The buchholz relay is factory calibrated. You should perform regular functional tests to verify that the contacts will still operate properly and actuate whatever function you want, such as an alarm or a trip. When I examine the old spare buchholz relay in my office, I see that there is a hinged plate connected to the lower float. There is a small hole in the hinged plate to allow for the slow flow of oil back and forth between the tank of the transformer and the conservator during normal expansion and contraction of the the oil that results from changes in load or ambient temperature. The sizes of the plate and the hole are fixed and cannot be changed without changing the operation of the relay.
On the older buchholz we have installed on our 30 year old transformers, there is a test button under a protective threaded cap. When the cap is removed and the button pressed half way down, the upper float in the buchholz relay will travel downward simulating low oil level, and should alarm only. When the button is depressed fully, the lower float in the buchholz relay will travel downward simulating either low-low level or sudden pressure, and should trip.
It is important to keep the valves on both sides of the buchholz relay fully opened to allow for the free flow of oil between the tank and the conservator during normal operation of the transformer. Last year, when we took our unit off line, we de-gassed one of our GSU transformers. Being sensitive to recent events with mispositioned valves on one of our transformers, the maintenance people doing the degas verified three times that the isolation valve was open. Only problem was that the arrow on the handwheel of the valve was backwards, and we actually closed the valve. Several days later, after we went back to full power and the ambient temperature rose, the oil expanded sufficiently that the pressure relief device actuated and dumped a significant amount of oil on the ground. (Turns out about half of the handwheel arrows on similar valves on that transformer and its companions were incorrect.) When we opened the improperly closed isolation valve, we got an actuation because the pressure buildup caused the oil to flow into the conservator so rapidly that the relay actuated. The rapid oil flow probably pushed the hinged plate and depressed the lower float and actuated the relay. Fortunately the maintenance foreman had suggested that we cut out the trip signal, or we would have dumped 1100Mw!
