Hot water temperature probe (thermistor or RTD)

[nikens]

Hi, I have a hot water boiler that comes with some electronic control unit, but this unit does not have switching for electric heater inside a boiler. So I am building my own control unit, where I would like to use temperature probes that are allready installed (4). The problem is that I don't know a thing about what kind of probes are there. Only think I can say is that it has two wires and metal housing of a probe is an inch long and it has 1/5 of inch diameter.
So can I measure the resistance to know what kind of probe this is? Does those probes have linear relation between temp and resistance?

 

[mvs sarma]

Hi, I have a hot water boiler that comes with some electronic control unit, but this unit does not have switching for electric heater inside a boiler. So I am building my own control unit, where I would like to use temperature probes that are allready installed (4). The problem is that I don't know a thing about what kind of probes are there. Only think I can say is that it has two wires and metal housing of a probe is an inch long and it has 1/5 of inch diameter.
So can I measure the resistance to know what kind of probe this is? Does those probes have linear relation between temp and resistance?

I assume that they used PT100 probe.

 

[FvM]

Thermistors are often used with this kind of applications, unlike RTD, they don't have standarized characteristics, unfortunately. By measuring the resistance at two known temperatures, you should be able to clarify the sensor type.

I posted some thermistor sensor data in a recent thread:
https://www.edaboard.com/threads/189527/

 

[pranam77]

Usually the one you mentioned may be simple J or K type with a bimetalic junction inside. You may use such simple sensors which will have a differencial output which can then be compared by a comparator and a output be taken.

 

[harimelath]

For boiler applications it is recommended to use RTD or PT100 sensors with high temp cables. Thermistors may not be suitable. PT100 probes for water heaters are readly available in the market, popular with the solar water heater suppliers.these sensors have four wire connections for accurate measurement of tempreature over a long distance probe.
thanks
Hari

 

[FvM]

For boiler applications it is recommended to use RTD or PT100 sensors with high temp cables. Thermistors may not be suitable.

How do you know? I have seen various products, that use thermistors.

 

[harimelath]

I am with this industry for the last 15years and we have experimented various sensors, PT100 was the best. I only shared my experiance......
not for an arguement...
Thanks
hari

 

[FvM]

PT100 was the best. I only shared my experiance

I completely agree. But the original poster was asking about the instrumentation of an existing device, not the best way to do it. Your statement sounds like you wanted to doubt, that thermistors are used for hot water equipment. They actually are, very frequently, no matter if RTD would be better.