# Resistance thermometers

Resistance thermometers (RT) are temperature sensors whose principle of operation is based on the change in electrical resistance from temperature. How to figure out which RT do you need? Today we will consider the main characteristics of the most commonly used thermal resistances.

1. NSC is the most important characteristic of the RT!

The Russian Federation has а National standard 6651-2009 "State system for ensuring the uniformity of measurements. Thermal converters of resistance from platinum, copper and nickel. General technical requirements and test methods" (The NS 6651-2009).

The NS 6651-2009 has a concept "Nominal Static Characteristic" (NSC). The NSC is a function or table of values that defines the resistance-temperature relationship.

The figure shows that the temperature measurement range depends on the NSC.
What will the NSX tell you about?
• Nominal resistance R0 (50 Ohm, 100 Ohm, 500 Ohm, 1000 Ohm)
• Sensing element material (copper, platinum)
• Temperature coefficient α,
where:
α=0,00428 ⁰С-1 – for 50М/100М
α=0,00391 ⁰С-1 – for 50P/100P
α=0,00385 ⁰С-1 – for Pt100/Pt500/Pt1000

2. Which resistance should I choose: 50 Ohm, 100 Ohm or 1000 Ohm?

The main principle of the RT is the change in resistance with temperature.
But the resistance of the communication lines from the sensor to the device is added to the resistance of the sensitive element. Based on this, it is better to use a RT with a higher resistance than the resistance of the communication line. Therefore, 1000 ohms is best.

Attention! Many devices cannot work with this type of NSС, so the standard is 50 ohms and 100 ohms.
Previously, 50-ohm sensors were standard because they required less material to manufacture than 100-ohm ones.

The best solution today is 100 ohm sensors, but it all depends on the needs.

3. Which is better: copper or platinum?

The temperature measurement range depends on the material of the sensing element

Copper: -50…+180 ⁰С
Platinum: -196…+500 ⁰С

Copper is cheaper, platinum is more expensive.

In the NS 6651-2009 there is a table of the dependence of the accuracy class on the material and the measuring range for the correct selection of the sensor.

ЧЭ - sensing element.

ТС - resistance thermometers.

Copper is better for measuring temperatures up to 150 ⁰С.
Platinum is used to measure high temperatures up to +500 ⁰С or low temperatures down to -196 ⁰С. At the same time, Pt100 is used for high temperatures, and 50P/100P for low temperatures.

4. What is the difference between 100P and Pt100?

There are the following technologies for the production of sensitive elements: winding / wire and thin-film.

• 100P, used by Russian manufacturers, are manufactured using wire technology. These RT operate in a wider range, but the sensor itself is rather bulky.
• Pt100 have thin-film sensing elements. A thin metal layer is sprayed onto the ceramic substrate, which forms a conductive track, the so-called meander. These sensing elements are small in size, which allows them to be used in models with a small diameter. Also, the production technology allows making 500- and 1000-ohm sensors.

Attention! 100P and Pt100 have different temperature coefficient α, therefore it will not work to replace one with the other without changing the settings of the secondary device / PLC.

5. How to choose a sensor with the required accuracy requirements?

The tolerance class determines the maximum permissible deviation from the nominal characteristic, and this deviation is set as a function of temperature - at zero degrees, the smallest permissible deviation is fixed, and with a decrease or increase in temperature, the range of permissible values increases linearly.
In the NS 6651-2009 there are 4 tolerance classes: AA, A, B, C.

The graph shows:
Tolerance class B is a general industrial version, and it is he who is the standard in the OWEN company.
Tolerance class A and AA - ordered for laboratory measurements.

6. Which output signal is better: resistance, current or digital?

The choice of output signal comes down to reliability and cost. Reliability is considered in the context of noise immunity:
• The current signal is more noise-immune than the resistance signal.
• A digital signal, for example, RS-485, is more noise-immune than a current one.

As for the cost, the cheapest is "resistance", current is average in price and "digital" is expensive.
If you buy a set of equipment, then sensors with RS-485 allow you to use cheaper PLCs, since there is no need for analog inputs.