What are Mass Flow Meters​?

A mass flow meter is a fast and accurate instrument that measures the flow rate of a fluid.  Thermal mass flow meters can be used to measure any fluid (gas or liquid); however, Teledyne Hastings only manufactures thermal mass flow meters for fluids in gas phase.  The term “mass flow” can be a bit misleading; the term actually refers to the flow rate of gas molecules or molecular flow rate. 

A traditional mass flow meter uses a thermal sensor to perform mass flow measurement.  The flow sensor is heated, and the flowing gas molecules transfer or “carry” the heat downstream.  The rate of heat transfer is proportional to the mass / number of gas molecules flowing through the sensor which is used to determine the flow measurements.​

How does a Mass Flow Meter work?

Basic Design:​

Cutaway Image.jpgA mass flow meter consists of four basic components:

  1. Electronic Circuit Board
  2. Flow Sensor
  3. Bypass Shunt
  4. Base
*See Figure on right

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®, VCO®, etc). A majority of the gas will move through the bypass shunt; however, a certain fraction flows through the thermal mass flow sensor capillary tube. 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. The gas flow must be laminar to ensure a linear percentage enters the mass flow sensor.  After passing through the thermal mass flow sensor, the gas then exits the flow meter via a downstream port. ​


Thermal Mass Flow Sensor: T​he Heart of the Flow Meter

Heater_Meter Image.jpg

Tube Graph.jpgThere are numerous thermal mass flow sensor designs.  The Teledyne Hastings’ 200 Series sensor is shown in image on left.  This thermal mass flow sensor consists of a small 316SS capillary tube with a heater winding located in the center.  A thermocouple (TC-1) is located on the inlet side and another thermocouple (TC-2) is located on the outlet side.  At zero flow (no gas flow), the heat is transferred through the capillary tube in both directions towards the two thermocouples, each of which has the same temperature (see image on right, ZERO FLOW condition).  As gas flow moves through the capillary tube (inlet to outlet), heat is then transferred downstream by the gas molecules.  The temperature of TC-2 will increase, while the temperature of TC-1 will decrease.  This temperature differential correlates to the molecular flow rate of the gases (mass flow). 

THCD-401_THCD-101 Group.jpgA majority of thermal mass flow meters provide an analog output signal (0-5vdc, 4-20mA, etc.) that is directly proportional to the gas flow rate.  System integrators can directly acquire this signal for process control.  

If the installation is not configured for data acquisition, Teledyne Hastings offers convenient power supplies with integrated displays (see models THCD-101 and THCD-401 in image on right) and ready-to-use connector cables for quick start-up.

Some mass flow meters offer digital communication to convey the flow rate, while other models have a built-in color touchscreen display (See model HFC-D-302B Vue in image on left).​

How do you use a Mass Flow Meter​?

Just like any piece of instrumentation, it is important to read the product user manual prior to operation and to follow the mechanical and electrical connection procedures specified.  Additionally, an electrical pinout reminder is found on the mass flow meter label.  As with any flow device, ensure that mechanical flow connections are leak free. 

For monitoring and control, some mass flow meter models offered by Teledyne Hastings have an optional built-in display and can operate using a commercial off-the-shelf AC/DC power convertor. For those models without integrated displays, Teledyne Hastings offers simple power supply/display controllers with installation-ready connection cables for straight-forward, out-of-the-box analog mass flow meter configuration. 

It is recommended to allow the thermal mass flow meter to warm-up in order to achieve high accuracy.  Additionally, after the flow meter has been allowed to warm up, it is recommended to zero the instrument prior to use.​

What are typical applications for Mass Flow Meters?

Common applications for thermal mass flow meters include but are not limited to:

  1. Leak Testing
    Historically, leak testing of components was accomplished using bubble tests, rotameters, and pressure decay tests (pressure loss over time).  With the introduction of the mass flow meter, leak testing with air was faster, more accurate, and cleaner than other methods.  Thermal mass flow meters have become the clear choice for high-throughput automation and have proven their reliability to leak test automotive and aircraft components, respirators, valves, medical devices, filtration media, and even hard disk drive enclosures.
  2. Custody Transfer
    When industrial gas from a common source is used in large facilities such as factories, universities, national laboratories, and hospitals it is common to set up custody transfer mass flow meters.  Mass flow meters used for custody transfer allow individual departments to monitor and be held accountable for their gas usage, as well as troubleshooting costly leaks and waste.
  3. Environmental Monitoring
    The exceptional accuracy of mass flow meters is a key part of the environmental monitoring processes.  Mass flow meters allow monitoring equipment to calculate contamination levels in real time and because mass flow meters measure molecular flow, compensation for changes in ambient conditions are not necessary.

What is a Mass Flow Controller?​

A mass flow controller is essentially a mass flow meter with a proportional control valve and a proportional integral derivative (PID) loop for controlling the valve.  The mass flow controller not only measures the gas flow rate, but also allows the user to set and control that rate.  The controller’s proportional control valve continually adjusts position to achieve and maintain the desired gas flow rate, providing accurate, autonomous flow control.



​Q: Can Teledyne Hastings’ mass flow meters be used with liquids or any fluid? 
A:  No. Teledyne Hastings’ mass flow meters cannot be used with liquids.  A fluid is defined as “a substance without a shape and can be either a gas or liquid”.  Fluids used in our mass flow meters must be in the gas phase (gases only).

Q: What is mass flow? 
A:  When measuring gases, mass flow is the amount of gas, measured in units of mass, flowing per units of time. Examples of true mass flow units are kg/hr and gm/sec. However, mass flow is often measured in standardized volumetric units. ​

Q: What is volumetric flow? 
A:  When measuring gases, volumetric flow is the amount of gas, measured in units of volume, flowing per unit time. Examples include l/s and cfm. Mass flow can be calculated from volumetric flow only if the density (ρ) is known. ​Qmass = Qvol*ρ.

Q: What is standard temperature and pressure (STP)? 
A:  In standardized volumetric flow units (sccm, slm, scfm), the reference conditions, or “STP” pressure and temperature, define the quantity of gas by determining the number of molecules according to the Ideal Gas Law. In most cases, the selected reference conditions are 0°C & 760 Torr. But, other reference conditions are also used, such as 20°C & 760 Torr, or 70°F & 760 Torr. 

Q: How frequently does a mass flow meter require calibration? 
A:  While each individual user’s quality manual will dictate calibration frequency requirements, it is fairly common to calibrate mass flow meters yearly.  Teledyne Hastings has authorized calibration service centers in the United States, United Kingdom, South Korea, and China.

Q: Is a thermal mass flow meter safe to use with hydrogen and other flammable gases? 
A:  Yes.  All Teledyne Hastings’ mass flow meters are designed and tested for hermeticity (gas tight) and the bypass shunt and sensor gas path are isolated from all electronics to eliminate sources of ignition.​