Resistance temperature detectors -- or RTDs, as they are more commonly known -- are a common way to measure temperature. RTDs were developed in Europe about a century ago but have become popular in the United States only in the last 25 years. RTDs are very similar in appearance to thermocouples, but they function completely differently.
As you may know, thermocouples produce a very small voltage when heated. An RTD does not produce any voltage and so it relies on an instrument for power. RTDs are electrical resistors that change resistance as temperature changes. With all of the common types of RTDs, the resistance increases as the temperature increases. This is referred to as a positive temperature coefficient.
RTDs are manufactured using several different materials as the sensing element. The most common by far is the platinum RTD. Platinum is used for several different reasons, including a high temperature rating and its very stable and very repeatable performance. Other materials used to make RTDs are nickel, copper and nickel-iron. These materials are becoming less common as the cost of platinum RTDs has fallen.
Platinum Thin Film RTD. The thin-film style of RTD (figure 1) is probably the most popular design because of its rugged design and cost. The thin-film element is manufactured by coating a small ceramic chip with a very thin (0.0001") platinum film and then laser cutting or chemically etching a resistance path in the film. The element then is coated with a thin layer of glass to protect it from harmful chemicals and gases. Larger extension lead wires are spot welded to the chip, and this junction then is covered with a drop of epoxy to help hold the wires to the element.
Inner Coil Wire Wound RTD. This type of element (figure 2) normally is manufactured using platinum wire. A very small platinum wire (0.0002") is coiled and then slid into a small two-hole ceramic insulator. Larger extension leads are spot welded to the ends of the platinum wire and cemented in place. Some manufacturers backfill the bores of the insulator with ceramic powder once the coils have been inserted. This keeps the coils from moving and shorting against each other. The end opposite the extension leads also is capped with ceramic cement.
Outer Wound RTD Element. The outer wound RTD element (figure 3) is made by winding the sensing element wire around a center mandrill, which is usually made of ceramic. This winding then is coated with glass or some other insulating material to protect and secure the windings. The winding wires are spot welded to extension leads and secured to the body with ceramic cement or epoxy.
Each of the types has their advantages. The thin film is the least expensive to manufacture and the most rugged. They also can be manufactured in very small sizes. The inner coil wire wound style is the most accurate. It is however, more expensive to manufacture and does not perform well in high vibration applications. The outer wound element is similar in cost to the inner coil element. It is not as accurate as the inner coil style but is more rugged.
FunctionTo use an RTD, a small voltage is passed through the element and then measured. The resistance of the RTD element reduces the voltage, and this voltage drop can be converted into a temperature measurement. With most RTDs, the higher the temperature, the higher the resistance. Figure 4 represents a simple two-wire RTD circuit. An instrument is hooked to one red wire and sends a voltage through the red wire, the element and then back through the other red wire. This reading is converted to a temperature by the instrument. The only problem with this simple two-wire circuit is that you read the resistance of the lead wire along with the resistance of the element. There is no way to separate the three resistances.
The three-wire circuit (figure 5) does allow for compensation of lead wire resistance, which is normally done by the measuring instrument. The instrument measures the resistance between the red and the white leads and then subtracts the resistance between the two reds.
The problem with the three-wire circuit is that the formula assumes that all three wires are the same resistance. This is not a problem on short lead wire lengths, but it can become a problem as the length of the extension lead wires increases. The four-wire circuit (figure 6) is a true four-wire bridge circuit that eliminates any differences in lead resistances. The four-wire bridge circuit eliminates lead wires resistance electrically instead of mathematically.
Coefficient, Rating And ToleranceAnother common term used with RTDs is temperature coefficient. This refers to the change in resistance vs. change in temperature. There are two common coefficients for platinum RTDs and several others for the copper and nickel types. The most common platinum RTD has a temperature coefficient of 0.00385 //oC. This means that a 100 platinum RTD will increase in resistance 0.385 for every 1.8oF (1oC) increase in temperature.
Temperature Rating. The maximum temperature rating for RTDs is based on two different factors. The first is the element material. Platinum RTDs can be used as high as 1,202oF (650oC). Other materials are much lower in temperature rating and vary from material to material. The other determining factor for temperature rating is probe construction. Construction considerations used in each style determine the temperature range of the device. No one style is good for all ranges.
In conclusion, RTDs are generally more expensive to manufacture or purchase than thermocouples. Because the RTD circuit is just a resistance circuit, no special extension leadwires or connectors are required, making this portion of the circuit less expensive than that of a thermocouple.
Although some types of RTD elements are rated to fairly high temperatures (1,202oF), they become quite fragile at temperatures above 600oF (320oC). An RTD sensor will not hold up well at these elevated temperatures if there is any vibration present. The tolerance or accuracy of an RTD generally decreases as temperature increases. PH