Measuring The Cooling Effect Of Transformer Oil

Transformer oil has long served two purposes, cooling and insulating. The large, steel encased transformers we see connected to the electrical grid are filled with transformer oil which is circulated through radiator fins for dumping heat to the surrounding air. In the hacker world, we use transformer oil for cooling RF dummy loads and insulating high voltage components. [GreatScott] decided to do some tests of his own to see just how good it is for cooling circuits.

Thermal measurement resultsHe started with testing canola oil but found that it breaks down from contact with air and becomes rancid. So he purchased some transformer oil. First, testing its suitability for submerging circuits, he found that he couldn’t see any current above his meter’s 0.0 μA limit when applying 15 V no matter how close together he brought his contacts. At 1 cm he got around 2 μA with 230 VAC, likely from parasitic capacitance, for a resistance of 115 Mohm/cm.

Moving on to thermal testing, he purchased a 4.7 ohm, 100 watt, heatsink encased resistor and attached a temperature probe to it with Kapton tape. Submerging it in transformer oil and applying 25 watts through it continuously, he measured a temperature of 46.8°C after seven minutes. The same test with distilled water reached 35.3°C. Water’s heat capacity is 4187 J/kg∙K, not surprisingly much better than the transformer oil’s 2090 J/kg∙K which in turn is twice as good as air’s 1005 J/kg∙K.

He performed a few more experiments but we’ll leave those to his video below.

We’ve run across a number of tests running boards submerged in various oils before. For example, we’ve seen Raspberry Pi’s running in vegetable oil and mineral oil as well as an Arduino running in a non-conductive liquid coolant, all either overclocked or under heavy load.

36 thoughts on “Measuring The Cooling Effect Of Transformer Oil

    1. Water is usually better at transferring heat. The downside for a transformer though is that water freezes and conductivity is sensitive when impurities are present in it. In many cases the oil is a better choice because it is not all about heat transfer as much as it is about stability. The oil changes temperature more slowly and helps keep thermal swings dampened in utility transformers. A lot of utility transformers do not actually move the oil, the oil is just present to conduct the heat to the outer finned case of the transformer.

      1. Yeah on reflection seems to be one of those old wives tales I picked up years ago. Possibly originating due to exhortations not to use teflon tape on hydraulic systems or else little shreds of teflon could block small valve orifices and clog up valve bodies etc.

  1. Can you use transformer oil to cook food? Seems like it would be an ideal liquid for pan or deep frying food with its excellent thermal qualities. Ingesting it should help you resist electrocution in the future and help cool off your insides better if its a hot day. And because its not derived from natural sources it should be both vegan friendly and gluten-free.

  2. I think I miss the point in many of his videos…yes water has a high heat capacity than oil, in fact it probably has a higher heat capacity than most common materials…yes transformer oil has a very low electrical conductivity, that’s why it exists.

    Similarly I didn’t get the build vs buy an SSR…of course you can buy on that performs better than one you built from discrete components.

    I don’t know…I must be missing something.

    1. That’s been an old party trick among the custom pc folks for a while. Stick it in a fish tank, use the same case fans to circulate the oil, drop in the evap coil of a mini fridge if you’re feeling crazy and want to keep it real chilly.

      The problem comes when it’s time to upgrade your graphics card and you have to clean all the little contacts in your PCI slot. People end up putting the mobo in the dishwasher. Yeah, really. Not many people think it’s worth it, understandably.

        1. Because the oil is non-conductive and the pin slide over the thin layer rather than cutting through the oil. Remember that oil in most cases is intended to forms a thin film barrier over metals. That’s why it is used as a lubricant. You would probably get some kind of connection on something like a bus connector but the resistance of that connection is non-determinant and buses don’t like impedance mismatches.

          1. I’ve always wanted to run a decent computer in oil. If changing components could you not just use wd40 on the contacts or actual switch cleaner or degreaser before changing?

  3. Transformer oil is mineral oil. Baby lotion without the lavender scent. I work in the transformer industry. It’s dielectric properties are great until it absorbs moisture or is otherwise contaminated.

    1. When I took on responsibility for a load HV gear (power supplies, transformers, rectifiers) at work, one of the 1st things I asked to see was the maintenance and oil sampling records. It turned out one of the units had been overlooked. When we had the results back, one had about 30 ppm water in the oil. That was enough to lower the dielectric withstand to nearly half what it should have been.

        1. Yes, exactly because the water does not want to combine with the oil it will live in the insulation instead. I have done some work with high power transformers and one of the ways to correct the issue was to run the transformers at a high load increasing the temperature above the boiling point. That would cause the water to become vapor and vent from the transformer. Just changing the oil does not help much because the water remains in the spongy insulation.

  4. I worked in a place that used Fluorinert for checking chips for fractures. You dunk a flat of chips in a glass vat of very hot Fluorinert and look for bubbles. One of the girls tripped and one of the very expensive containers of the Fluorinert hit the ground and spilled all over. Outside of the cost, which back then was over $400 for a 1 or 2 liter container, the stuff in non wetting. It was funny watching her try to clean it up with paper towels.

    Depending on what you are going to be dunking into it, deionized water has a lot of good characteristics. The trick is keeping the ions out of it.

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