Thermal-type mass flowmeters were conventionally designed for measurement of gases. However, thermal units for liquid flow measurements are also available. These types of flowmeters are similar to Coriolis meters in numerous ways. Their working is also independent of density, pressure, and viscosity variations just like Coriolis meters.
A basic thermal mass type flowmeter consists of two temperature sensors and an electric heater mounted between them. This heater can be either projected into the liquid stream or can be positioned at the periphery of the pipe. This unit operates in two ways:
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Either introduces a fixed quantity of heat into the flowing fluid stream and then finds the related changes in temperature.
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Or keeps a probe at a constant temperature and then detects the energy needed to achieve so.
The temperature sensing element is never in direct contact with the liquid. The whole package of an electronic thermal type mass flowmeter consists of a flow analyzer, a temperature compensator, and a signal conditioner which usually gives a linear output that is directly proportional to mass flow rate.
Advantages -
- Accuracy of these meters depends on various factors like calibration integrity of the actual process and changes in temperature, pressure, flow rate, heat capacity and viscosity of the liquid being measured.
- These types of flowmeters are frequently employed for monitoring and controlling of mass-related processes like chemical reactions. These processes mainly depend on the relative masses of non reacting ingredients.
- Major capability of thermal mass type flowmeters is accurate measurement of low gas flowrates or velocities. These meters are capable to detect gas flows much lower than can be detected via any other metering device
- They offer good rangeability in the range of 10:1 to 100:1, only if they are designed to work in constant-temperature-difference mode.
- Although thermal mass type flowmeters are most appropriate for very low flow measurements, yet they can be applied in large flow applications also. For example combustion air, natural gas, or the distribution of compressed air.
Limitations -
Major problem faced with the use of thermal mass flow devices is the condensation of moisture on the temperature detecting element. This happens in case of saturated gases only. However, this condensation causes the thermometer to give low readings. Also this can result in corrosion of the device.