The reference markings on the face should somewhat correspond to the 26Q, top oil temperature indicator.
Some guages will have smaller "Tick" marks above and below the 20 or 25 deg mark. These are usally a value of 5 deg C for the smaller marks and 10 deg for the larger. The markings will vary between vendors. I'd refer to the vendors cut sheets in the operations manual that came with the transformer for detailed information on your guage.
Just keep in mind that the level guage is not a direct reading of the units temperature but more of a reference point to it's oil level as a result of it's Temperature.
This level will change with the units temperature or ambient if the unit has been offline for some time. Low level alarms in sub zero climates can happen.
There's several vendors who make level guages..ie Qualitrol, Messko, Orto to name a few.
Look at the transformer oil temperature gauge... look at the oil level gauge. If they're about the same, then you're at about the right level. If the level gauge reads a higher temperature then you've too much oil, if it reads lower then you've not enough oil.
One important point-in case you fill more than indicated by oil temperature,say oil is at 20C,but you fill up to 60C, then oil will overflow through breather pipe with loading ( incase of free breathing conservators) or operate PRD or rupture tank weld or cause radiator fin rupture( in case of conservator with air cell)Remember oil expands by about 0.8 % by volume for every 10 C rise in temperature.If there is no space for this expansion,excessive pressure will develop causing operation of pressure relief device or weld crack on tank.
Some manufcaturers mark only as Empty, 1/4,half,3/4,full-then you have to find out the range of temp Min to max that may occur in service and then decide appropriate level.
