What is a Thermal Mass Flow Meter & What is its Working Principle? - Part 1 of 2

Fri, Mar 20, 2015 @ 10:59 AM | Thermal Flow What is a Thermal Mass Flow Meter & What is its Working Principle? - Part 1 of 2

Thermal Mass Flow Sensors and their desired characteristics

ThermalMassFlowSensorCutawayThis is part 1 of a 2-part blog on the Thermal Mass Flow Meter.  In Part 1 we will explain the desired characteristics of a thermal mass flow meter (and its sensor). Part 2 will discuss the operation of the Teledyne Hastings 300 Series flow meters (Patent #6,125,695) and how the 300 Series thermal mass flow sensor meets each of the desired characteristics described below.

What is a thermal mass flow meter:

  • Electronic Circuit Card

  • Flow Sensor

  • Bypass Shunt

  • Base

A cutaway is shown in the image on the right.

A thermal mass flow meter measures the amount of fluid that passes through the meter. At Teledyne Hastings we design our mass flow meters for dry and clean gases.  

In a typical thermal mass flow meter, gas enters the meter via an upstream port connected to the process to be measured (by Swagelok®, VCR®    or other fitting).  A majority of the gas passes through the meter’s bypass shunt; however, a certain fraction flows through the meter’s thermal mass flow sensor.

The meter’s thermal mass flow sensor measures the gas molecular flow that passes through its capillary tube by quantitating thermal energy transfer. The mass flow rate is a function of the gas flow and the specific heat of the gas. The thermal mass flow sensor then provides accurate measurements which can be referenced back to standardized volumetric flow units.  Reference conditions (standard temperature and pressure) are based upon the amount of gas flow, which is determined by the number of gas molecules, using the ideal gas law. The meter’s shunt is selected such that the amount of gas moving through the flow sensor is approximately the same at full-scale flow. After passing through the thermal mass flow sensor, the gas then exits the flow meter via a downstream port.

Thermal Mass Flow Meter Characteristics

FlowSensorOutput

Ideally, a thermal mass flow sensor will exhibit the following characteristics:

  • Linearity: Linearity means that the sensor’s electronic output is directly proportional to the rate of gas flow that is moving through the sensor (within its range). Linearity of the flow sensor leads to the second attribute: Accuracy.
  • Accuracy: Accuracy is dependent on the sensor’s Linearity. An accurate flow sensor provides the benefits of better gas flow measurement, flow control and a thorough understanding of the system’s parameters.
  • Fast Response:  Ideally, the flow sensor would respond instantaneously to a change in the flow rate. Aside from the obvious benefit of instant real-time oversight of the process flow, fast response becomes critical when the flow meter is coupled with a proportional control valve to create a thermal mass flow controller.
  • Low Differential Drop: For a thermal mass flow sensor to be ideal for leak testing, it should have a low differential pressure drop across the meter. 
Typically, a thermal mass flow meter is calibrated using nitrogen gas (or in the case of very large flows, it may be calibrated in air). The output of the flow meter can then be scaled for use with other process gases. This means the flow meter technician can calibrate a flow meter for use with a corrosive process gas, such as silane (SiH4), without having to use that specific type of gas. A linear flow sensor will retain its linear behavior as the gas is switched from the calibration gas (N2) to the process gas.

Application

Measuring gas flow has become increasingly critical to many processes and the thermal mass flow meter achieves those results with a high level of accuracy.  Accurate readings must reference  standard temperature and pressure (STP) conditions, without having to correct for temperature and pressure using volumetric flow meters.

Thermal mass flow meters exhibiting low pressure differentials are ideal for measuring mass flow in leak testing applications and must provide fast response and accurate gas flow readings.

Teledyne Hastings designed its first fast-response flow meter for leak testing applications in the automotive industry. The low-pressure differential and response speed proved to be highly successful. Today, Teledyne Hastings' Thermal Mass Flow Sensors are used globally in a variety of diverse industries and applications.  For more information on Best Practices for Mass Flow Controllers and Mass Flow Meters download our whitepaper.

Download Whitepaper

Be sure to visit our website for additional information on Teledyne Hastings Mass Flow Controllers and Mass Flow Meters

In our next blog, we will discuss the thermal mass flow sensor at the heart of Teledyne 300 Series of mass flow meters. We will also look at how the 300 Series thermal mass flow sensor meets each of the desired characteristics described above.

Written By: Will Harrison