Calibrating a temperature transmitter equipped with an RTD sensor:
Temperature transmitters equipped with an RTD are usually calibrated at the manufacturer’s factory on the temperature range engraved on the name plate of the device. When the function of the temperature transmitter is disturbed and requires recalibration, the transmitter is usually calibrated using the Resistance Decade box.
To calibrate an RTD transmitter, the following equipment is needed:
- A digital voltmeter with proper accuracy and very high resolution (preferable with 1 mv resolution)
- A 24 VDC power supply
- A Resistance Decade box with five high precision dials that allow 100Ω steps.
The calibration steps of an RTD transmitter using a Resistance Decade box are as follows:
First, connect the above equipment to each other.
- Locate the transmitter terminals by removing the body kit.
- If an RTD is already connected to the transmitter terminals, disconnect all the RTD wires from under the terminals.
- Connect the resistance decade box leads to the transmitter terminals.
- According to the desired measurement range, determine the RTD resistance in LRV and URV using the table or formula.
- Turn on the power supply.
- Set the Resistance Decade box to a resistance that is equivalent to the resistance of the RTD at LRV temperature. Adjust the Zero potentiometer so that the output is 4 mA.
- Set the Resistance Decade box to a resistance that is equivalent to the resistance of the RTD at the URV temperature. Adjust the span potentiometer so that the output is 20 mA.
- Repeat steps 5 and 6 until the output current value is 4 mA in LRV and 20 mA in URV, and there is no need to adjust the potentiometers.
The following formula can be used to calculate PT100 resistance at different temperatures:
R T =R o (1+AT+BT 2 +C(T-100)T 3 )
C=-4.183*10 -12 below 0 0 C, and zero above 0 0 C
RT = RTD resistance at temperature T
RO= RTD resistance at 0 0 temperature C (for example, if our sensor is PT100, RO will be equal to 100)
T = temperature
A = coefficient of thermal resistance (it is the same as a)
In the list below, you can see different values of a in different standards:
Common standards for platinum RTDs*
Organization Standard ALPHA(a): Average temperature nominal resistance
British standard bs1904:1984 0.003850 100
Deutschenlnstitut fur DIN 43760:1980 0.003850 100
International Electrotechnical IEC 751:1995 0.00385055 100
Commission (Amend. 2)
Scientific Apparatus SAMA RC-4-1966 0.003923 98.129
Manufacturers of America
Japanese Standard jis c1604-1981 0.003916 100
American Society for Tasting
And Materials ASTM E1137 0.00385055 100
* Sensing Devices, lnc. Manufactures platinum RTDs to these thermometry standards.
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