There are many types of resistance thermometer, often called RTD’s or PRT’s and they all measure temperature through a change in electrical resistance of a length of wire. A variety of materials are used in these types of sensor including Copper, Nickel and Platinum. The vast majority however are Platinum based so we will cover these in more detail.
Platinum has been chosen as the standard material for resistance thermometers as it gives an extremely linear resistance change in relation to temperature and is very stable. It is therefore much easier to control accuracy over a wide temperature range. It also has the advantage of having a very wide operating temperature range.
What are the different types?
Most users of temperature probes will be familiar with the terminology Pt100, Pt500 or Pt1000. These are the standard values for this type of sensor, but what does it mean?
Pt is the chemical symbol for Platinum and denotes its use in the sensor. Other sensors may use Cu (copper) or Ni (Nickel) accordingly.
The number relates to the resistance value at 0°C. So a Pt100 will have a resistance of 100Ω at °C, and it follows that a Pt1000 will be 1000Ω at 0°C.
How does the sensor work?
Quite simply as the temperature in your application changes then the electrical resistance will also change. An increase in temperature will increase the resistance of the sensor.
The measurement is completed by your instrument by applying a small measurement voltage and utilising a bridge type circuit. You normally don’t need to worry about the electronics as long as the sensor is connected correctly.
Are RTD’s interchangeable?
Yes. All sensors manufactured by Process Parameters Ltd conform to the relevant British and International standards that apply. In the case of Platinum Resistance Thermometers we apply BS EN 60751 which covers the characteristics, accuracy and colour coding systems used. This means that you can take a Pt100 and replace it with another and it will simply work, there is no need for any configuration. In some applications there is a requirement for a procedural calibration check.
This also means that probes manufactured by other suppliers can be replaced by probes made by us. This can be extremely economic as OEM parts can often be expensive. We can usually supply like for like probes.
How accurate are RTD’s?
Part of the standard that we operate to governs accuracy. In theory PRT’s can be extremely accurate but you need to consider a few factors when thinking about accuracy.
First of all there are five accuracy bands; Class B, Class A, 1/3 DIN, 1/5 DIN and 1/10 DIN. Class B is the least accurate, 1/10 DIN is the most accurate. The table below gives accuracies for each tolerance band at different temperatures. Note that accuracy is best for all classes at 0°C and worsens for increasing and decreasing temperature.
The next thing to consider is the probe immersion. It is extremely important to ensure that your Pt100 temperature probe is completely immersed to ensure that accuracy is maintained. Incomplete immersion can introduce measurement errors due to stem conduction.
Finally, it is also important to consider the wiring system utilised in the sensor. For the lower accuracy sensors it is quite acceptable to use a 3 or even a 2 wire system but for the higher accuracy 1/5 DIN and 1/10 DIN types we strongly recommend a 4 wire system. See below for more information.
There is a myth that PRT’s are always more accurate than thermocouples but this is not necessarily the case. Take a Class B Pt100 operating at 300°C, it should be accurate to ±1.8°C. A type K thermocouple however will have an accuracy of ±1.5°C at the same temperature. There are other factors to consider but it should not be assumed that PRT’s are “best”.
What is meant by 2 wire, 3 wire and 4 wire?
The resistance change of a Pt100 sensor is just 0.385Ω per 1°C temperature change so it is easy to see that errors can be introduced fairly easily.
When using a Platinum Resistance Thermometer you clearly need to be able to connect it to your instrumentation which involves using a cable. The simplest way is to use a 2 core cable, but this introduces additional resistance into the measurement circuit and therefore potentially large measurement errors. In general therefore we sell the majority of our PRT’s with either a 3 wire or 4 wire configuration. The addition of the third and fourth wires allows your instrumentation to compensate for the resistance of the extension cabling and negate most or all of the effects of this ensuring you get the most accurate measurement.
Note that you can use copper based cables with PRT’s. You do not need special cables as with Thermocouples.
Wiring Colour Codes
BS EN 60751 also specified the colour coding used for the wiring of PRT’s and is extremely simple. See the chart below for wiring colour coding for each wiring system.
What temperature range are PRT’s suitable for?
Many measuring instruments will often quote a measuring range of up to 850°C but in reality this is extremely optimistic for an industrial PRT.
The maximum range we recommend for an industrial PRT is -200 to +650°C but this is very much dependent on the construction and materials used in the assembly. We can give advice on sensor assemblies should it be necessary.