Pt100 temperature sensor – useful things to know
RTD temperature sensors
As the Pt100 is an RTD sensor, let’s look first at what an RTD sensor is.
The abbreviation RTD is short for “Resistance Temperature Detector.” So it is a temperature sensor in which the resistance depends on temperature; when the temperature changes, the sensor’s resistance changes. So, by measuring the RTD sensor’s resistance, an RTD sensor can be used to measure temperature.
RTD sensors are most commonly made from platinum, copper, nickel alloys, or various metal oxides and the Pt100 is one of the most common.
PRT temperature sensors
Platinum is the most common material for RTD sensors. Platinum has a reliable, repeatable, and linear temperature-resistance relationship. RTD sensors made of platinum are called PRT, “Platinum Resistance Thermometer.” The most common PRT sensor used in the process industry is the Pt100 sensor. The number “100” in the name indicates that it has a resistance of 100 ohms at 0°C (32°F) temperature. More details on that later.
PRTs versus thermocouples
Thermocouples:
Can be used to measure much higher temperatures
Very robust
Inexpensive
Self-powered, does not need external excitation
Not very accurate
Requires cold junction compensation
Extension wires must be made of suitable material for the thermocouple type
Attention must be paid to temperature homogeneity across all junctions in the measurement circuit
Inhomogeneities in wires may cause unexpected errors
PRTs:
More accurate, linear, and stable than thermocouples
Do not require cold junction compensation
Extension wires can be made of copper
More expensive than thermocouples
Need a known excitation current suitable for the sensor type
More fragile
In short, thermocouples are more suitable for high-temperature applications and PRTs for applications that require better accuracy.
Measuring an RTD/PRT sensor
Since an RTD sensor’s resistance changes when the temperature changes, it is pretty clear that when measuring the RTD sensor you need to measure resistance. You can measure the resistance in Ohms and then convert that manually into a temperature measurement according to the conversion table (or formula) of the RTD type being used.
More commonly nowadays, you use a temperature measurement device or calibrator that automatically converts the measured resistance into a temperature reading. This requires the correct RTD type to be selected in the device (assuming it supports the RTD type used). If the wrong RTD sensor type is selected, it will result in incorrect temperature measurement results.
There are different ways to measure resistance. You can use a 2, 3, or 4 wire connection. The 2-wire connection is only suitable for very low-accuracy measurement (mainly troubleshooting) because any wire resistance or connection resistance will introduce errors in the measurement.
Sure for some high-impedance thermistors, Pt1000 sensors, or other high-impedance sensors, the additional error caused by the 2-wire measurement may not be too significant.
Any normal process measurement should be done using 3 or 4 wire measurements.
For example, the IEC 60751 standard specifies that any sensor with higher than class B accuracy must be measured using a 3 or 4-wire measurement. More on the accuracy classes later in this article.
Just remember to use a 3 or 4-wire measurement and you are good to go.
Measurement current
As explained in the above-linked blog post in more detail, when a device is measuring resistance it sends a small accurate current through the resistor and then measures the voltage drop generated over it. The resistance can then be calculated by dividing the voltage drop by the current according to Ohm’s law (R=U/I).
Self-heating
When the measurement current goes through the RTD sensor, it also causes the RTD sensor to warm slightly. This phenomenon is called self-heating. The higher the measurement current and the longer it is on, the more the sensor will warm. The sensor’s structure and its thermal resistance to its surroundings will also have a big effect on the self-heating. This kind of self-heating in a temperature sensor will cause a small measurement error.
The measurement current is typically a max of 1 mA when measuring a Pt100 sensor, but it can be as low as 100 µA or even lower. According to standards such as IEC 60751, self-heating must not exceed 25% of the sensor’s tolerance specification.
