Jon Klima, Owner
360 Trout Lane, PO Box 23, Guffey, CO 80820
Phone: 719 479-2281,  Email:
In business for 39 years


• Article 2 - May, 1990


Using 30 K ohm sensors with an IE C-30 controller©

A few days ago, I was asked why 30 KΩ sensors would not work with an IE C-30 controller. The truth is, they will, as long as certain things are accounted for. First of all, the C-30 is designed to be most accurate when the storage sensor (a 10 KΩ thermistor) temperature range is roughly between 60° F to 180° F. Within this range, the 20°/5° turn on/turn off temperature differentials are usually within one or two degrees of specified values. If 30 KΩ sensors are used, the controller is seeing much colder temperatures (higher resistances) and the accuracy of the turn on/turn off differential temperatures is less than when using 10 KΩ sensors. The big difference though is in the accuracy of the high limit setting. If the controller is adjusted to a high limit of, say 180° F, the controller is looking for a sensor resistance of 1168 Ω, at which point the controller turns its output off. If a 30 KΩ sensor were in use, the sensor would be at a temperature of about 248° F before the controller saw the 1168 Ω. You could however compensate for this by turning down the C-30 high limit adjustment. But the question is, how far? The trick here is to select a high temperature setting such that when the 30 KΩ storage sensor is at 180° F, the controller sensing circuitry actually interrupts the sensor as being at 180° F. At 180° F a 30 KΩ sensor resistance is about 3510 Ω. This means the controller high limit adjustment should be set up to turn the controller off when it detects a storage sensor resistance of 3510 Ω. This happens with a 10 KΩ sensor at a temperature of about 123° F. So the bottom line is that you would need to set the high limit adjustment at 123° F in order to achieve a 180° F high limit cutoff when using 30 KΩ sensors.


If for one reason or another, you must use 30 KΩ sensors with a C-30 controller, make sure you compensate for the high limit setting and above all, don't mix a 30 KΩ sensor with a 10 KΩ sensor. One more thing, you could also use 3 KΩ sensors with a C-30. Here again though, the high limit setting would be incorrect. How many of you can figure out what storage temperature the system would shut down at if the high limit were set at 180° F? Does the answer to this question give you any ideas as to a practical application?


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