Flare Gas Meter

There are so many things that could be happening here. I'm not familiar specifically with any technique used with flare gas monitoring so I will have to answer you in a generic fashion.

There maybe a loose or poor connection somewhere in the connection to the ultrasonic transducers. Troubleshooting where this type of an intermittent connection might be is the nightmare of any technician.

I also did a quick Google of your two models and noticed that both models are listed as a nebulous "fast response time" device. If the FGM160 is actually faster then possibly you are now seeing something that was ignored by the previous model. If this is the case then an authority in your company must decide if the gas flow must be corrected or if these fluctuations should be filtered out of the system.

Good Luck

I have no experience with this either, but there is some correlation with light and dark conditions, or hot and cool.

Are the sensors using IR? Did they get re-aimed in some way? Is the new control board possibly incompatible with the sensor and requires a sensor upgrade as well?

Perhaps the sensor signal on the sensor is delivering a higher current or voltage than the new controller requires. It may be all you need is a series resistor in the line to reduce the sensitivity. If the new control board can "see" night and day differences when the old one did not, then I would suspect a sensitivity issue related to the sensor requirements of the new board.