The importance of flow measurement in industrial and chemical processes cannot be disputed. There are many applications for flow meters, but it is important to note that different flow meters are meant for different applications. The three main types of mass flow meters are differential pressure flow meters, thermal mass flow meters and Coriolis mass flow meters. These three types use different principles in the measurement of fluid flow and therefore the factors affecting their accuracy may differ.
- Differential pressure flow meter
In this type of mass flow meter, the temperature and pressure of the fluid are used to determine the mass flow rate. Therefore, if there are any errors in the temperature and pressure readings, the final result will also be inaccurate. Additionally, some assumptions are made about the viscosity of the fluid. The flow is also assumed to be laminar, which is often not the case because an obstruction is used to create a pressure difference in the flow. The size of the orifice may increase over time due to wear and this also leads to inaccurate readings. Lastly, if the fluid is very dense, it can lead to partial blockage of the orifice.
- Thermal mass flow meters
In the thermal mass flow meter, some assumptions are made about the heat capacity of the fluid. Incorrect assumptions may be made about the composition of a gas and this then leads to inaccurate readings. Something else that may influence the accuracy of this mass flow meter is condensation of gases on the temperature sensors. This mostly happens when the gas is saturated. The temperature readings will therefore be inaccurate and this affects the final reading of the mass flow rate. One mistake that people make when using thermal mass flow meters is taking the readings as soon as the device is turned on. Readings should only be taken once a steady-state operating temperature is reached and this takes some time.
- Coriolis mass flow meters
Coriolis mass flow meters are considered to be the most accurate. However, the readings taken can be inaccurate if there are bubbles in the fluid. In this flow meter, the mass flow rate is measured through inertia. Since the tube through which the fluid is flowing is vibrated using an actuator, this can easily lead to splashing within the pipe or tube and some noise is therefore generated. Remember, noise is also a form of energy. This influences the amount of energy needed for the vibration of the tube, leading to inaccurate readings.
Most devices are prone to inaccuracies due to one reason or another. However, it is important to know what causes the inaccuracies so that you can do something to improve the accuracy where this is possible. Additionally, knowing what causes the inaccuracies can help in the selection of an appropriate flow meter for the application at hand. For instance, if you are unsure about the composition of a gas, then you should probably avoid using a thermal mass flow meter because any incorrect assumptions about the composition will affect the accuracy of the readings.