Using 10k Thermistors as Temperature Sensors

[neillt]

OK guys, I have been scouring the Google and not come up with an answer on this.
 
I know the OmniStat remote sensors are 10k thermistors.  But what about the old "Standard range" temperature sensors (part 31A00-1) that go to the HAI panels themselves?  Are they also just 10k thermistors?
 
The main reason I ask is that I have a domestic hot water solar collector that has an ancient (70s vintage) Robertshaw differential temperature controller to turn the heat exchanger pump on and off.  The sensors in the hot water tank and up in the collector panel are 10k thermistors.
 
I would rather get rid of the controller that has been wonky and just let the Omni record the temperatures in the tank and the collector... but I can only do that if I can get the existing sensors fed in to it.
 
Does anyone know what the original HAI sensors were doing?

 

[ano]

I think you are confusing things a bit here. The temp. sensors that hook to an HAI zone input are not just thermistors. There are two different kinds, the "standard" kind and the "extended range."  The standard type returns a voltage proportional to the temperature. The "extended range" type returns a digital signal that varies with the temperature.  Each of these sensors contains an adjustment to calibrate them.
 
So no, you likely can't just feed some thermistor into your panel and expect it to read back some temperature with any accuracy.  You may be able to buy a Leviton extended range temp. sensor, 31A00-7, and remove THAT thermistor to use as a sensor. That can measure -40 to 120F.  They also make this 14A00-8 which is a standard temp. sensor designed to measure water temps., 0-120F.  It sounds like you want to use YOUR thermistor with the 14A00-8, but its very unlikely you'll get it to be very accurate.  Also, a heat exchanger likely produces a temperature range much wider than 0-120F.  
 
So what is the "standard" temp sensor doing?  Its returning a voltage of, I THINK, 0 to 5 volts based on a temperature of 0 to 120F.
Look at this: http://www.electronics-tutorials.ws/io/io_3.html
Then look under the section "Temperature Sensors Example No1" 
 
You need to design a circuit over the range YOU want to measure.  If its not 0-120F, then you will need to build a conversion table. For example, Omni Pro II reads "75 degrees" my temp is actually "250 degrees."
 
 
 
 

 

 

[neillt]

Yeah, that's the missing piece of the puzzle, it's voltage it's going off of, not resistance.  I was playing with it today with a couple of different resistors and a potentiometer to see if I could get anything meaningful.  But the panel just says "NOT RDY" and won't even show a temp.  Of course, then I looked in the installer manual and the temp sensors only go to the positive side, which should have been a bigger clue as well.
 
I knew the extended range ones are pulse width (which is also why you can't use them on an expansion enclosure, but that's a whole different rant)... but I had hoped the non-extended range ones were something simpler.
 
Welp, short story is I am giving up on this pipe dream and I will have to figure out something else.

 

[ano]

Each zone has a primitive A/D converter which reads the voltage and converts it to a digital 8 bit number. So that gives you 256 voltage levels. (And you can read these levels with PC Access.)  So for 0 to 120F, that gives about 1/2 degree resolution.  I think it goes from 0V to 5V, so the "temp" would be (voltage * 24) degrees F.

 

[pete_c]

The sensors in the hot water tank and up in the collector panel are 10k thermistors.
 
Curious whether you can purchase the same type of extended thermocouple for the HAI temperature sensor that would fit in place of the 10k thermistors? 

 

[ano]

The sensors in the hot water tank and up in the collector panel are 10k thermistors.
 
Curious whether you can purchase the same type of extended thermocouple for the HAI temperature sensor that would fit in place of the 10k thermistors? 

So a "thermocouple" and a "thermistor" are different. They are not interchangeable. Thermocouples produce a small bit of voltage based on the temp. The great part about them, is with a certain type of thermocouple, this voltage is standardized.  Like a "J type" thermocouple will ALWAYS put out a certain voltage at a certain temp. from 32 to 1382°F. 
 
A thermistor is a negative resistance device, and can vary from one to the next.  You may have two that are both 10K at 70 degrees, but they could be completely different resistances at 120 degrees, maybe 5K, maybe 500 ohms.
 
If you DID have a certain type of thermocouple in this heatpump, then you could accurately read the temperature, but with thermistors, its not so simple.

 

[a0128958]

... The standard type returns a voltage proportional to the temperature. ...

 
I don't think this is correct.  These sensors convert a temperature to a frequency.  I.e., the HAI temp sensors are variable frequency generators, with frequency a function of temperature.
 
Interestingly, the design hasn't changed for a standard temp sensor.  The HAI units (and PESMs) I used in the 90s remain compatible with my OP2 today.  Most HAI zone inputs remain capable of reading the variable frequency generated by the temp units.
 
I don't believe there's any A/D work going on inside an HAI panel.  It's all done at the sensor.  Inside the panel it's a frequency counting exercise.
 
Best regards,
 
Bill

 

[pete_c]

The sensors in the hot water tank and up in the collector panel are 10k thermistors.
 
Remove the sensors and install an HAI temperature probe or put a sleeve on the HAI temperature sensors so that they fit where the 10k thermistors are at.  Three wires on it and I am guessing two are for 12VDC.  You can just use a voltmeter on a 14A00-8 to check for yourself.
 
What I was writing above was to make the temperature probe on the HAI temperature sensor fit your scenario or see about purchasing a similar in function temperature probe that you can install in your hot water tank?
 
Maybe there is a part number on the temperature probe.  I have regular temperature and temperature combos here but have never purchased an HAI model 14A00-8 temperature sensor with a probe connection.
 

 

[ano]

I don't believe there's any A/D work going on inside an HAI panel.  It's all done at the sensor.  Inside the panel it's a frequency counting exercise.

Hmmm. So you put a 1K resister across a zone, then go to PC Access and read the zone value and its "147" so your telling me the resistor is generating a "frequency?" If not, where does the "147" come from?
 

The sensors in the hot water tank and up in the collector panel are 10k thermistors.
 
Remove the sensors and install an HAI temperature probe or put a sleeve on the HAI temperature sensors so that they fit where the 10k thermistors are at.  Three wires on it and I am guessing two are for 12VDC.  You can just use a voltmeter on a 14A00-8 to check for yourself.
 
What I was writing above was to make the temperature probe on the HAI temperature sensor fit your scenario or see about purchasing a similar in function temperature probe that you can install in your hot water tank?
 
Maybe there is a part number on the temperature probe.  I have regular temperature and temperature combos here but have never purchased an HAI model 14A00-8 temperature sensor with a probe connection.
 

temperature probe.png

It sounded from the OP that the 10K thermistors he had in place couldn't be removed. If that wasn't a problem, the 14A00-8 could work, but its temp range is fixed at 0 - 120F.  Now you could use the 10K that was installed and fix it to the 14A00-8. That would work, but you would have to determine a mapping of OMNI read temp vs. real temp.  Its worth a try.  Any idea what temp range you need to measure in the water tank and solar panels? You might get lucky where the temp you read is close enough. I think the 14A00-8 does allow some adjustment.

 

[neillt]

Yeah guys, I can't change out the 10k thermistors, they are deep inside the service tank and the solar collector.

 

[cobra]

Industrial temperature measurement has all kinds of converters available, but it'll probably be a little expensive to do. For example, you can get a 10k thermistor to 4-20mA converter in the $100-250 range. There would be some more work to interface it to an HAI temp board. Probably easier to use something like that with RP or other hub for input to Omni.

An example device is like this one: http://www.datexel.com/Dual-Thermistor-Temperature-Converter-DAT4532C.html