Thermal Mass Flow Meters and Controllers

What are Thermal Mass Flow Meters and How Do They Work?

For example, the McMillan Model 50X Series Mass Flow Meters utilise thermal sensing technology. Flow enters the unit, and a portion of the flow is redirected into a small tube. This tube has two coils, one downstream from the other. The first coil introduces a small amount of heat into the gas stream. As the gas passes through the tube, the smart electronics sense the amount of heat transferred from one coil to the other. McMillan’s proprietary design system ensures that the zero remains stable and the sensor is extremely repeatable.

The output of the thermal mass flow sensor is directly related to the specific heat characteristic of the gas being measured. Therefore, if a unit is calibrated for air, it is a relatively simple calculation to figure the calibration for nitrogen or some other similar gas. This advantage offers flexibility not found on many other types of flow sensors.

Mass Flowmeters vs. Mass Flow Controllers

The McMillan RHODIUM Series Mass Flow Controllers utilise MEMS thermal sensing technology.

MEMS technology utilises a compact, reliable CMOS (Complementary Metal Oxide Semiconductor) sensor. This no-drift sensor module is comprised of both the electronic and mechanical elements on a single silicon chip, similar to the process used for integrated circuits.

Flow enters the unit, and a portion of the flow is redirected into a chamber and across the MEMS device. This device has two temperature sensors, one downstream from the other. Between the coils sits a small heating element, as shown in the image to the right. As the gas passes through the device, the smart electronics sense the difference in heat between the two sensors, since the upstream coil is not exposed to the heated gas. The downstream coil senses the heated gas and McMillan’s advanced sensing algorithm calculates the flow rate based on the difference in value. The precision manufacturing and compact size of the MEMS device insure that the zero remains stable and the sensor is extremely repeatable.

Flow then passes into the proportional solenoid valve. This valve is controlled by the active servo electronics, which compare a setpoint (supplied via analogue input signal) to the actual flow rate provided by the flow sensor and adjust the valve accordingly.

How to Select a Flow Measuring Device

• Flow Ranges - consider what flow range is required for measurement and/or control (eg. 0 - 20 sccm or 0 - 500  L/min)
   
• Power - what supply power is required? Units may be required to operate with either 12 VDC or 24 VDC power.
   
• Signal Inputs & Outputs - for example 0 - 5 VDC or 4 to 20 mA linear outputs are common options.
   
• Accuracy/Linearity - are there any specific accuracy requirements for the device? Do you
  require NIST traceable calibration certificates?
   
• Fluid Connections - typical options include compression type tube fittings or flare fittings (sizes usually ranging from 1/8" - 1/2" depending on flow range).
   
• Electrical Connections - typical units are utilising 36" output cables terminating with a 6-PIN PS/2 style connector, with optional mating cable assemblies available.
   
• Wetted Materials - consider what is the flowing media and chemical compatibility. We can offer units in Stainless Steel, Brass, PTFE and more.
   
• Displays - do you require an integral LCD display?
   
• Calibration Gases - units may be calibrated for virtually any clean, dry gases.