Electronics > Metrology

DIY Metrology Oil Bath for Standard Resistors

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Most of us volt or ohm nuts have surplus standard resistors sitting on the book shelf. Old resistors settled over the decades can be extremely stable, less than 1 ppm/year is not exceptional (but to be honest there are a lot of bad ones too). However the temperature coefficient is relatively high, 10-20 ppm per degree typical. It means that the normal variations of the room temperature increase the uncertainty to a magnitude worse than the long term stability making the resistors more or less useless for accurate work.

The resistor manufacturers could have optimised the tempco by carefull wire selection or using wires of opposite behaviour. But the low tempco was not the primary design parameter, because the resistors were going to be used in a temperature stabilised oil bath anyway. The benefit would have been minimal and required a lot of extra work.

From the ohm nut point of view this is a real problem, because metrology grade oil baths are not easily available. Occationally you can find surplus units for a low price, but we are talking about instruments of a bath tub size, with freon filled refridgerator engine and remarkably high power consumption. And everyone ever worked with precision baths know that they develop faults very often, even the modern ones.

There are plenty of small water baths available, but circulating system doesn't work with oil and even if it does there is a risk of fire. And the temperature stability and uniformity of these units is usually very poor.

Many good reasons for designing and building my own version. And to start this thread for collecting information about the subject.

It is not going to be an easy project because expertise from a number of areas is required. Not only elecronics but also control systems, thermal design and fluid mechanics, just to mention a few. But as usual, easily covered by the members of this community. And I wouldn't be surprised if someone here has already built something similar.

You could use an ice-water bath. You get no temperature change even if you add or remove thermal energy to the bath.

The length of the old International Prototype Meter was specified in water at the freezing point - this special temperature is easily reproduced anywhere.

Pick up a deep fryer, some baby oil, add resistors, ???, profit. As a bonus it will act like a diffuser and make your lab smell like a baby. (As an aside, I've always wondered how they extract the oil from the babies...)

On a more serious note, for actually moving the oil, I would look into food safe pump, I forget the exact term but basically you run a silicon tube through a set of rollers and it actually squeezes the tubing to produce the pumping action.

I'd run the oil through a water block, which I'd attach a heating element to. That would keep the warm oil circulating in the tank without resorting to some sort of mechanical stirrer. If you wanted to go the other direction and cool the oil bath, just use a peltier attached to the water block. You could even attach a second water block to the hot side and have that hooked up to a water tank+radiator instead of simply a heat sink, for increased efficiency. You can only get so cold compared to ambient with this method, so it might be worth tearing apart a mini-fridge for the compressor, etc.

I'm not sure if it's a joke or not, but what is this baby oil?

Metrology papers state ultra-pure mineral oil, no baby stuff.  :-DD

Going to build small oil bath for vref/resistor tests, but what oil I need?

I did something similare two years ago, a DIY-Enclosure for a unsaturated Weston Cell which may of interest.

I got unexpected good 1st results (32°C +/- 0.005 °C), ~24 hours needed to stabilize (due to low heating power for the accu operation). It is runnning constantly since that time in my cellar @ below 18°C ambient.
Just the long time drift due to the drift of the PT1000 sensor is not yet clear.
But in the case of an resistor measurement it may not a critical point, only the short term drift.

The 32° are choosen for the Weston Cell and may up to 60°C (maximum for the isolation and the electronics).

The regulator is a Resistor Bridge controlled by an Auto-Zero opamp as a simple on/off-regulator/comparator. The sensors are automotive grade PT1000 sensors (cheap and well specified). For primary protection of the Weston Cell there is an 35° +/-1° temp-switch.

I did many tests before this state of design to optimize the regulation in case of an sudden ambient tep-change and minimum overshoot during power on (critical for the cell).


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