Are it not expressed in the flow rate of the oil during a oil surge? Thus, at a certain minimum flow rate the contacts will open. I think it will depend on the size and type of Buchholz relay, somewhere between 60 and 150cm/sec.
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Bucholz relays have 3 alarms/trips. The first is accumulation of gases (usually set to 150 mm^3), the second one is total absence of oil in the Bucholz gas accumulation chamber (that means the conservator is out of oil) and the third one is the oil speed, even when the Bucholz is full of oil this contact (normaly wired as a trip signal) will respond to a sudden pressure wave going from the main tank to the conservator (a common set point is 1.5 m/s).
Hope this helps.
