That does seem to be too high.

I have 2 High Flow Next in my test loop. Coolant is distilled water treated with Primochill Liquid Utopia. HFN #1 reports 45.5uS/cm. HFN # 2 reports 42.8uS/cm. Liquid Utopia contains an antimicrobial agent and corrosion inhibitor. The MSDS sheet does not say what is actually in it, but I suspect the corrosion inhibitor increases the conductivity of the water. I originally had "pure" distilled water in the loop and regret that I did not record the conductivity values I was getting then. If memory serves, it was around 20 to 25 uS/cm. When I added the Liquid Utopia, I was getting about 32uS/cm. It is interesting that it seems to be increasing over time. I know that DP Ultra's conductivity starts out low and increases over time whereas EK Cryofuel's conductivity starts out very high and decreases over time. So the trend depends on what additives they are using.

Google says pure distilled water should have a conductivity of 0.5 tp 3 uS/cm. I don't think I ever got readings that low. I also don't know how pure distilled water from the supermarket really is. The conductivity sensor can measure from 2 to 200 uS/cm and may not be as accurate at the extreme low end. I don't know why you are getting over 70uS/cm. I would say either the distilled water you are using has some impurity in it that it shouldn't, there is something in your loop that is causing the conductivity to increase (unlikely if you have filled it with fresh distilled water twice), or the sensor is messed up.

The conductivity measurement of the high flow NEXT is used to detect when the coolant should be changed, it is designed for DP Ultra.
Outside of this function, it has no significance, since each coolant has a different conductivity.

Since the conductivity does not refer to heat, but to current, it also has no relevance for cooling.

That the conductivity increases over time is due to the fact that ions dissolve from the metals and pass into the coolant.

My DP Ultra is currently at 24.9

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The 'Conductivity' is what it is, it is not really designed for DP Ultra
The 'Coolant quality'upper/lower limits are defaulted for DP Ultra, but can be set to accommodate most any coolant's conductivity
The initial Conductivity of any coolant should be considered a baseline, and increased conductivity over time indicates an increase of contaminants
Some coolants (Cryofuel) have additives intended to be electrically deposited over time, and I do not know how conductivity performs with that scenario

I use DP Ultra clear, and my conductivity reads 7.4 µS/cm (conductivity.jpg)

one thing is we rarely have any specs for the DI water we use.
The one we buy at the store made for ironing clothes and other niceities can vary widely in quality. They mostly care about having very low mineral content, but it may not be treated like industrial grade deionised water. One batch may be low, one batch may be high, depending on if it was produced with fresh filters and ion exchangers or not.
Also, if your bottle is old, the water conductivity can increase, as the water will happily absorb the CO2 in the air, from when you opened it up. It's very corrosive, and wants those ions back. (by the way, don't run a loop on DI water alone. it will eat your blocks..)

If you have friends working in big factories that may have use for DI water, it can be a way to have certified low conductivity water. At the semicon fab where i used to work, i can go get some 14, and 20 megohms water, on the tap. At least you know what you should be getting if you know the spec to begin with.

Citation

The 'Conductivity' is what it is, it is not really designed for DP Ultra

I meant the function of high flow NEXT, everything is configured for DP Ultra.
The conductivity itself was not meant.