Monday, February 8, 2016

Information Required When Specifying a Thermal Mass Flowmeter

thermal mass flowmeters
Thermal mass flowmeters
inline style (left) and insertion style (right)
(courtesy of Eldridge Products, Inc.)
When determining the proper thermal mass flowmeter, you must have details of your flow conditions so the flowmeter will be optimally matched to your application. Beyond the normal information such as gas, line size, temperature and pressure, power input, etc., be prepared to know the following:

Process Gas Composition — This is simple if you are flowing air or a pure gas such as hydrogen, nitrogen, etc. Gas mixtures should be provided with each gas listed as a percent of the total, with the sum equalling 100%. It is possible to calibrate your thermal mass flowmeter with the actual gas your sensing. When this is not possible, a gas or gas mixture with equivalent heat transfer characteristics is used instead.

Full Scale Flow Rate — Although its difficult to know the exact maximum flow rate (Full Scale), its important to provide an estimate for the calibration. If the Full Scale is not known, it is best to over estimate a flow. This information can be in units of mass flow (SCFM, Lbs/Hr, NCMH, etc.) or velocity (SFPM, MPS, etc.).

Line Size — Irrespective of using an in-line or an insertion style thermal mass flowmeter, its mandatory to know your process line size.

Process Gas Pressure and Temperature — Thermal mass flowmeters must be calibrated to conditions as close to the actual process conditions as possible. Therefore, knowing the process pressure and temperature conditions is required.

inline thermal mass flowmeter
Inline style thermal
mass flowmeter
Ambient Conditions for Sensor Electronics — Making sure the thermal mass flowmeter electronics are in the proper environment assures performance and long life. Conditions such as high temperature, hazardous locations, exposure to dirt or spray-down all need to be considered. A wide variety of options exist to compensate for differing environmental conditions.

Input Power Supply — What power is available for the thermal mass flowmeter? AC voltage? DC? Typically thermal mass flowmeters are powered by 24 VDC or 115 VAC, with 230 VAC as an option.

Probe / Body Configuration — Inline configuration or an insertion style flowmeter? Inline styles have the sensing elements mounted directly in the flow section for exposure to the process gas. With insertion style thermal mass flow meters, the probe assemblies is inserted directly into any suitable flow section, pipe, duct, etc.

insertion thermal mass flowmeter
Insertion style thermal mass
flowmeter with BVR

Inline style flowmeters typically are provided in line sizes from 1/4" to 4" with a number of installation options such as MNPT ends, ANSI of DIN flanges, etc. Insertion style flowmeters can be used in line sizes of 2" or greater and can be mounted with ball valve retractor assemblies, compression fittings, pipe nipples, etc. When using inline style, a device called a “ball valve retractor assembly” (BVR) is used when it is desirable to provide a means of flowmeter removal while keeping the process line running. This so called “hot-tap” device is an excellent way to minimize downtime when servicing or calibrating the flowmeter.

One final consideration for selecting a thermal mass flowmeter is the use of integral or remote electronics. Integral electronics configurations are generally less expensive, while remote style electronics allows for easy access when the actual process line installation is in an inconvenient location, or in inhospitable environments.

For more information on any thermal mass flowmeter question or application, contact:

Arjay Automation
1178 E. Cliff Road
Burnsville, MN 55337-1577
Phone (800) 761-1749
Fax (612) 861-4292