Thermal Mass flow meters

The thermal principle operates by monitoring the cooling effect of a gas stream as it passes over a heated transducer (PT100).
Gas flowing through the sensing section passes over two PT 100 RTD transducers one of which is used conventionally as a temperature sensing device, whilst the other is used as a heater. The temperature sensor monitors the actual process values whilst the heater is maintained at a constant differential temperature above this by varying the power consumed by the sensor. The greater the mass flow, the greater the cooling effect and power required to maintain the differential temperature. The measured heater power is therefore a measure of the gas mass flowrate.

 

As gas flows by the heated sensor (flow sensor), molecules of the flowing gas transport heat away from this sensor, the sensor cools and energy is lost.

Flow measurement is arguably the most complex, most varied and most interesting of measurements that an Instrument Engineer is likely to encounter.
For example flow can be measured in volumetric or mass terms e.g. m3/hr or kg/sec. These measurements being related by the material's density. And since gases are compressible and change volume when placed under pressure, are heated or are cooled then the volumetric flow rate may change under differing process conditions, the mass flow rate may not. This introduces the need for terms like "actual" flow rate through a meter and "standard" flow rate. So flow rate that you report may in fact not be the flow measured. 

Then there is the task of how to measure flow. And here the choice can be huge:
  - mechanical flowmeters like positive displacement meters or oval gear meters,
  - pressure based meters like the ubiquitous orifice plate or equally popular venturi meter,
  - thermal mass meters,
  - vortex meters, electromagnetic, ultrasonic and coriolis flow meters
  - and not forgetting laser and acoustic doppler methods.

All of these meters have their place; some being more suited to an application than others. Many factors may influence your choice when selecting a meter, including:
  - Cost,
  - Required accuracy (or uncertainty),
  - Whether mass or volume flow rate is required,
  - Properties of the fluid to be measured e.g. pressure, temperature, viscosity, conductivity etc,
  - Installation restrictions e.g. straight length requirements, vertical or horizontal pipeline etc,
  - Familiarity of operators with the flow meter technology

 

The level sensor works exactly the same way-measures level. The primary device (flume or weir) measures flow. The flow transmitter takes the level signal and produces a flow value based on the primary device.