PT100 temperature probe: Our buying advice
What is the PT 100 probe used for?
Pt100 sensors are the most common type of platinum resistance thermometer. Resistance thermometers are mostly Pt100, Pt500 or Pt1000. The term "Pt" refers to the fact that the sensor is made of platinum. 100 means that at 0 ° C, the sensor has a resistance of 100 ohms (Ω).
A resistance thermometer is a type of temperature sensor. It is composed of an element whose resistance varies according to the evolution of the temperature. Common names for resistance thermometers include RTD (short for resistance temperature detector), RT, Pt100, Pt500, Pt1000.
A Pt100 element consists of a coil of wire or a deposited film of pure metal. The resistance of the element increases with temperature in a known and repeatable manner. The Pt100 have excellent accuracy over a wide temperature range.
- Temperature range: -200 to 700ºC
- Sensitivity: the voltage drop across an RTD provides a much larger output than a thermocouple.
- Linearity: Platinum and copper RTDs produce a more linear response than thermocouples or thermistors. RTD nonlinearities can be corrected through the proper design of resistive bridge networks.
The most commonly used material is platinum with a resistance of 100 ohms at 0ºC and a temperature coefficient (Alpha) of 0,00385 ohms / ºC. Other elementary materials also used are copper, nickel and nickel- iron. The elements of platinum are predominant because of their wider range, and because platinum is the most repeatable and stable of all metals.
The characteristics of PT100 probe
Tolerance of PT100; (Alpha = 0.003850 @ 0ºC)
|CLASS B||± 0,12; OR ± 0,30ºC|
|CLASS A||± 0.06; OR ± 0.15ºC|
|1/3 B (1/3 DIN)||± 0,04; OR ± 0,10ºC|
|1/10 B (1/10 DIN)||± 0.012; OR ± 0.03ºC|
The linearization equation of a PT100 probe
Rt = R0 * (1 + A * t + B * t2 + C * (t-100) * t3)
Rt is the temperature resistance t
R0 is resistance at 0 ° C
A = 3,9083 E-3 B = -5,775 E-7 C = -4,183 E-12 (below 0 ° C) or C = 0 (above 0 ° C)
For a PT100 sensor, a temperature change of 1 ° C will cause a resistance change of 0,384 ohm. Even a small error in resistance measurement (for example, the resistance of the wires leading to the sensor) can cause a large error in the temperature measurement. For precision work, the sensors have four wires: two to carry the detection current and two to measure the voltage across the sensor element. It is also possible to get three-wire sensors, although these work on the assumption (not necessarily valid) that the resistance of each of the three wires is the same.
Connection / Wiring details:
Different types of connection. Standard color code; A is white, B is red.
|2 Yarn : Basic connection where the conductor is short. No compensation wire.||3 wires: The most common with 3 connection wires, the instrument measures the resistance of wire B and deduces it from its measurement.||4 Yarn : The 4-wire connection is the most accurate measurement. The instrument measures the resistance of the four conductor wires and deduces it from its measurement.||Double Pt100 : Double 3-wire RTD connection with two different sensitive elements.|
No maintenance is required for RTD sensors, however, scheduled ice point (0ºC) calibration checks are recommended.
Method for determining the resistance to the ice point (0 ° C)
Prepare an insulated containerat least 300 mm deep and with an internal diameter of 100 mm.
The procedure should be as follows:
(a) Fill the insulated container with finely divided ice made from distilled water
Note: If the chilled water of distilled water is not available, the transparent part of a block of commercial ice will suffice, provided that all surfaces are first washed with distilled water.
b) Mix the ice with distilled water previously cooled using the stirrer, then drain the excess water. The ice should be glassy but there should be no remaining open water.
c) Connect the thermometer to a suitable resistance measuring device and adjust so that the electric power dissipated in the element does not exceed 1 m W.
d) Immerse the thermometer in ice so that the element is at a depth of at least 150 mm. Make sure the bottom of the thermometer is at least 30mm from the bottom of the container. Note: Thermometers with stem lengths less than 150mm should be submerged to their maximum possible depth.
e) When the element reaches equilibrium with the ice, action can be taken. Measurements made with direct current should be made with current in both forward and reverse directions. Note: The time it takes for the element to reach equilibrium is normally about 3 minutes.
f) Decrease the immersion depth of the element by 50 mm or 20% of the length of the rod, whichever is less.
g) Repeat step (e). If the change in reading is more than one-third of the appropriate tolerance, the entire procedure should be repeated with fresh ice.
Precision: RTD PT100 sensors; at 0 ºC = Class B +/- 0,3 ºC, Class A +/- 0,15 ºC, 1/10 DIN = +/- 0,03 ºC
Risk of self-heating of the Pt100
The current through the sensor will cause heating: for example, a sense current of 1 mA through a 100 ohm resistor will generate 100 μW of heat. If the sensing element fails to dissipate this heat, it will report an artificially high temperature. This effect can be reduced either by using a large sensor element or by ensuring that it is in thermal contact with its surroundings.
Using a sense current of 1 mA will give a signal of only 100 mV. Because the change in resistance for one degree Celsius is very small, even a small error in measuring the voltage across the sensor will produce a large error in measuring the temperature.
For example, a voltage measurement error of 100 μV will give an error of 0,4 ° C in the temperature reading. Likewise, an error of 1 μA in the detection current will give a temperature error of 0,4 ° C.
Due to the low signal levels, it is important to keep cables away from electrical cables, motors, equipment and other devices that may emit electrical noise.
Using a shielded cable with the screen grounded at one end can help reduce interference. When using long cables, it is necessary to verify that the measuring equipment is able to withstand the resistance of the cables.
More information on the values of the Pt100 probes below!
|Precision class||R / T conversion table|