THERMOCOUPLE & RTD COMPARISON

In using both thermocouples and RTD's, direct readout devices are readily available.  However, the indiscriminate use of either type of sensor should be avoided.  All temperatures below 1000° F, and RTD is very sensitive, able to take the wear, and can be more accurate than a thermocouple; however, the reliability of RTD's in service above 1000° F is poor.  In addition, an RTD's resistance versus temperature relationship is more linear than the thermocouple's EMF-to-temperature ratio.

For almost all other requirements, however, thermocouples are preferable to RTD's.  First, due to its ability to withstand extreme bending and vibration, the thermocouple is more durable than the RTD.  Thermocouples also have a wider temperature range (i.e., -350° to 4200° F), as opposed to the RTD's range of -435° F to 1000° F.  While in theory RTD elements are good to temperatures well in excess of 1000° F, in practice, contamination of the element (and thus, loss of accuracy) is common above 1000° F.

RTD elements are, in fact very vulnerable to contamination of all kinds, and must be protected from even the most benign conditions.  Moisture, dirt, oil or any oxidizing condition will seriously affect the accuracy of RTD's, they are, therefore, used only in hermetically sealed probes for any industry application.  Their sensitivity to contamination is apparent in the fabrication of the RTD probe, which is never touched with bare hands so as to avoid contact with moisture or skin oils.. And, since an RTD element must be insulated from the environment in a probe, the response time is longer that that achievable by a thermocouple.

Another difference in the industrial applications of the two systems is the relative probe sizes attainable between the two.  Before exploring this subject, however, it is first necessary to understand the types of thermocouples and RTD's available.

Industrial RTD's can be made out of any conductive material, however, they are commercially available in platinum, nickel and copper in a variety of resistances.  The 100-ohm platinum RTD's are the most accurate and have the widest working range of the three, followed closely by nickel.  Copper RTD's fall behind the other two in accuracy, but they are cheaper "in the element form," i.e., one incorporated into a probe, the percentage of difference between the three types of materials is radically reduced.

No matter which material is chosen for RTD elements, they are available in two basic types of construction: wire-wound and thin-film.  Wire-wound RTD's are just that ... a wire coil of the chosen metal that will give a desired resistance at 0° C.  The thin-film typed is constructed from the chosen metal in a sheet approximately 0.020 inches thick.  This sheet, which is cemented to an insulator block, is then laser-cut to form a ever-increasing path of resistance until the desired resistance is achieved.

Since thin-film RTD's can be fabricated more quickly and use less material, it is the cheaper of the two types.  In the past, the thin-film RTD was considerably more delicate than the wire-wound type; now, however, such problems have been rectified.  Due to the size of even the smallest RTD's, it is extremely expensive and difficult to wire an RTD probe with an O.D. of less than 1/8-inch, additionally, probes of this size are subject to fragility.  Keeping this in mind, and remembering the range in probe size for sheathed thermocouples (0.020 inches to 0.75 inches), it becomes apparent that a thermocouple can be applied in many more cases than an RTD probe.