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Resistance thermometers or thermistors, what you need to know

Temperature measurement. Thermal resistance

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

1. NSC - the most important characteristic of the vehicle!

NSC (nominal static characteristic) is a function or table of values that defines the relationship "resistance - temperature».
1.jpg

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


2.jpg

2. What resistance to choose: 50 om, 100 om or 1000 om?

The main principle of operation of the TC 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 RTD with a higher resistance than the resistance of the communication line. Therefore, 1000 om is best.

NB! Many devices cannot work with this kind of NSH, so 50 om and 100 om are the standard.
In the past, 50 om sensors were the standard because they used less material to manufacture than 100 om sensors.

3.jpg

3. Which is better: copper or platinum?

The temperature measurement range depends on the material of the sensitive element (SE)

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

Copper is cheaper, platinum is more expensive.

For the correct choice of sensor in GOST R 6651-2009 there is a table depending on the accuracy class on the material and measurement range.

Диапазоны измерений

Полезный совет

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 made using wire technology. These vehicles operate in a wider range, but the sensor itself is quite bulky.
• Pt100 have thin-film sensing elements. A thin layer of metal is sprayed onto the ceramic substrate, which forms a conductive path, 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 you to make 500- and 1000-om sensors.

Виды намоток

Полезный совет

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

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

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


Класс термосопротивлений

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 terms of noise immunity:
• The current signal is more noise-resistant than the "resistance" signal.
• A digital signal, such as RS-485, is more resistant to interference than current.

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

Стоимость

The author of this article is the company OWEN: https://owen.ru/forum/showthread.php?t=31526


#Thermalresistance, #TS, #thermometer, #resistance, #basicdesign, #temperature, #measurements

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