How do I wire a switch to select one or other NTC controllers

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Well-Known Member
Jun 5, 2019
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Paremata New Zealand
I have an All in one brew system that I have fitted a SmartPID controller replacement.
This works well but the kettle sensor is in the bottom of the boiler and reads a much lower temperature when in the boiling phase, probably due to the trubtrapper and poor thermal currents. I have to keep an eye on the wort as it heats up because the reading can be 80celsius whilst the wort is visibly boiling. This has issues as the PID software doesn't move into boil mode and allow me to then reduce the power to control the boil rate.

I plan to add another NTC sensor of the correct type but on a long wire that I can put into a thermowell at the top of the liquid this would give a real temperature.

I would need to fit a switch to select between the base sensor for mashing and the thermowell one for boil and cooling phase but I'm not sure what type of switch to get and how to connect up the two sensors, switch and back to the PID.
So just an update, found out that I needed an SPDT switch and worked out the wiring was fairly simple.
Tested the NTC 3950 probe and it worked, so then I wired the toggle switch in and the screwed in kettle base probe and then the put anywhere probe.

Turned on and I got readings from each one when I flipped the switch to select. But the temperature seemed to fluctuate up and down about 0.5 degrees on the probe reading even though stable, they did react to temperature change and had good parity when in the same environment.

I looked in the settings for the PID controller and noticed that the Beta for the NTC in the kettle base was 3977
wheras the other probe was 3950. I'd been told my fitted one in the kettle was a 3950.

So I disconnected the 3950 and have placed an order for the appropriate 3977.

Does this seem likely as the cause of the temperature variance I was getting or do I need something else in the circuit or am I trying to achieve the impossible?
I've had another look at the specifications for my in kettle NTC and in fact it was supplied with a sensor with a beta of 3950. I must have had it on the wrong setting all this time. Still seemed to be working well though.

So I'm back to my question is it possible to switch between two separate NTC sensors so they can utilise the same input to the control board?

When I tried this my temperature values weren't stable and I wondered whether they were somehow interfering with each other? Do I need a diode in the circuit to each sensor perhaps?

I'm keen to find an answer to this issue if possible.