Introduction: a flow meter or thermal
conductivity meter is that kind of meter by which we can measure the quantity of water , liquid , gas , air going through a tube. An air flow meter measure the
flow rate of air in the pipe.
Description:
A flow meter or a thermal conductivity meter may be constructed using the following circuit diagram
Operation principle: From the figure we can see that two
thermistors RT1 and RT2 are used adjacent to each other. For the
flow measurement, one thermistor is sealed in a small cavity copper cylinder and other installed in a small copper pipe. The bridge can be balanced so that the output voltage is zero. When air follows over the thermistor, it temperature decreases and in turn the
resistance increases, causing he bridge to be unbalanced. The unbalanced voltage is then amplified by the differential instrumentation
amplifier and applied to the meter. Thus the amount of mater deflection is proportional to the flow rate of the air in the pipe.
The equation indicate that the output voltage is directly proportional to the change in resistance ΔR of the transducer. Since the change in resistance is caused by a change in physical energy. A meter connected at the output can be calibrated in terms of the unit of physical energy.
Conclusion:
One potential disadvantage of
flow meter occurs when the fluid
temperature and pressure deviate from the
calibration temperature and pressure. Because temperature and pressure variations will cause a gas to expand and contract, thereby changing density and
viscosity, the calibration of a particular variable-area flow meter will no longer be valid as these conditions fluctuate. During operation, the flow meter accuracy can quickly degrade once the temperatures and pressures start fluctuating from the standard calibration temperature and pressure. Meters used for water tend to show less variability, since water viscosity and density changes very little with normal temperature and pressure
fluctuations
