RTD Technical Data:
All of Tempco’s Accu-Ohm RTDs comply with the following specifications:
- IEC publication 751 issued by the International Electrotechnical Commission (dated 1983).
- This is the widest international scope of any RTD standard. This publication sets the tolerance for platinum RTDs with a value of 100 ohms at 0°C with a temperature coefficient of resistance (TCR) of 0.00385 ohms/ohm/°C in one of two classes:
- Class A: Plus or minus 0.06% at 0°C
- Class B: Plus or minus 0.12% at 0°C
- All Tempco RTDs meet class B; class A is optional.
- DIN 43760 issued by Deutsches Institute fur Normung (Germany), dated 1987. The platinum resistance curves are now covered under DIN IEC 751.
- JIS 1604-1989 issued by the Japanese Standards Association (dated 1989). The Platinum resistance curves are in accordance with IEC 751 but there is also a provision for TCR 0.003916 ohms/ohm/°C which can be supplied in most of Tempco’s standard designs on special request.
- BS 1904-1984 issued by the British Standard Institute (dated 1984). This specification is identical to IEC 751.
What is the Temperature Coefficient of Resistance (TCR?)?
Temperature coefficient differentiates between resistance/temperature curves of RTDs. It is also called ALPHA and may be specified in various ways by different manufacturers. Here TCR is the RTDs resistance change from 0 to 100°C, divided by the resistance at 0°C, divided by 100°C:
||R 100°C - R 0°C
|R 0°C x 100°C
Example: A platinum RTD measuring 100 Ω’s at 0°C and 138.5 Ω’s at 100°C has TCR 0.00385 Ω/Ω/°C
||138.5Ω - 100Ω
||= 0.00385 Ω/Ω/°C
|100Ω x 100°C
Stated another way, TCR is the average resistance increase per degree of a hypothetical RTD measuring 1 ohm at 0°C.
The most common use of TCR is to distinguish between curves for platinum, which is available with TCRs ranging from 0.00375 to 0.003927. The highest TCR indicates the highest purity platinum, and is mandated by ITS-90 for standard platinum thermometers.
There are no technical advantages of one TCR versus another in practical industrial applications. 0.00385 platinum is the most popular worldwide standard and is available in both wire-wound and thin-film elements.
In most cases, all you need to know about TCR is that it must be properly matched when replacing RTDs or connecting them to instruments.
Interchangeability and Repeatability:
Interchangeability and accuracy are commonly cited as the RTDs most distinguishing attributes. Because of the tight tolerances of the Class A and Class B, RTDs are quite interchangeable. Their accuracy is also very good because of the RTD’s repeatability over the standard temperature scale from –260°C to 630°C. Ordinary industrial RTDs tend to show a drift of less than 0.1°C per year in normal use.
Because RTDs are exactly what the name implies (Resistance Temperature Detectors), a resistance type sensor, any resistance introduced by the addition of extension wires between the RTD and the control or measuring instrument will add to the readings. This added resistance is not constant since the extension wires, usually copper, change their resistance values with changing ambient temperature.
Extension wire errors can be significant, particularly with small gauge wires or elements with low sensitivity. Fortunately most of these errors may be nearly canceled by using a three wire system.
The majority of RTDs in today’s industry are 3- or 4-wire systems; the 2-wire lead system is the least efficient unless the leads are heavy gauge, very short, or both.
In 3- or 4-wire circuits, common leads, connected to the same end of the RTD element, are the same color.
Tolerances for 100Ω RTDs
Tolerance Values as a Function of Temperature for 100Ω RTDs