Flow Control’s thermal mass flow measurement technology portal provides important information about flow instrumentation and measurement. Tom Kemme, our thermal dispersion product manager, answers questions about the technology in the portal’s Ask the Expert column. This week’s blog shares some recent Q&As.
Question: Does a thermal mass flow meter require temperature compensation during gas flow measurement?
Answer: Yes, thermal mass flow requires temperature compensation. However, this is not the same as the temperature correction you would utilize with a multivariable transmitter or external to flow technologies such as differential pressure in order to obtain Nm3/h, SCFM, SCFH, etc. Thermal manufacturers understand that gas properties that effect heat transfer vary with temperature. The process temperature is already being measured (using a Resistance Temperature Detector) and is accounted for in the calculation.
Question: Can thermal mass flow meters be used with combustible gases?
Answer: Thermal dispersion flow meters can be used in combustible gases. Typically very little heat is added to the system (how much heat depends on the manufacturer). Sometimes customers ask us this question, as they are worried about exceeding the auto-ignition temperature of the particular gas. The maximum temperature rise of our sensor recorded by FM during the approvals process was 4K above ambient. Therefore, this little temperature added to the process temperature does not cause ignition, but it should be evaluated as needed on an application basis with the proper manufacturer.
Question: For wet gases/vapors (e.g., compressed air), what is the maximum allowable moisture content that will allow the thermal mass flow sensor to work without incurring significant error due to heat loss from surface condensation? What size droplets (microns) & mist eliminator efficiency would have to be required to post-treat acid-laden vapor before reaching the temperature sensors? Can the temperature sensor pins be coated with PFA or ETFE for use in corrosive acid vapor service?
Answer: Thermal flow meters are used in applications that have vapors present, as well as gases that are considered “wet,” such as digester gas. If actual condensation is present and it comes into contact with the probe tips, then this can cause a spike in the reading due to the additional cooling of the liquid. There are no specific specs on droplet size.
Liquid drops themselves would not damage the sensor, but corrosion is a different issue. Most manufacturers offer a standard stainless steel probe option, but many have a Hastelloy® option as well, for more corrosive gases. Upon request, other sensor materials can be looked into.
An option when going into an application where condensation could be present would be to install the probe at an angle to prevent the liquid from dripping down the probe and coming into contact with the pins. Thermal flow meters do not have to be installed top dead center of a pipe or duct and can go into horizontal or vertical lines. An example of this is shown here.
We would typically not recommend coating of the sensor given the fact it would change the thermal characteristics of the sensor. This could affect sensitivity and achievable flow rates.
As important as accuracy is many times in compressed air applications customers are looking for a relative indication of the amount of leakage. Knowing this measurement assists in calculating the approximate savings by eliminating such leaks. Every flow meter has advantages and disadvantages. For thermal, some of the main advantages are:
- Direct mass flow measurement
- Strong signal at low flows or low pressures
- High turndown
If you have more questions about thermal mass flow meters, you can check out previous answers in the technology portal.