There are different sensors for different spas, and you omitted any information on what you might have. But generally, the sensors are "thermistors", which change resistance depending on the temperature. The most common is a 10k ohm thermistor...which will have a resistance of 10,000 ohms at 77 degrees Fahrenheit. As the temperature increases, the resistance decreases. At 104 degrees, the resistance is 5,326 ohms. At 60 degrees, the resistance is 15,314 ohms
There are also 3k ohm sensors...which will have a resistance of 3,000 ohms at 77 degrees Fahrenheit.