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Pt100 Sensor Explained | Working Principles

Learn about the working principles of the Pt100 RTD and how it's used in industrial measurement.

Platinum 100, or Pt100, resistance temperature detectors are an important part of many process control installations.

Accurate and repeatable measurement of temperature is a requirement for many processes, including heating and cooling, chemical reactions, pasteurization, and many others.

In this article, we will:

– Introduce you to the working principles of a Platinum 100 resistance temperature sensor,

– Describe the physical properties of a Pt100 sensor that make it valuable for process control,

– Describe the ways Pt100 RTD sensors can be integrated into a measurement and control system.

What is a Pt100 sensor?

Resistance temperature detectors, or RTDs, are a class of sensors that change resistance when the temperature of the medium they are inserted into changes.

This change of resistance is proportional to temperature and varies in a somewhat linear fashion with temperature.

This means that as the temperature increases, the resistance of the RTD also increases. So, if we are able to measure the RTD’s resistance, we can determine the temperature. Why is this? It is due entirely to the physical properties of the material from which the RTD is constructed.

Resistance temperature detectors, or RTDs

While RTDs can be manufactured from many metals, including nickel and copper, platinum exhibits physical properties that make it ideal for use in RTD temperature sensors.

RTD material physical properties

Physical properties of platinum

1) Basic element

Let’s take a look at the physical properties of platinum. First, platinum is a basic element, with the chemical symbol Pt. That is the first part of the designation of the Pt100 RTD.

Platinum has a molecular weight of 195, which makes it a rather heavy metal with free electrons to make it a good conductor of electricity, although not as good as copper or silver.

Platinum linear fashion electrical resistance

2) Linear fashion

Platinum exhibits an electrical resistance that varies in a nearly linear fashion with temperature and has a resistance of exactly 100.00 ohms at zero degrees Celsius. This is where the second part of the designation Pt100 comes from.

Platinum linear fashion electrical resistance

3) Inert property

Another property of platinum that makes it highly valuable to temperature measurement is that it is quite inert. It does not react with other compounds to any great extent.

Platinum inert properties.

The alpha coefficient

So how much does the resistance of a platinum change with temperature? The purity of the platinum used affects the change in resistance when the temperature changes.

The most common Pt100 RTD used in industry is one that changes resistance at the rate of about 0.385 ohms for every degree Celsius rise in temperature.

Commonly used Pt100 RTD

We know the resistance of a Pt100 sensor at zero degrees Celsius is 100 ohms, so the resistance we would expect at 100 degrees Celsius would be 138.5 ohms.

Pt100 sensor at zero degrees

The 385 factor comes from the equation that approximates the resistance of an RTD based on its physical properties.

The equation relates the resistance of the RTD at the temperature being measured to the resistance at zero degrees Celsius. The coefficient alpha in this equation describes the rate of change of resistance with temperature.

RTD equation.

For the Pt100 RTD, we have been describing if we substitute the resistance values of the Pt100 RTD at zero and at 100 degrees Celsius, we find that the value of alpha is 0.00385.

Knowing alpha, we can calculate the approximate resistance the RTD will exhibit at any temperature within its range.

The Pt100 RTD is often referred to as the Pt100 (385) RTD. There are platinum RTD’s that exhibit different values of alpha, and those would be designated with their respective alpha values, such as with the Pt100 (391) sensor.

Pt100 (385) and Pt100 (391) RTD

Pt100 (385) RTD standard table

The equation is only approximate, so to know the true temperature at any measured resistance, we will need to consult a published standard table of resistance for a Pt100 (385) sensor, like the one shown here.

Pt100 (385) RTD standard table.

A Pt100 RTD is typically constructed by winding a thin platinum wire around a non-conductive core which helps support the thin wire. The entire assembly is encased in a sheath to protect the sensor and to give it stability.

Industrial Pt100 Sensors

In industrial applications, RTDs are commonly placed inside protective metal tubes called thermowell. The length of the RTD and the design of the thermowell are design parameters determined by the instrument engineer.

Thermowells

PT100 RTDs can be constructed from a single platinum wire, giving a sensor with two leads.

These leads can be connected to a special I/O card designed to accept RTD inputs, or the leads can be connected to a temperature transmitter, which will output a standard 4-20 milliamp signal.

In either case, the I/O card or the transmitter will have firmware that will determine the temperature read by the RTD from the measured resistance.

The table we showed you before is programmed into the transmitter and the RTD analog input card.

RTD analog input card.

2-wire Pt100 RTD

In order to determine the resistance of the RTD, a special bridge circuit is used, called a Wheatstone bridge.

In this diagram, there are four resistors. Resistors A, B, and C are of equal value. The fourth resistor is the RTD itself and its resistance can be deduced from the voltage measured across the two legs of the bridge.

Wheatstone bridge

This 2-wire RTD design is not very accurate because the platinum leads themselves have an electrical resistance due to the length of the wire and the connection points, in addition to the resistance from the temperature detected at the point of measurement.

2-Wire Pt100 RTD

3-wire Pt100 RTD

To compensate for this added resistance, a second platinum wire is added to the sensor at a third lead.

This third lead is used to determine the resistance of the lead itself, and the resistance is subtracted from the overall measure resistance to give the true resistance due to the change in temperature alone.

3-Wire Pt100 RTD

These 3-wire RTDs are the most widely used in the industry. Although more expensive than a 2-wire RTD, the added stability and accuracy are well worth the added cost.

Summary

In this article, we introduced you to the working principles of the Platinum 100 resistance temperature sensor, its physical properties, and how Pt100 sensors are used in industrial measurement and control systems.

We demonstrated the linear relationship between temperature and resistance for platinum. This property of Pt100 sensors makes these sensors reliable, accurate, and affordable for most places that temperature measurements are required.

So, the next time you come across a Pt100 (385) RTD in the field, you will have a much better understanding of how the sensor works.

If you have any questions about the Pt100 Sensor, or about sensors in general, add them in the comments below and we will get back to you in less than 24 hours.

Got a friend, client, or colleague who could use some of this information? Please share this article.

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Posted on Sep 13th, 2021

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