Replacement IC-40 flow, temp sensor with aftermarket

[Flying Tivo]

Forgot to mentioned i tested the old thermistor and it read 8.67kOhms, the new on read 9.57kOhms. The thermistors are rated at 10kohms.

 

[JamesW]

Temperature Sensors - Further Reading

www.troublefreepool.com

https://www.aetsolar.com/literature/Manuals/TempVsResistChart.pdf

The resistance is not a fixed value. The resistance depends on the temperature of the sensor.

 

[Flying Tivo]

Thats what i was implying, that just 1kohms diffrence is enough to render it unusable, instead of a wider range of ohms.

 

[JamesW]

When you measured both sensors, were the sensors at the same temperature?

The circuit just measures the resistance of the sensor and converts the resistance into temperature based on the chart/formula.

 

[Flying Tivo]

Yes side by side.

 

[Flying Tivo]

The circuit just measures the resistance of the sensor and converts the resistance into temperature based on the chart/formula.

Thats what has me guessing that its just not the thermistor, but the combination of thermistor and resistance across the plates to figure out the salinity level. But the mod works and all is good.

 

[JamesW]

The resistance across the plates is not measured as part of the temperature measurement.

 

[JoyfulNoise]

It’s probably not a simple resistance measurement with the IC electronics as self-heating issues can arise. It’s likely part of a voltage comparator circuit where an op amp is driven to output when the load is unbalanced. The output voltage then becomes proportional to temperature of the thermistor. Because op amp inputs have very high input impedance, there’s minimal current draw which would reduce self heating. A Wheatstone bridge could also be used but it has to be designed to reduce current flow as much as possible.

 

[Flying Tivo]

That is well above my pay grade Matt, but thanks.

 

[JoyfulNoise]

That is well above my pay grade Matt, but thanks.

I once worked in a low temperature physics lab...any random excess microjoule of energy could potentially throw off an experiment. Carefully designed measurements were a must. I suspect the folks at Pentair are far less concerned with self-heating of thermistors but I often wondered if the way they are utilizing the thermistor in their circuitry isn’t actually the cause of the failure itself If the thermistor is the weak link and you don’t take care to design the measurement circuitry properly, then you’ll fry it or prematurely age it into failure.

 

[Flying Tivo]

Now, that is plain English, thanks again.