Variable Area (VA) Meter
This remains a good simple solution for some processes but it is important to be aware of the limitations of these units.
VA limitations – changes in pressure and temperature
When using VA meters for gas flow they can become extremely sensitive to changes in pressure and temperature! As an example, a change from 0 to15 Psi (100 kPa or 1 Bar) commonly generates an error in indication of 40%. The addition of piping, fittings, filters and your process all create pressure drop. The lower the pressure inside the VA meter combined with an increased flow creates an even bigger pressure in your VA meters and a bigger error.
A VA meter is a great simple indicator for gas flow, but if you application calls for accuracy and repeatability you need a mass flow meter.
Below are 3 graphs which demonstrate how pressure influences the reading of a VA meter.
For processes that require a higher accuracy, Vogtlin provides the Compact and soon to be launched, Compact 2 mass flow meter. A battery operated (2 years on one battery) flow meter with ranges from 0-25 SCCM to 0-450 SLPM.
There are three main types of thermal mass flow meters in industry today.
Industrial thermal mass flow meters
Also known as thermal dispersion or insertion mass flow meters comprise a family of instruments for the measurement of the total mass flow rate of a fluid, primarily gases, flowing through closed conduits and pipe lines.
Capillary-tube type thermal mass flow meter
Many mass flow controllers (MFC) which combine a mass flow meter, electronics and a valve are based on this design.
MEMS type thermal mass flow meter
MEMS technology creates micro-heaters and thermal sensors on a microchip, CMOS (complementary metal oxide semiconductor). Advantages of MEMS thermal mass flow sensors is its small size, high speed response time, low power consumption and high sensitivity to low flow rates.
Thermal Mass Principle
The thermal mass flow sensor typically consists of upstream and downstream temperature sensors (thermopiles) and a heater located between the two temperature sensors as shown below.
Thermal mass flow meters are mostly used for gas flow applications. However this technology is also being applied to liquid micro flow applications. As the name suggests, thermal mass flow meters use heat to detect and measure flow. Thermal mass flow meters introduce heat into the flow stream and measure how much heat dissipates using one or more temperature sensors. The amount of heat lost from the sensor is dependent upon the sensor design and the thermal properties of the fluid.
The thermal properties of the fluid can (and do) vary with pressure and temperature, however, these variations are typically small in most applications. In applications where the thermal properties of the fluid are known and relatively constant during actual operation, thermal flow meters can be used to measure the mass flow of the fluid because the thermal flow measurement is not dependent upon the pressure or temperature of the fluid.
This method works best with gas flow measurement due to the difficulty in getting a strong signal using thermal mass flow meters in liquids, due to considerations relating to heat absorption.
While all thermal flow meters use heat to make their flow measurements, there are two different methods for measuring how much heat is dissipated across the sensors.
1) Thermal dispersion
Constant electric current through a heated element (e.g. wire) à Direct relationship between the temperature (resistance) of the element and the flow rate.
Alternative: Constant temperature at the heating element à Direct relationship between the current and the flow rate.
2) Thermal profile
In a measuring tube, heat is introduced into the medium with constant heating output. In the presence of flow, temperature sensors arranged symmetrically before and after the heating system detect a shift in the temperature profile towards the sensor downstream of the heating system. If there is no flow, both sensing elements measure the same temperature. In many cases, a differential temperature is determined instead of the two absolute temperatures.
Summary of Variable Area and Thermal Mass
Variable Area Advantage
+ Simple indicator, useful for some processes
Variable Area Disadvantge
– Extremely sensitive to changes in pressure and temperature
Thermal Mass Advantages
+ Mainly for gases
+ Indirect mass measurement (medium-dependent)
+ No moving parts
+ High precision (when calibrated correctly)
+ Independent of flow profile
+ Fast, dynamic measurement
– Susceptible to soiling / contamination with some sensors.
For further information and application support on Thermal Mass flow meters and controllers please consult our engineers.