plc
- Thread starter chakku
- Start date
leitmotif
Member
Couple things to look out for on thermistors I have learned from my readings and evaluations
1. Temperature resistance is inverse ie resistance goes down as temp goes up. RTD and Thermocouple are the opposite - voltage or resistance are proportional to temperature.
2. RTD and TC are built to universal standard so no matter who made it a particular unit (100 ohm RTD or K TC) will be the same. Apparently there is no such standard for thermistor so they may vary with the maker.
3. Thermistor have a linear portion but it is in a temperature band much narrower than that for RTD or TC - limits them to about 0 to 100 C.
The moral of the story is
make sure your PLC and the sensor match
Dan Bentler
1. Temperature resistance is inverse ie resistance goes down as temp goes up. RTD and Thermocouple are the opposite - voltage or resistance are proportional to temperature.
2. RTD and TC are built to universal standard so no matter who made it a particular unit (100 ohm RTD or K TC) will be the same. Apparently there is no such standard for thermistor so they may vary with the maker.
3. Thermistor have a linear portion but it is in a temperature band much narrower than that for RTD or TC - limits them to about 0 to 100 C.
The moral of the story is
make sure your PLC and the sensor match
Dan Bentler
Paul B
Member
Some thermistors I have worked with were digital. The signal was interfaced to a relay. The relay provided the plc with an input.
If yours are analog remember the thermistor is inside the motor windings and subject to high-voltage (motor voltage) if the motor fails. I'd plan on using a device to isolate the PLC from the thermistor.
If yours are analog remember the thermistor is inside the motor windings and subject to high-voltage (motor voltage) if the motor fails. I'd plan on using a device to isolate the PLC from the thermistor.
Why bother? Thermistors are crude, unreliable devices for temperature control. I never use them except as a 'switch' via a thermistor relay for overheated windings in motors.
I will also not use 2 wire RTDs - they are also inacurate.
Only ever use 3 wire RTDs or type 'k' thermocouples (unless temperature is really high of course).
Will not use type J thermocouples as the iron wire rusts too easily.
I will also not use 2 wire RTDs - they are also inacurate.
Only ever use 3 wire RTDs or type 'k' thermocouples (unless temperature is really high of course).
Will not use type J thermocouples as the iron wire rusts too easily.
danw
Lifetime Supporting Member
One problem you're likely to run into is discovering what the curve is for the temperature response of the thermistor. You have to know its resistance response to a change in temperature in order to make sense of what you input to the PLC. That relationship between the resistance and temperature, as mentioned previously, is not linear.
But maybe you'll figure that out.
Us old farts think alike. I echo BobB's sentiments, and would recommend using a thermocouple or RTD. But if you can't substitute a thermocouple or an RTD, and really need to use that thermistor, then you'll need an analog input card for your PLC that can read resistance over the range of your thermistor. No, none of us can tell you what the range is for your thermistor. There's no 'standard'. You can put an ohmmeter across it and see what resistance it has at whatever temperature it's at.
I suspect that the thermistor resistance is high enough that the resistance of the copper wire used for connecting is not a factor (unlike 100 ohm platinum 2 wire RTD's where it is a factor)
Or, if you can find a constant current source you could pump current through the thermistor and read the voltage drop across the thermistor with a dc voltage analog input card.
Or you can try to find a transmitter to convert to a conventional 4-20mA signal that is might or might not be linearized, which would require a 4-20mA analog input card for your PLC.
Cole Parmer sells a 'thermistor' transmitter, but check out the caveats:
"Thermistor Transmitter is compatible with 400 series thermistor probes for excellent accuracy over the ambient temperature range."
http://www.coleparmer.com/catalog/product_view.asp?sku=9477110&pfx=
Here's another thermisotor to 4-20ma transmitter, but be warned that it's for specific thermistor curves.
http://www.robertoweninc.com/produc...ng/4-20ma/resistance/thermistor/hardware.html
But maybe you'll figure that out.
Us old farts think alike. I echo BobB's sentiments, and would recommend using a thermocouple or RTD. But if you can't substitute a thermocouple or an RTD, and really need to use that thermistor, then you'll need an analog input card for your PLC that can read resistance over the range of your thermistor. No, none of us can tell you what the range is for your thermistor. There's no 'standard'. You can put an ohmmeter across it and see what resistance it has at whatever temperature it's at.
I suspect that the thermistor resistance is high enough that the resistance of the copper wire used for connecting is not a factor (unlike 100 ohm platinum 2 wire RTD's where it is a factor)
Or, if you can find a constant current source you could pump current through the thermistor and read the voltage drop across the thermistor with a dc voltage analog input card.
Or you can try to find a transmitter to convert to a conventional 4-20mA signal that is might or might not be linearized, which would require a 4-20mA analog input card for your PLC.
Cole Parmer sells a 'thermistor' transmitter, but check out the caveats:
"Thermistor Transmitter is compatible with 400 series thermistor probes for excellent accuracy over the ambient temperature range."
http://www.coleparmer.com/catalog/product_view.asp?sku=9477110&pfx=
Here's another thermisotor to 4-20ma transmitter, but be warned that it's for specific thermistor curves.
http://www.robertoweninc.com/produc...ng/4-20ma/resistance/thermistor/hardware.html
danw
Lifetime Supporting Member
Yeah, but for the next 'thermistor' post, I'll just reference this one.
A dissenting old phart here. IMHO thermistors give the best accuracy bang per buck in their applicable temperature range (up to maybe 150'C). Betatherm and others have "interchangeable" products that come in at down to 0.05'C accuracy out of the box. They are also dead set easy to interface if you can wire a resistor. At around 10K resistance lead resistance is generally not a problem.
What you do need is to do some math. Excel can work out the equation for you. I have a useful tutorial here complete with sample spreadsheet.
What you do need is to do some math. Excel can work out the equation for you. I have a useful tutorial here complete with sample spreadsheet.
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