How does an averaging rtd work

Its Pt sensor measures temperature along the entire length of the wires, which minimizes the risk of getting a too-hot or too-cold reading in gas compressors, air exchangers, and HVAC assemblies. The sensors are sheathed in a flexible armor made of stainless steel, which means they can be inserted directly into the process.

The 3-wire or 4-wire configuration offers different levels of accuracy, precision, and repeatability. The sensors can come with or without a connection head. An optional Crastin terminal block reduces the risk of loose connections. This model also has the option of a digital programmable transmitter to convert the resistance output signal. Typically, RTDs contain either platinum, nickel, or copper wires, as these materials have a positive temperature coefficient.

This means that a rise in temperature results in an increased resistance — this change of resistance is then used to detect and measure temperature changes. Nickel RTDs are less expensive than platinum and have good corrosion resistance.

However, nickel ages more rapidly over time and loses accuracy at higher temperatures. Copper RTDs have the best resistance to temperature linearity of the three RTD types, and copper is a low cost material. However, copper oxidizes at higher temperatures.

In a wire wound RTD , a resistance wire is wound around a non-conducting core, which is usually made of ceramic. Next, lead wires are attached to the resistance wire, and then a glass or ceramic coating is applied over the wire for protection. As temperature increases, the length of resistance wire increases slightly. Care must be taken in the design to ensure that the resistance wire does not twist or otherwise deform as temperature increases.

This is because mechanical strain causes a change in wire resistance. Laboratory-grade RTDs used by calibration and standards laboratories eliminate this source of error by loosely winding resistance wire around a non-conducting support structure. This type of RTD can be extremely accurate, but is fragile and not suited for most industrial applications.

In a coiled element RTD , the resistance wire is rolled into small coils, which loosely fit into a ceramic form that is then filled with non-conductive powder.

The resistance wire is free to expand and contract as temperature changes, minimizing error caused by mechanical strain. The powder increases the rate of heat transfer into the coils, thereby improving the response time. They are smaller, and have a faster response time than the others, which is desirable in many applications.

They are made by depositing a thin pathway of platinum on a ceramic base. Thin film RTDs are not as accurate as the other types because:. The expression for resistance ratio is:. Resistance ratio is affected by the type and purity of the metal used to make the RTD. In general, RTDs that have a high R 0 value combined with a high resistance ratio are easier to measure accurately, but other characteristics of the metal used in the resistance wire still affect the inherent accuracy of the RTD.

These RTDs have a resistance ratio of Both types have a 0. These qualities also minimize error due to lead wire resistance. For an RTD, the approximate error due to lead wire resistance is:.

A 2-wire nickel RTD measures an air duct temperature. Each lead wire has a resistance of 0. Because a nickel RTD is so sensitive, a low-cost, low-accuracy transmitter can measure the RTD with acceptable accuracy. For these reasons, copper RTDs are sometimes used to measure winding temperature. Copper also has an extremely linear temperature vs.

Because of this, it is possible to accurately measure a narrow temperature span without additional linearization. According to published Resistance vs. Temperature tables, the RTD creates resistance of The non-linearity of copper does not become apparent unless measuring a wide span. Note that IEC specifies a maximum temperature range for each class. When operated outside this temperature range, the sensor accuracy might default to class B.

This table shows the calculated tolerance for each class and grade of RTD. The tolerance of these high accuracy RTDs is usually described as a fraction of class B tolerance. This tolerance is five times better than a class B RTD. The Callendar van Dusen equations describe the temperature vs. There are two Callendar van Dusen equations:. The 2-wire HART temperature transmitter , the 2-wire transmitter with HART protocol , and the 2-wire HART transmitter can be programmed with these coefficients, precisely matching the transmitter to a characterized RTD for exceptional measurement accuracy.

Book Training or Demo. How do RTD temperature sensors work? Applications Sensor , Temperature. Daniel Pregrasso October 25, Share on linkedin. Share on twitter.

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