DIY Metrology Oil Bath for Standard Resistors

[ltz2000]

If you need to run 24/7 or 10W of heat input is going to be a problem, it might be worth trying an air lift pump injecting air at the bottom of a fully immersed vertical open-ended 1" tube, 90%  of the bath depth.   If you have a permanent compressed air supply its going to be the cheapest/easiest way of stirring the oil within the bath without adding significant heat energy.  If you don't have a permanent air supply, aquarium air pumps are designed to run 24/7 and are reasonably affordable.

Not a good idea. There is always humidity in the air supply.

[ltz2000]

Yes, the viscosity of the oil is going to need to be as low as possible, the pump is effectively a synchronous motor with a surprisingly small impeller so it might not be too much of a problem

Another sort of geeks who don't sink their resistors in mineral oil, but whole computers instead, have actually tried that:

As expected, the aquarium pump (even though a very powerful one at 1600L/H) was utterly inadequate. It gave almost no flow when the oil was cool, only at ~60C+ did the oil become fluid enough to generate some flow, but still way too little.

[Ian.M]

Not a good idea. There is always humidity in the air supply.

Hmm.  Although air driers can be bought or made, they significantly increase the complexity and require regular maintenance.  So unless the pump air intake can be taken from over the bath under a sealed cover, WITHOUT putting the pump inside the cover where you dont want the heat,  we are back to a long shaft mechanical stirrer and a motor to drive it mounted over the bath

[PA4TIM]

I repaired a few oil baths for a customer the last month, and an other one is waiting on it's turn. I think they are not very practical for private use. They draw a lot of current while heating (I measured 14Aac)
They use a colorless transparant silicon oil. If you want I can ask him the type of oil they use. I fill them with water while working on them. (I think they need 4 or 5 liters. I did not measure it)

Repairing calibration gear has the advantage that I can calibrate my own gear. This way I calibrated my 4 wire PT100 on a 7,5 prema, and two TC's. A few days ago I calibrated (more or less) the flow of a computer fan (I use that to do a sanity check on thermal mass flowmeters after repair.)

[ltz2000]

I repaired a few oil baths for a customer the last month, and an other one is waiting on it's turn.

Please take photos, dimensions etc. If we are talking about a metrology grade bath, it could give usefull tips how arrange the circulation, heating and cooling.

[lars]

I have a question if standard resistors like LN4040B that are supposed to be in oil might drift if taken out of a bath (or put in)? I have several LN resistors that was in a cal lab in Sweden before in a bath. During the six years I have had them they seems to have started to drift upwards slightly, maybe 1ppm/year but I am not sure. The LN4040B was +47ppm when I got it and now I think it is +53ppm.

Lars

[ltz2000]

I have a question if standard resistors like LN4040B that are supposed to be in oil might drift if taken out of a bath (or put in)?

The normal room temperature variations don't affect the very long term drift, but can trigger changes which need several months to recover.

During the six years I have had them they seems to have started to drift upwards slightly, maybe 1ppm/year but I am not sure.

How do you know that your reference (whatever it is) doesn't for example drift +3 ppm/year and the resistor +4 ppm/year?

Also the temperature coefficient of these resistors is in the 10 ppm/C range making the measurements without a bath more difficult.

The LN4040B was +47ppm when I got it and now I think it is +53ppm.

Because the resistors were manufactured a long time ago, you can deduce something by comparing the current value with the assumed initial value. Which usually was very close to the nominal value, but unfortunately varies a lot between individual resistors and the time on manufacture.

I have several LN resistors that was in a cal lab in Sweden before in a bath.

Did you get any historical calibration data from the previous owner?

[lars]

How do you know that your reference (whatever it is) doesn't for example drift +3 ppm/year and the resistor +4 ppm/year?

I don´t know for sure, this is one of the most interesting topics for nuts I think. But I have calibrations with about 5ppm uncertainty for the GR1440 during the last 15 years and also Vishay and Alpha hermetic that are within +-0.5ppm/year of the GR1440 but the question mainly is if the in and out of bath changes the drift for the LN´s?

Did you get any historical calibration data from the previous owner?

No I got no historical data.

I have also measured the alpha and beta coefficients and the LN4040 is best at 22-23C so that makes it easier to compensate outside of a bath. It is also in a styrofoam container with PT100 sensors.

Lars

[ltz2000]

but the question mainly is if the in and out of bath changes the drift for the LN´s?

I think I already answered that question:

The normal room temperature variations don't affect the very long term drift, but can trigger changes which need several months to recover.

The stress caused by the small temperature changes will eventually release.

If the natural drift of the resistor is extremely low, the relative change can be very dramatic and certainly ruins the predictability for a long time. But now we are talking about the best of the best standard resistors used in the primary metrology.

The natural drift of the L&N 40xx type is normally so high, that it much exceeds those effects in mid or long term.

[sarepairman2]

how about putting the oil bath in a peltier minifridge with some desiccant packets?

[branadic]

In "Long term characterization of voltage references" they used "...a bath of perfluoropolyther heat transfer fluid (Galden HT 110) which has a high resistance (about 10e15  ? · cm) and a low viscosity (hence a good temperature homogeneity). This configuration also prevents any influence of the hygrometry and reduces the thermo-electric voltages at the soldered junctions on the electronics boards...".

[zlymex]

Attached some pictures of my 'oil bath' (inside, front and rear view).

Very nice
The markings on the second photo, was is laser engraving?
One small thing about arranging 4W is a straight line is it won't allow current pair and sense pair at the same time, and a standard ‘wind screen’ won't fit, of course that situation may rare to happen.

I have some photo of real oil bath from web.

[acbern]

The markings are engraved. The front panel is from schaeffer-ag.de. Not cheap, but quite convenient. Not cost efficient for multiples.

[Robert763]

Where did you get that enclosure? And what is the part number or series of the Vishay resistors you used?

The case is from a Wandel & Goltermann instrument, very distinctive. Possibly from a PDA3 or DF64