Just came across this old post, and I thought I could add a tidbit for anyone who comes across it in the future. I had a malfunctioning sensor on the my Sundance Cameo spa, so I purchased a new one. Now the temperature was more accurate but still seemed to be reading at least a few degrees hotter than the water was. Unfortunately my TWO manual thermometers read the water as even hotter. Apparently both electronic and non-electronic spa thermometers are quite inaccurate.
The temperature sensor produces a resistance that is inversely proportional to the temperature of the water. So the higher the resistance, the cooler the water. 59F should produce a resistance of around 48,840 ohms, and 104F should produce a resistance of around 16149 ohms.
This means that if you have a new temperature sensor that still is reading low, you can compensate by adding a resistor to it to help increase your high temperature a bit. If your hot tub is reading 104F when it should read 99F then you could compensate by adding approximately a 2k Ohm resistor in series with the temperature sensor (i.e., to one of its leads - NOT across the leads). This should work because the sensor should be outputting around 18000 ohms at 99F. Therefore, when you spa was seeing 16149 ohms and assuming it's 104 degrees, now it will see 18149 ohms, assume it's around 98 degrees and therefore keep heating. Meanwhile the temperature sensor will continue producing lower and lower resistance values. When the sum of the output of the sensor and your 2k ohm resistor is 16149 ohms the spa will feel it's reached it's 104F limit.
Do this with caution because you want to ensure you're not heating it about 104F, but this is a way of re-calibrating a sensor that is working consistently but that is off calibration.
