding the velocity sensor. Notably, this calibration validation method only measures the resistance of the platinum wire wrapped around the platinum mandrel. This method does not measure calibration drift because it does not measure factors related to heat transfer and gas flow.
CALIBRATION VALIDATION: ZERO FLOW
Another calibration validation method is to use a reproduce zero flow. Zero flow is only reproducible point between calibration and the site where the flow meter is being used. For this reason, manufacturers will usually provide data needed for checking zero flow at a set of reproducible conditions, such as zero flow at atmospheric pressure and temperature. This method requires the flow meter to be completely removed from the process and allowed to come to equilibrium in ambient conditions.
FIELD ADJUSTMENT: K-FACTORS
Using k-factors for field adjustments is another way to validate flow meter calibration. Manufacturers will enable k-factors that work as a multiplier to the observed flow value. This acts as a linear offset and ensures the meter reading agrees with the other device. However, one issue that is associated with k-factors is the inherent response curve from the thermal sensor in relation to the flow is non-linear. In the case a larger item is being tested, manufacturers will allow several points on the calibration curve to be adjusted.
CALIBRATION VALIDATION: FULL-FLOW
Another technique to validate calibration beyond zero is to use full-flow. This checks the full-flow range by generating a series of known flow rates, usually from zero to full scale. A small sonic nozzle is used for this method and is used to direct a known flow past the velocity center. The diameter of this nozzle should be fixed and should have differential pressure applied. This will help calculate the flow through the nozzle. Notably, this type of validation method is only accurate when the nozzle is not plugged or dirty. It also requires a full precision pressure gauge that is properly calibrated.
CALIBRATION VALIDATION: ACTUAL FLOW AUDIT METHOD
The calibration validation technique is the Flow-Audit method. This method uses a high-accuracy flow to verify accuracy of the flow device under test. Ideally, a meter used for the flow audit should have the application flexibility to use a variety of gases and pipe sizes.