NTC thermistor / sensor question

Thread Starter

MattBuzzBurbank

Joined Nov 14, 2019
21
Hello, in my espresso machine repair business I frequently replace 10K NTC boiler sensors on small automatic coffee machines. These are LUG 5M SENSORS and they go bad quite OFTEN, probably because the boilers regularly exceed 150 degrees C when steaming, which for most of these sensors is their upper capacity. So I have been using used OEM sensors for this, but even used are hard to get, and new are too expensive so I went to Digikey to see what they have and they DO have Vishay Lug 5M bolt on sensors - NTCALUG54A M5 to be exact however the temperature curve is significantly different. At boiling 100C the OEM ones measure .7K ohms and the Vishay are like 1.1K Ohms. However...the VISHAY seem to work, the Espresso machine with the VIshay sensor creates either hot water or steam as requested and does NOT put up any error message (like it would for a failed sensor) but does this mean we are good to go here? I do not see any other lug sensors with different temperature curves to even choose from. The boilers are protected with thermal fuses so there's no serious hazard if something goes wrong .......but again.......many questions abound. Will they go bad sooner? Perhaps work just fine? I have no idea. The limit of my electronics knowledge is basic use of a multimeter to measure things only.
The interesting question here is HOW do electronic boards determine exactly WHAT the termperature is with an NTC sensor? Do they correlate the exact resistance to some embedded database table? Or is it some general scale times some multiplier? Hmmmm. Ideally, I would want the replacement NTC sensor to measure .7K ohms at 100C! Is this NTC sensor available on Digikey? I do not know how to search for it! I am not about to construct a temperature curve for my OEM sensor to compare! All responses appreciated.
 

MrChips

Joined Oct 2, 2009
25,919
I think it strange that the thermistors would frequently go bad. We need to look as to why this is occurring.

In any case, thermistors are fabricated and characterized by their resistance vs temperature characteristic curves. These are very well established and documented. Once the curve is selected the thermistor is characterized by its resistance at 25°C.

A 10kΩ thermistor is a very common choice.

How a circuit makes use of the thermistor is entirely up to the circuit designer. There are many different schemes being used. The fundamental premise is that whatever parameter is being acquired is a function of resistance. That parameter might be voltage, time, frequency, or simply a trigger logic signal. As the temperature changes so does the resistance, of course. This translates to a changing parameter which could be analog or digital.

The basic thermistor resistance varies non-linearly with temperature. There are circuit schemes that attempt to make the recorded parameter linear with temperature. In lieu of this, the system internals, whether it is analog or digital may need to transform the data to real temperature. Again, there are multiple ways of doing this. One straight forward method is to use a look-up table. A second method is to implement a transformation formula using known parameters derived from the characteristic curve.
 

Thread Starter

MattBuzzBurbank

Joined Nov 14, 2019
21

MrChips

Joined Oct 2, 2009
25,919
I have not looked at the data but the 10kΩ thermistor suggested by the other member is 0.7kΩ at 100°C, according to the member. Isn't this your criteria?
 

Thread Starter

MattBuzzBurbank

Joined Nov 14, 2019
21
I have not looked at the data but the 10kΩ thermistor suggested by the other member is 0.7kΩ at 100°C, according to the member. Isn't this your criteria?
Yes, absolutely! But as mentioned above I directly measured it in boiling water and it clocked in at 1.1K Ohms! Is that a valid test?
 
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