The primary differences between the two thermocouples are:

　　Measuring Range...

　　K Type Thermocouple = -330°F to +2460°F (-200°C to +1250°C)

　　J Type Thermocouple = -40°F to +1382°F (-40°C to +750°C)

　　Sensitivity...

　　K Type Thermocouple = approximately 41 µV/°C

　　J Type Thermocouple = approximately 55 µV/°C

　　If you already own a gauge it is important that you use the correct thermocouple sensor type, otherwise the result will be incorrect CHT temperature readings due to mismatched sensitivities.