Transformer Oil

For lubricating oils we do a complete suiteof tests designed to ensure compatibility.

All run at different percentage mixes. 100 - 90/10 - 75/25 - 50/50 - 25/75 - 10/90 - 100

Foaming, Walter seperation, I forget what else, it's not a quick and dirty look, it's very thorough.

I used Herguth labs the last time I needed it done.

Not sure what extra stuff you might need for transformers, I'd assume that you'd want to look at the elctrical and off gass properties of the various blend ratios.

Do you know what Groups the oil basestocks are?
The antioxidants in a Group II/III oil (new oil) may not be compatible with the added and natural antioxidants in a Group I oil(old oil).
My experience has been that many older formulations have much higher levels of antioxidants than current formulations.
In addition to compatability testing may want to have www.Herguth.com or other lab do oxidative stability test on expected ratio to ensure long term antioxidant interactions are minimal.

The original oil is group 1 and the new is group 2. One of our fleet transformer "experts" was contacted and says they are compatible to be mixed. So thanks for the help guys.

These oils are naphthenics, and don't fit comfortably within the base oil groups. In fact, going to a higher "group" is bad for transformer oil. A higher group number generally means higher VI. Naphthenics are preferred for transformer applications specifically because of their very low VIs (I've seen naphthenics with negative VIs.) As the oil nearest the coils heats up, the vis & density both drop, allowing the warmer oil to rise quickly, inducing a convection current that brings cooler oil in contact with the coils. Naphthenics also have great low temperature properties, which is good in Type II applications like the transformers we see on utility poles.

These oils should all be compatible. A Type I transformer oil (Univolt 60) is going to be base oil with the tiniest dusting of antioxidant. If memory serves, the maximum allowable concentration by the Type I spec is 0.05%. The antioxidant will typically be a phenolic, almost invariably BHT.

In fact, back when Univolt was still on the market, lots of suppliers bought it to blend with their own transformer oils. Gassing tendency is a problem for these oils, gas doesn't transfer the heat from the coils as well as oil. Univolt actually had a negative gassing tendency. So, suppliers would add Univolt to get their oils down to an acceptable gassing level. (My wife would like me to try this approach.)

Since many competetive transformer oils already had Univolt 60 in them and they are ~99.95% base oil, I doubt compatibility will be an issue.

If Shell Diala is actually a Grp II parafinnic, you might want to look at some of the naphthenic suppliers like Nynas, Ergon, or Calumet.

There is no issue on mixing transformer oils made domestically in the United States (Ergon, Calument, Cross, San Joaquin, etc) as they are all of nearly the same viscosity, just mix them well and do not allow any bubbles if filling directly into the transformer. Always fill on the top, not the bottom through the drain valve at a slow flow rate usuall 1-3 gallons per minute

I do not know very much about transformer oils, and would like to know more. What periodic monitoring is done on them, and how. Is there any need for, or is there already in place, in-situ monitoring, and if so, what does it/should it measure?
Thanks for your help.