For technical information about thermal dispersion flow meter technology, Flow Control magazine’s thermal mass flow measurement technology portal provides a comprehensive resource. Tom Kemme, our thermal dispersion product manager, answers questions about the technology in the digital portal’s Ask the Expert column. This week’s blog shares some recent Q&As.
Question: What kind of pressure drop can I expect across a thermal dispersion flow meter?
Answer: Most thermal flow meters you’ll find are insertion probes, and the outside diameters are typically less than one inch. Very little pressure drop can be expected in comparison to other technologies that may block more of the flow path or need to impose a pressure drop in order to make a flow measurement.
Question: What type of configuration is needed in the field when a thermal flow meter is being installed?
Answer: This will vary by manufacturer, but typically the only thing needed in the field is to wire the unit up and install it into the pipe. Thermal dispersion flow meters are calibrated by the manufacturer for the individual application. This way the 4-20 mA output, pipe size and type of gas is already taken into account. There are always ways to communicate with the instrument if needed, but it will vary what configurations can be changed. Some suppliers have a display that the user can operate; sometimes, external hardware/software is needed. There are many settings that can be accessed and modified in our unit, although typically this isn’t needed.
Question: The rotatable housing—why is it needed, and how do I do it?
Answer: Thermal mass flow meters measure flow in one direction. When the flow meter is being installed, the flow arrow on the probe should be in the direction of the flow in the pipe and parallel to the piping. At Magnetrol®, we calibrate with flow going from left to right. Depending on the application, the display may not be facing the user or they would be inclined to install with the flow arrow in the wrong direction in order to see the display. The NPT threads connecting the probe to the housing did not allow for rotation. With the straight threads we use, now the user can loosen a set screw at the bottom-front of the housing and rotate the head to the desired location, and then tighten it back down after rotation. A standard 2.5mm Allen key can be used. There is a second set screw in place to prevent over-rotation of the housing. The rotatable head allows the user to place an order without having to specify direction of flow, simplifies installation and adds ease-of-use when working from the transmitter display.