What are RTD temperature sensors?

RTDs or Resistance Temperature Detectors, are temperature sensors that contain a resistor that changes resistance value as its temperature changes. The most popular RTD is the Pt100. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability, and stability.

Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material whose resistance at various temperatures has been documented. The material has a predictable change in resistance as the temperature changes; it is this predictable change that is used to determine temperature.

The Pt100 is one of the most accurate temperature sensors. Not only does it provide good accuracy, it also provides excellent stability and repeatability. Most OMEGA standard Pt100 comply with DIN-IEC Class B or Class A. Pt100 are also relatively immune to electrical noise and therefore well suited for temperature measurement in industrial environments, especially around motors, generators and other high voltage equipment.

RTD standards

There are two standards for Pt100 RTDs: the European standard, also known as the DIN or IEC standard and the American standard. The European standard is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0 and 100°C.

There are two resistance tolerances specified in DIN/IEC751:
Class A = ±(0.15 + 0.002*t)°C or 100.00 ±0.06 Ω at 0ºC
Class B = ±(0.3 + 0.005*t)°C or 100.00 ±0.12 Ω at 0ºC

Two resistance tolerances used in industry are:
1/3 DIN = ±1/3* (0.3 + 0.005*t)°C or 100.00 ±0.10 Ω at 0ºC
1/10 DIN = ±1 /10* (0.3 + 0.005*t)°C or 100.00 ±0.03 Ω at 0ºC

The larger the element tolerance, the more the Pt100 sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability).

What types of RTDs are available?

Resistance Temperature Detectors (RTDs) available today can generally be categorized into one of two basic types of RTDs, depending on how their temperature sensing element is constructed. One type of RTD contains thin-film elements and the other type of RTD contains wire-wound elements. Each type is best suited for use in certain environments and applications.

The invention of a resistance thermometer was made possible by the discovery that the conductivity of metals decreases predictably with increases in their temperatures. The first-ever resistance thermometer was assembled from insulated copper wire, a battery and a galvanometer in 1860. However, its inventor, C.W. Siemens, soon discovered that a platinum element yielded more accurate readings at a much wider range of temperatures. Platinum remains the most commonly used material in temperature measurement using RTD sensing elements today.

Difference between a 2, 3, and 4 wire

Because every Pt100 element in the circuit containing the sensing element—including the lead wires, connectors and the measuring instrument itself—will introduce additional resistance into the circuit.

How the circuit is configured determines how accurately the sensor’s resistance can be calculated, and how much the temperature reading may be distorted by extraneous resistance in the circuit. Since the lead wire used between the resistance element and the measuring instrument has a resistance itself, we must also supply a means of compensating for this inaccuracy.

There are three types of wire configurations, 2 wire, 3 wire, and 4 wire, that are commonly used in RTD sensing circuits. A 2-wire configuration with a compensating loop is also an option.

Pt100 vs Pt1000

The RTD PT100, which is the most commonly used RTD sensor, is made of platinum (PT), and its resistance value at 0°C is 100 Ω. In contrast, a PT1000 sensor, also made of platinum, has a resistance value of 1000 Ω at 0°C.

The RTD Pt100 and Pt1000 are available in a similar range of tolerances, and both can have similar temperature coefficients, depending on the purity of the platinum used in the sensor. When comparing the Pt100 vs Pt1000 in terms of resistance, keep in mind that resistance value readings for the Pt1000 will be higher by a factor of ten than resistance value readings for the Pt100 at the same temperature. For most applications, the Pt100 and Pt1000 can be used interchangeably depending on the instrument used. In some cases the Pt1000 will work better and be more accurate.

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