Connecting Themocouple to Plc Via Isolation Amplifier

replytosun

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i want to connect Thermocouple to Plc through Isolation Amplifier.
My purpose is to control /monitor Heater temprature.I want heater to stop reaching 750 F Temp.
Component detail are.

1)Thermocouple Range= 0-2192 F (Type K)
(0-1200 C)
VOLTAGE OUTPUT =0-48.875 mv

2) isolation amplifier
Input 0-500 mv DC
output 0-10 VDC
how i need to configure Zero and offset on Isolation Amplfier.

3) Input card is
1746-NIB CARD (AB)
how to configure SCP ( input max,output max,ScaledMin, Scaledmax)instruction in Ladder Logic

Thx in Advance
 
welcoe to the site
Why dont you have a Thermocouple (temperature) Card for the PLC
instead.
 
I would look at using an isolation amplifier that is designed to take a thermocouple input, and I would look at using one with a 4-20mA output, assuming that your input card can take a current rather than voltage signal. Using a Voltage in and out is making work for yourself. Also the terminals on the voltage/voltage amplifier won't be made of the correct metals so you will get interference or even a failed system.

Examples of Thermocouple to 4-20mA converters:

http://www.status.co.uk/product/32_programmable-pt100,-thermocouple,-mv-and-passive-ma.html

http://www.status.co.uk/product/28_...-din-rail-4-20ma-temperature-transmitter.html
 
Thx for quick reply.I have system already with hardware mention in my first post i.e thermocouple with mv output.My requirement is my Heater should stop at 750 F .Temprature, in betweem temprature 0 to 760 F is insignificant.
 
Thx BraynG for quick reply.
my input card is 1746-NI16V ANALOG VOLTAGE INPUT CARD -CLASS-3.which take volatge as input not current.
 
Your isolation amplifier idea has a big problem - it has no cold junction compensation. Measuring temperature with a thermocouple DEMANDS cold junction compensation.

Your temperature conversion using the isolation amp will be in error by the difference between the isolation amplifier's input terminal temperature and the freezing point of water.

The devices BryanG recommends are called temperature transmitters, designed to convert a thermocouple input to a 4-20mA output with cold junction compensation.

Your card is for voltage inputs, but all 4-20mA signals are converted to a voltage at the input card by the use of a resistor. For instance, a 250 ohm resistor develops a 1.0 to 5.0Vdc signal.

There are isolated transmitters and non-isolated transmitters. I always use isolated transmitters.

A temperature transmitter can provide cold junction compensation AND isolation.

Your isolation amplifier can not provide cold junction compensation.
 
1)Thermocouple Range= 0-2192 F (Type K) (0-1200 C)
VOLTAGE OUTPUT =0-48.875 mv
Click to expand...
That is an unusual range for a Type K thermocouple. A more usual range would have a negative minimum, such as a -330 °F to +2460 °F or −200 °C to +1350 °C range. (Simplified for a theoretical student problem, I wager)!

If your TC milivolt range is correct, then you could convert it to a 0-10 volt range with this formula:

Slope = (10-0)/(48.875-0) = 0.204604
Transmitter Offset or Zero = 0 - (0 x 0.204604) = 0

Transmitter Output = Milivolt Input X 0.204604 + 0
Test: When Milivolt Input = 48.875, then Transmitter Out = 48.875 X 0.204604 = 10 volts
3) Input card is 1746-NI8 CARD (AB)
How to configure SCP (Input min, Input max, Scaled Min, Scaled max)instruction in Ladder Logic?
Click to expand...
Your 1746-NI8 module has several modes. If you use the Engineering Units mode, then its output counts range for 0 to 10 volts will be 0 to 10000 counts. (On the other hand, if your 1746-NI8 module is configured for Scaled-for-PID output, then its voltage count range is 0 to 16383. Check your 1746-NI8 User Manual - link below).
http://search.rockwellautomation.co...&entqrm=2&ud=1&exclude_apps=1&site=literature

Therefore with the Engineering Units configuration:
Input Min = 0 volts (or 0 counts)
Input Max = 10 volts (or 10000 counts)
Scaled Min = 0 degrees F
Scaled Max = 2192 degrees F

Slope = (scaled max - scaled min) / (input max - input min)
= 2192/10000 = 0.2192
Offset or Zero = scaled min - (input min X slope)
= 0 - (0 x 0.2192) = 0

Test: Scaled value = input value X slope + offset = 10000 x 0.2192 = 2192 degrees F.

Test: Scaled value = input value X slope + offset = 3422 x 0.2192 = 750.1 degrees F.

Here is a scaling spreadsheet that might be useful.
 
Lancie1 said:
That is an unusual range for a Type K thermocouple. A more usual range would have a negative minimum, such as a -330 °F to +2460 °F or −200 °C to +1350 °C range. (Simplified for a theoretical student problem, I wager)!

If your TC milivolt range is correct, then you could convert it to a 0-10 volt range with this formula:

Slope = (10-0)/(48.875-0) = 0.204604
Transmitter Offset or Zero = 0 - (0 x 0.204604) = 0

Transmitter Output = Milivolt Input X 0.204604 + 0
Test: When Milivolt Input = 48.875, then Transmitter Out = 48.875 X 0.204604 = 10 volts

Your 1746-NI8 module has several modes. If you use the Engineering Units mode, then its output counts range for 0 to 10 volts will be 0 to 10000 counts. (On the other hand, if your 1746-NI8 module is configured for Scaled-for-PID output, then its voltage count range is 0 to 16383. Check your 1746-NI8 User Manual - link below).
http://search.rockwellautomation.co...&entqrm=2&ud=1&exclude_apps=1&site=literature

Therefore with the Engineering Units configuration:
Input Min = 0 volts (or 0 counts)
Input Max = 10 volts (or 10000 counts)
Scaled Min = 0 degrees F
Scaled Max = 2192 degrees F

Slope = (scaled max - scaled min) / (input max - input min)
= 2192/10000 = 0.2192
Offset or Zero = scaled min - (input min X slope)
= 0 - (0 x 0.2192) = 0

Test: Scaled value = input value X slope + offset = 10000 x 0.2192 = 2192 degrees F.

Test: Scaled value = input value X slope + offset = 3422 x 0.2192 = 750.1 degrees F.

Here is a scaling spreadsheet that might be useful.
Click to expand...

Unless I am misunderstanding what you are doing, the problem is that T/Cs are not linear so you can't just apply slope and offset.

Also, it seems like the OP is needing a High Limit Cutoff. I don't know what the requirement in India is, but using a PLC won't fly in the United States. You need a dedicated FM approved High Limit Controller.
 
Yes, you are correct about the linearity. The simple fix is look at the input signal count for the 750 degree heater stop condition and determine that count by observation, and use that point to cut off the heater at 750 degrees F. Then any concerns about the TC linearity go away.

Any high-limit safety device would have to be extra in series with the PLC control logic.
 
The signal from the isolation amp when converted according to thermocouple tables or a polynomial will read low, possibly 40-80°F low, depending on what the terminal block temperature is.

That's an error in the wrong direction for a high limit app.
 
your isolation is not correct type, but it is possible, just connect everything heat your sensor up and when limit is reached read the value on the PLC.
No need for any difficult linearization etc, CJC is in the amplifier.
 
Looking at the range (0 to 1200 degrees C), that happens to correspond to the NIST coefficient tables 2 and 3 for converting Type K thermocouple milivolt output to Degrees C for the range 0 to 1372. I think this is a student problem, and the PLC program is supposed to calculate the thermocouple temperature using the 20 NIST Type K coefficients. See Step 5 in this link for the NIST equations and coefficients. The milivolt input will need to be converted to 0-10 volts, but then the program must make two sets of calculations, based on input range, using the XPY (X to the Power of Y) advanced math function to calculate each of the 10 c*V coefficient X milivolt terms and then find the sum to get the Temperature in Degrees C.

http://www.home.agilent.com/upload/cmc_upload/All/5306OSKR-MXD-5501-040107_2.htm?&cc=US&lc=eng
 
Last edited:

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