Resistors Used in an LM399 Circuit

[alm]

In the concept that iMo proposed, making a continuous WW resistor and tapping the appropriate point for the divider, the power is dissipated equally along the physical resistor because each incremental segment sees the same current and thus the same dissipation per length because of constant effective resistance per length of the resistor.

So if our understanding is correct, then iMo's concept wouldn't experience any change in voltage division ratio due to temperature, either ambient or induced (self heating). Of course in reality there will be some variations in this due to resistor non-uniformity, especially at the tap and ends.

I agree that that would mitigate the problem, but it is important to stay aware of it and not blindly assume that matching tempco of resistors means no net tempco on the output of the divider, especially because the TC of manganin can be quite substantial. Dissipation at the ends near the leads will be much higher than in the middle.

[iMo]

The power dissipation is not a big issue, imho, as it will be constant. We talk here the feedback divider - the biggest trouble maker with the 7->10V amplifier.
With say (R1+R2) in a typical range 5k to 20k the Ptot will be 20mW to 5mW. That is not small, but constant (as the 10V output will be +/- couple of ppm worst case).
With both R1 and R2 wound on the same bobbin the temperature difference between R1 and R2 should stay constant, afaik. With changing the ambient the temperature of (R1+R2) will change accordingly, but the difference stays the same.

The ratio will be

Ratio = R1*(1+TC*(Tr1-25)) / R2*(1+TC(Tr2-25)), where (Tr1+Tamb)-(Tr2+Tamb)=const.

Basically the Vishay divider with 3 legs 

PS: I would even try with lower valued (R1+R2), like 1k. We use usually transistor buffer at the output, thus the 10mA via the divider cannot hurt (100mW), imho, the elevated temperature of the divider may even help to repel the humidity inside the resistor off.

[Conrad Hoffman]

Totally OT, but manganin is also used to make high pressure sensors- https://strainblog.micro-measurements.com/content/manganin-pressure-gages

[iMo]

Totally OT, but manganin is also used to make high pressure sensors- https://strainblog.micro-measurements.com/content/manganin-pressure-gages

Thus we should not use the manganin resistors in ref sources inside a submersible made of carbon fibre

[iMo]

Some first hand info on Manganin. The TC in Graph 1 and 2.
It looks like they produce a specific compositions for specific box with minimum TC..

[Conrad Hoffman]

The above data sheet refers to technical info regarding aging (and soldering/brazing), so here's that. I also consider the thermal emf against copper. There are some alloys that would be excellent, except they have a high thermocouple voltage.

[Kleinstein]

The use in pressure sensors is not because of special, extra strong reaction to pressure or strain. This are normal DMS. The relative resistance change with strain is quite similar between metal alloys. The choice is more to look for alloys with low TC and a low tendency for mechanical creepage and a suitable thermal expansion depending on the material of the elastic element.