What is a Sanitary RTD?

March 6, 2014

Measuring and accurately monitoring process conditions is essential in high purity applications. In the over sixty years Holland has spent applying process equipment for the high purity industry, we’ve learned that the process is the product. If we can’t control the process, we can’t control the final product. Today, we’ll take a little time to talk about sanitary RTDs, what they do, how they work, and how we can apply them.

Resistance temperature detection or RTD is a century old technology. The application of the tendency of electrical conductors to increase their electrical resistance with rising temperature was first described by Sir William Siemens in 1871. This phenomenon is known as thermal resistivity.

Sir William Siemens

Sir William Siemens

RTD’s leverage this concept through the use of a resistive material attached to leads. The resistive material most often used, especially in sanitary and critical high purity applications, is platinum. Other materials used include copper and nickel. Copper and nickel are generally used in less critical applications due to limited resistive linearity and narrow temperature ranges which can lead to inaccuracies.

For this post, we will focus on platinum RTDs or PRTDs. The common values of resistance for a platinum RTD range from 10 ohms to over 1000 ohms. The most common value is 100 ohms of resistance at 0 C. The temperature coefficient of platinum wire is 0.00385. For a 100 ohm wire, this corresponds to a gain of 0.385 ohms/ degree C.

In a PRTD the platinum sensing element is located in the tip of the temperature sensor that is exposed to the process and is connected to wire leads. The sensing element responds to temperature by generating a measurable resistance change that increases as the temperature increases. An RTD is usually provided with either one or two elements in one sensor sheath. Dual elements provide a redundancy that can be used by a temperature transmitter for hot back up, drift monitoring, or to provide inputs to two independent controllers. The sensor sheath is generally made of stainless in a food or pharmaceutical application. The RTD sheath can be directly immersed into the process or inserted into a thermowell. Future posts will take a closer look at thermowells.

The sensing element is connected to wire leads. A single RTD can have two, three, or four leads and a dual RTD can have four, six, or eight leads. The sensor leads connect the sensor to the terminal block, transmitter, or any other termination point. The lead length usually varies by vendor and user requirements.

As mentioned previously, RTD lead configurations are generally offered in three standard configurations: two, three, or four leads. Two wire configuration is simple, but offers no compensation for resistance losses in the lead wire since the lead wires are in series with the element and appear to the transmitter as part of the sensor’s resistance. This leads to inaccuracies.

In three-wire configurations, a third wire is used to compensate for lead wire resistance. The idea is that the third wire will have the same resistance as the other two and the same compensation can be applied to all three wires.

Anderson SA RTDs and CT Wiring Heads

Anderson SA RTDs and CT Wiring Heads

For the highest accuracies, a four-wire RTD is used. A four wire design uses a small amount of current applied to two of the wires and the voltage developed across the sensor is measured over the other two wires with a high-impedance, high resolution measuring circuit. The high impedance eliminates current flow in the voltage measurement leads and therefore the resistance of the leads is not a factor.

Much more can be said about RTD construction and options. Future posts will focus not only on the specific products we offer, but also on thermowell and transmitter selection. If you’d like to skip the wait, call us today and we’d be happy to help you with your next temperature application.