DIY Metrology Oil Bath for Standard Resistors

[ltz2000]

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.

[pmbrunelle]

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.

[timb]

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.

[TiN]

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. 

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

[pelule]

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).

[VK5RC]

How about a deep hole, 4-5m down soil temp is pretty constant?

[Ian.M]

For the agitation, use an immersable 12V fuel pump, or an inline one for circulation.

[PTR_1275]

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 think you are talking about a peristaltic pump. They work extremely well when you don't want any contamination. I think they use them on dialysis machines and the like too.

[timb]

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. 

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

Baby Oil is Mineral Oil + Fragrance. Obviously you wouldn't use that. ;-)

You can pickup Mineral Oil at any drug store, though you might have to find some online if you're after ultra pure. (Though I've successfully run a computer submersed in off the shelf mineral oil so I don't see why it wouldn't work?)

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 think you are talking about a peristaltic pump. They work extremely well when you don't want any contamination. I think they use them on dialysis machines and the like too.

Yes, that's exactly what I'm talking about, thanks! You're right in that they're used a lot in the medical industry, for pumping drugs and blood. The food industry uses them to pump non-pressurized liquids that would be too sticky (or have the potential for bacteria growth).

Spark Fun actually sell one if anyone wants to try it out.


Sent from my Smartphone

[CatalinaWOW]

A well equipped hobby shop will also have peristaltic pumps, marketed as fuel pumps for model airplane fuel.  They are available with electric motors and hand cranks.  Two flavors, with the tubing appropriate to resist gasoline (petrol) or glow fuel (castor oil, alcohol and nitromethane).  Only $10-20 here in the US for the manual variety, a little more for the electric ones.

[ltz2000]

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

But why not placing the Weston cell in the oil? That is how it was made in primary labs in the past.

Greatly reduces the thermal EMFs of the connections. And you also get rid of the temperature uniformity problems caused by the air convection.

[Ian.M]

Why would anyone mess with a peristaltic pump when you can use a cheap auto fuel pump suitable for Diesel oil?  Peristaltic pumps are expensive, typically low flow rate, and the pumping tube is a wear part that tends to split after many hours of operation.

The only reason to use a peristaltic pump for a circulating application is for highly agressive fluids or if a sealed sterile system must be maintained.

N.B. If, for ease of cleaning items removed from the bath, you use Fluorinert for the 'oil' bath fluid, it is likely to require a lubricity additive, and you should use a gasolene fuel pump.

[ltz2000]

The pump seems to be one of the most difficult parts of this project:

Acoustical noise

A car fuel pump running in your lab 24/7 may require strong medication.

Electronic noise

This is a real problem when measuring fractions of a ppm, usually with quite long cables.

Heat

A car fuel pump consumes probably 10 W and the heat is mostly transferred to the liquid it pumps. That needs to be somehow compensated.

Durability

If running 24/7 almost any commonly available pump will meet its maker quite fast. For example a rough estimatre for a car fuel pump:

200 000 km / 60 km/h = 3333 h = 139 days

Flow rate

Many pumps are optimised for pressure, not flow rate. I don't know if 70 litres/h (fuel pump again) is enough for keeping the temperature uniform enough.


That is why a stirrer with a low running motor is used in most metrology oil baths.

[PedroDaGr8]

Why would anyone mess with a peristaltic pump when you can use a cheap auto fuel pump suitable for Diesel oil?  Peristaltic pumps are expensive, typically low flow rate, and the pumping tube is a wear part that tends to split after many hours of operation.

The only reason to use a peristaltic pump for a circulating application is for highly agressive fluids or if a sealed sterile system must be maintained.

N.B. If, for ease of cleaning items removed from the bath, you use Fluorinert for the 'oil' bath fluid, it is likely to require a lubricity additive, and you should use a gasolene fuel pump.

Don't remind me of the tubes breaking. We use these pumps a LOT in biotech and I can say the tubes break and they break often. We actually have three month change intervals and sometimes that's not enough. Nothing like 2L of 1N NaOH all over your bench because the tube broke overnight and flooded your bench.

[Ian.M]

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.

[LINACboy]

Do you know what type of oil the resistor manufacturer recommends?
In an IETlabs datasheet I found a reference for Drakeol #9, white oil.
In any case, the insulation resistance ought to be pretty high, so you'll want to be very careful with any kind of circulating pump. There's a good chance that the build-up and entrainment of static charge from the moving fluid could ruin your precision measurements. I'd also be worried about vibration, both directly from the pump rotor and from the oil hitting obstacles as it flows.

I'd suggest to surround the oil bath with a water bath and let that circulate through a temperature controller.


Apart from that, I love using peristaltic pumps for pumping high voltage insulating oils. The stress on the liquid during pumping is minimal and with tubes correctly matched to both pump and liquid they can last virtually forever.

EDIT: For proper insulating oils, even those based on mineral oil, the fire hazard is minimal in an application like this, where the temperature is kept close to ambient. In case of a catastrophic failure of the pump, I wouldn't be too worried, either. Standard precaustions like motor thermal overload protection will be sufficient.

[acbern]

Attached some pictures of my 'oil bath' (inside, front and rear view). Actually without oil, using VPG hermetic 4-wire resistors and a precision temperature regulator, specified to 0.01K stability (that is actually a little overstated, as I measured with the internal PT100). But overall much cheaper than an oil bath with Guildline or whatsoever standard resistors and less messy .

[Dr. Frank]

Attached some pictures of my 'oil bath' (inside, front and rear view). Actually without oil, using VPG hermetic 4-wire resistors and a precision temperature regulator, specified to 0.01K stability (that is actually a little overstated, as I measured with the internal PT100). But overall much cheaper than an oil bath with Guildline or whatsoever standard resistors and less messy .

In your case, the oil bath is inside the hermetic resistors, probably. 
That interior oil should also provide a good thermal coupling between the resistance elements (BMF) and the aluminium bar, so that most of the heat flows over the case, than over the leads.

Looks very nice.

Frank

[quarks]

bookmarked!
Looks very interesting, thanks for sharing

[acbern]

In your case, the oil bath is inside the hermetic resistors, probably. 
That interior oil should also provide a good thermal coupling between the resistance elements (BMF) and the aluminium bar, so that most of the heat flows over the case, than over the leads.

Yes, the oil (primarily for long term stability, but also for thermal transfer) is inside the resistors. The thermal transfer to the resistor cases is achieved by a relatively thick AL bar, and thermal transfer pads. Since essentially no power (<10mW) is consumed in a single resistor at a time max, the thermal conductivity is not even critical. The temperature is measured directly at the (in the end isothermal) bar by a ceramic PT100.

[timb]

Attached some pictures of my 'oil bath' (inside, front and rear view). Actually without oil, using VPG hermetic 4-wire resistors and a precision temperature regulator, specified to 0.01K stability (that is actually a little overstated, as I measured with the internal PT100). But overall much cheaper than an oil bath with Guildline or whatsoever standard resistors and less messy .

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

[acbern]

I had the enclosure in my lab, a one-off, not sure where it is from.
The resistors are from the HZ series of VPG.

[Gyro]

The pump seems to be one of the most difficult parts of this project:

Acoustical noise

A car fuel pump running in your lab 24/7 may require strong medication.

Electronic noise

This is a real problem when measuring fractions of a ppm, usually with quite long cables.

Heat

A car fuel pump consumes probably 10 W and the heat is mostly transferred to the liquid it pumps. That needs to be somehow compensated.

Durability

If running 24/7 almost any commonly available pump will meet its maker quite fast. For example a rough estimatre for a car fuel pump:

200 000 km / 60 km/h = 3333 h = 139 days

Flow rate

Many pumps are optimised for pressure, not flow rate. I don't know if 70 litres/h (fuel pump again) is enough for keeping the temperature uniform enough.


That is why a stirrer with a low running motor is used in most metrology oil baths.

Maybe you should be looking towards aquarium technology. I've had one of these filter / pumps running for 25 years+ continuously with no sign of deterioration:

https://www.eheim.com/en_GB/products/technology/external-filters/classic

The pump (in the top) is a small centrifugal one: The winding is sealed  in the plastic molding and the rotor is a small plastic covered cylindrical magnet and impeller with a carbon bush running on a short ceramic spindle. Insignificant wear (it uses the water as a lubricant) and low power consumption (5W).

You could maybe even use the filter chamber as the bath (or heater chamber). The only thing you need to check is material compatibility with whatever oil you're using. As long as the viscosity is low, I'm sure the pump lubrication would be vastly better than the 'biologically active' water that it's used to running in.

Of course you can also source all the 'plumbing' from aquarium suppliers too.

[ltz2000]

Maybe you should be looking towards aquarium technology. I've had one of these filter / pumps running for 25 years+ continuously with no sign of deterioration:

https://www.eheim.com/en_GB/products/technology/external-filters/classic

That was one of my alternatives. But the Eheim unit looks much better than the Chinese crap that I was able to find. Thanks!

Do you have any information about the exact materials used in plastic parts, rubber seal etc. All of these must withstand mineral oil, also long term.

The major problem when using pumps designed for water is that that the viscosity of mineral oil is so much higher which can seriosly overload the pump.

And runnning the pump without the original multistage filters also affects that fine balance. With good luck these two can compensate each other a little.

[Gyro]

Yes, the Eheim units are certainly built to last.

Hard to tell on the materials - I'm pretty sure the Top molding and pipe fittings are ABS, I would guess that the green tank is probably polycarbonate - it has to withstand the outward pressure of the O-ring without cracking. The O-ring on the new ones is advertised as silicone, the one on mine is black, probably nitrile - never had to replace it though.

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 - there is a simple rattling clutch arrangement to make sure the magnet is turning before it engages the impeller. You an see basic pictures in the manual on their site.

P.S. I don't think the absence of filter media would make any difference - you can put various types in there anyway and it has to cope with silting up etc. There is always a gap at the top regardless and the pump cavity itself is up in the lid (ie. there's no spinning impeller in the tank itself). Hope this helps.

P.P.S. They also do smaller in-tank ones that might be a good match.

https://www.eheim.com/en_GB/products/technology/internal-filters