MFC Pressure Specifications: Why are there so many?

Thermal Mass Flow Controllers (MFCs) control the mass flow (or more specifically molecular flow) of gases. If we are measuring flow rates, why are there so may pressure specifications? This blog will attempt to explain the various pressure specifications associated with MFCs.

Standard Temperature and Pressure (or STP):

The volume of a gas changes with temperature and pressure changes; therefore, basic volumetric flow units are almost useless when defining gas flow rates. When you pick up an MFC or look at the calibration report, you always find a value for Standard Temperature and Pressure (or STP. For most MFC applications, it will be listed as STP: 0° C & 760 Torr. This does not mean that the MFC must be used at that temperature and pressure. Some gases may not even be in gas phase at those conditions. STP is a way of defining molar volumetric units. STP allows your gas calculations to be correct when you measure all your experimental data using the same STP values.

Maximum Allowable Working Pressure:

The Maximum Allowable Working Pressure (sometimes expressed as M.A.W.P.) represents the maximum working pressure for which the end user is permitted to operate the MFC. There should be a label on the MFC that defines this value. For safety purposes, the end user should never exceed this limit.

Proof Pressure:

The Proof Pressure is the pressure value to which the MFC manufacturer proof tests the unit. This value is typically 1.5 times the Maximum Allowable Working Pressure. For example: if the Maximum Allowable Working Pressure is 500 PSIG, the manufacture may Proof Test the unit to 750 PSIG. Even though the manufacturer tests the unit to this value, the end user is still not permitted to exceed the Maximum Allowable Working Pressure.

Burst Pressure:

Burst Pressure is the maximum pressure that can possibly be applied to the MFC without physical damage to the pressure containment. It’s typically much higher than the Proof Pressure. MFC manufacturers will define this during product development. This can be modelled using 3D stress analysis and eventually tested using a hydraulic pump. Burst Pressure testing tends to be a favorite for most engineers.

Inlet Pressure & Outlet Pressure:

Some people forget that the MFC contains a proportional control valve. To proportionally control the flow, there must be a pressure drop across the MFC. Inlet Pressure (Up Stream Pressure) is the pressure measured exactly at the MFC inlet fitting. Outlet Pressure (Down Stream Pressure) is the pressure measured exactly at the MFC outlet fitting. To facilitate flow, Inlet Pressure must always be greater than Outlet Pressure. Some MFC manufacturers will define these values on the MFC Serial Number label.

Differential Pressure

Differential Pressure is often abbreviated as dP or ΔP.

dP = Inlet Pressure – Outlet Pressure

Pressure Coefficients:

Even though thermal mass flow values are independent of temperature and pressure changes, the thermal mass flow sensor performance is impacted at higher pressures. The flow sensor consists of a small 316SS capillary tube that is wrapped with several windings of heater wire. When the pressure increases, the capillary tube expands. When that occurs, the heater windings expand with the tube. The error associated with this expansion is represented by the Pressure Coefficient. For example, a typical Pressure Coefficient could be +0.0067%/psi. 

To learn more about the Teledyne Hastings Instruments mass flow meters and mass flow controllers, visit https://www.teledyne-hi.com/en-us/what-we-do/thermal-mass-flow 

If you have questions about mass flow controllers, feel free to contact us by phone (+1-757-723-6531 or 1-800-950-2468), email Hastings_Instruments@Teledyne.com, or via Live Chat on our website www.teledyne-hi.com.