What is the liquid in a radon U-tube Manometer? I'm assuming that it's not water because of evaporation. Mine is clear in color, but I've been reading that it's normally blue or red. Is it a particular type of oil? Can it be refilled/replaced? Some bubbles have appeared recently in my gauge that were not there last summer. Can they be purged out? It doesn't seem to be happening naturally with the vacuum produced by the fan and it makes reading the gauge very difficult.
It is oil of a specific density to correspond to the scale. I believe red is one density and blue another, at least for Dwyer manometers. I'm fairly sure replacement oil is available. If you don't care about the accurate reading and only that the fan is working, you could just use plain mineral oil, but you should post a notice next to the gauge that the oil is non-standard and what the last known good reading of the new oil was. I wouldn't use vegetable oil as it may get rancid.
From what I remember, the oil is typically vegetable oil with a food coloring added to it. I'm not too sure it can be replaced but I would call your local Radon specialist and ask over the phone. No need for someone to come out and make you pay to ask. If they won't answer the question over the phone, call someone else. They can't want your business too badly if they won't answer a simple question.
When my radon mitigation system was installed I was told the blue liquid would fade to clear in a few years. The company said it's not a problem since you can still read the meter by looking a bit closer, and definitely not worth a service call as long as you can verify the fan is working.