When we use flow meters to measure gases, they need to be checked from time to time to make sure the readings are correct. This process is called calibration.
Just like a weighing scale at home can show the wrong weight if it is not adjusted, a flow meter can also give wrong results if it is not calibrated. So to calibrate flow meter is very important.
In this post, we will explore how to calibrate flow meters with gases and learn why each method is important.
Soap Film Burettes Method to Calibrate flow meter
This method is suitable only for low flow rates, since it is difficult to form a stable soap film across a burette of more than about 50 mm diameter. The film is made to act as a frictionless ‘piston’ which travels freely with the flowing stream of gas, so that the velocity of the film is a good indication of the velocity of the flowing gas.
The soap film burette can be used to calibrate both flow rate and quantity meters. It will measure flow rate with an accuracy of ± 0.5% and volume with an accuracy between ± 0.5% and ± 0.2%.
Bell Prover System to Calibrate flow meter
Bell prover system for calibrating gas flow meter is shown in Figure below.
The gas flowing through the flow meter during a timed interval is trapped in gasometer bell and its volume is measured. Temperature and pressure measurements allow calculation of mass and conversion of volume to any desired standard conditions.
By filling the bell with gas, rising it to the top, and adding appropriate weights, such a system may be used as a gas
supply to drive gas through a flow meter as the bell gradually drops at a measured rate. By using a precision analytical balance to measure the mass accumulated in storage vessel over time, accuracies, of ± 0.02% were obtained for flow rate upto 9 kg/sec.
Sonic Venturi-Nozzles Method to Calibrate flow meter
The Sonic Venturi-nozzle is a very convenient device for calibrating a gas flow rate meter at one volumetric flow rate.
It depends upon the fact that in the throat of a nozzle, the gas cannot travel faster than the speed of sound. Provided the upstream pressure is sufficient to ensure that sonic velocity is actually reached, the flow rate through the nozzle will therefore always have a fixed value for a given gas at a specified temperature and pressure.
The tapered section of the nozzle plays no part in controlling the flow rate ; its function is merely to assist, in recovering some 90-95% of the initial pressure, thus conserving energy. If a number of sonic venturi nozzles of various sizes are used in succession a gas flow meter can be calibrated over a range of flow rates with them.
When the highest possible accuracies (about ± 0.5% on flow rate) are called for it is usual for venturi nozzles to be calibrated against a primary gas flow standard before they are used as calibration devices.
If slightly lower accuracies are acceptable their performance can be predicted fairly reliably from a knowledge of their dimensions.
Gravimetric System to Calibrate flow meter
The gravimetric calibration is a highly sophisticated and expensive device, but is regarded as the best available primary system for calibrating secondary high-pressure gas flow standards.
It can also be used directly for the calibration of high-pressure gas meters. It is broadly similar in principle to the gravimetric system with static weighing for liquids, but with one important difference. In the liquid system the meter being calibrated is at upstream of the flow diverted, but in the gas system it is at downstream and in a line venting to atmosphere.
This enables the meter to be read under steady state conditions, thus overcoming the problem of diminishing
flow rate, which occurs while the weighing vessel is being filled.
As shown in above Fig., a critical venturi-nozzle is used to maintain a constant flow rate through the test system. In the first part of the test, the flow at downstream of this nozzle is diverted through the meter being calibrated, while its reading is noted.
Then the flow is diverted into a light weight spherical pressure vessel for a measured time, and the measurements of the weight of this sphere before and after diversion are used to calculate the mass flow rate during the diversion period.
By varying the pressure upstream of the critical venturi nozzle, a fairly wide range of mass flow rates can be covered with this system, and by using several alternative nozzles of different sizes, an almost unlimited range can be covered. The accuracy of flow rate measurement in a system of this kind is between ± 0.5% and ± 0.2%.
PVT System Method to Calibrate flow meter
The PVT (Pressure-Volume-Temperature) method illustrated in Fig. 1.91 is used mainly as a primary standard, to calibrate reference meters and sonic venturi nozzles which can thereafter be used as secondary calibration devices.
In this system, a storage vessel of known volume is charged with gas at high pressure. The pressure and temperature of the gas in the vessel are first measured, then the gas is allowed to flow out through a regulating sonic nozzle in series with the meter on test, and finally the pressure and temperature of the gas in the storage vessel are measured again at the end of a measured period of time.
From these measurements the mass flow rate through the system during the test period can be calculated. Heat exchangers and an upstream pressure controller are used to control the conditions during flow and thus the performance of the meter under test can be determined over a wide range.
Accuracies of flow rate measurement between ± 0.5% and ± 0.2% can be achieved.
Difference between Flow Meter Calibration and Recalibration
Flowmeter calibration means checking a working flowmeter against a standard measuring device under the same conditions. If needed, the flowmeter scale is adjusted so its readings match the standard value.
Recalibration is done after the flowmeter has already been used. It is important to recalibrate regularly because, in real industrial processes, conditions change and the flowmeter can slowly start giving wrong readings.
The key difference is that calibration is done before the flowmeter is put into use, while recalibration is done after it has been running for some time. After calibration, special software tools can also be used to double-check the accuracy.
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