Callibrating RTD Sensors

In our power plants, many RTD sensors are used to monitor temperature in several locations. At least twelve sensors here. Each of them delivers temperature reading to CRT (yeah, it's too old but it's ok :D). The connection diagram is looked like this picture below.



We use Pt50 and I think it should be change to Pt100 since Pt50 is not ordinary used by now. Well, our power plants has a 20th century's technology. Actually, the cable from sensors to transducer is long enough (more than 10 meters) so it has a compensation of transducer reading in ohms.

To callibrate the system, we should have the following devices :
- Ohm meter and ampere meter, or AVO-meter to check the resistance and output current
- Precision sliding resistor to give nominal resistance we need
- Transducer holder, it usually comes with its transducer's box
- Some cable for wiring
- Power supply to energize the transducer
- Trimmer to trim span and zero of the transducer

Now we're ready to callibrate. First of all, don't forget to make a resistance table consists of resistance versus temperature reading of Pt50. I use the Callendar-Van Dusen equation below :

R(t) = R(0) [1 + At + Bt2]

where A = 3.9083.10-3 °C-1, B = -5.775.10-7 °C-2, R(0) = 50 Ω, and t = temperature in °C. The result is shown in table below :



From the transducer label, we can find many useful information i.e. pin connection number of the holder, the source that is used, input, and output of transducer. For CGP-2M (temperature transducer for Pt50) we can see that it has 0 - 150 °C (pin 3-4-5) correspond to 4 - 20 mA DC output (pin 1(+) and 2 (-)) and AC 100-110 V (pin 7-8). For our need, pin 4-5 can be shorted since pin 5 is used for compensation (it's relatively too small). So the connection diagram will look like this :



Determine the ZERO
Set the sliding resistor to have a resistance of 50 Ω as close as possible. Check it by a precision ohm meter on end point of cable. The cable also have a resistance itself. So 50 Ω is summation of sliding resistor and cables. Connect the cable from sliding rhesistor to transducer holder. Turn on the power supply and read the transducer output. It shold be read 4.00 mA. If it is not, turn the zero trimpot right or left carefully to get 4.00 mA reading. Congratulation, you have set the zero point for transducer.

Determine SPAN
Turn off power supply and unplug transducer to release resistance cable. Set the resistance to 78.66 Ω correspond to 150 °C which gives 20.00 mA output of transducer. Repeat the same procedure as determining zero. The difference is now amperemeter reading should be 20.00 mA. Turn span trimpot right or left to get 20.00 reading.

You can check the transducer output using varied resistance to see it has a linear output or not. If it is not, find the closest linear reading of transducer to your daily reading. So if it is used to monitor a daily temperature of bearing around 50 - 60 °C, make sure the transducer reading in this range is linear. But, you must make a note that this transducer should be retired off soon.

Checking the system
We have callibrate all transducer and the result is good. Now we have to check whole temperature system. It means from sensor that placed far away to the nominal shown in CRT. We need a good temperature callibrator to give a certain heat so that the sensor read the same or quiet same temperature all the time. For example, we use Ametek Jofra - Temperature Callibrator.

First of all, place the sensor, cable, and transducer in right place. After setting the temperature to Ametek - where there is sensor in it - we have to wait until the temperature shown in Ametek is steady. We can use other thermometer to compare the temperature in Ametek. Just to make sure.

When temperature is steadily set, read the nominal in CRT. It's usually different between the actual and the nominal. It can be considered since we have more than 10 meters cable connecting sensor to transducer which give certain resistance donated to total resistance reading of transducer. In earlier step we've sure that our transducer is linear. So all we can do is turn the trimpot for span to equalize the actual temperature and nominal reading in CRT. It is ok since the acceptable error reading is 2 °C.