Many thermal mass flow meters are of the insertion type. As a starting point, proper insertion depth and straight run per the manufacturer’s recommendations should be adhered to. There will also be a flow arrow located on the probe or flange depending on the process connection. This is because thermal mass flow meters are calibrated in one direction and are only accurate in the direction for which it was calibrated. If not installed properly it can still measure and be repeatable but accuracy will be affected.
Verifying flow meter performance is an essential step in ensuring your flow meter is meeting its specifications. Manufacturers design-in diagnostics tests that can be run in the field, in the pipe (in-situ) or in the instrumentation shop.