Temperature Sensors

Thermocouples

A thermocouple is a temperature sensor made of two dissimilar metal wires that form a junction at one end. As the temperature of the junction varies, the millivolt output (generated by the temperature gradient along the wires) is interpreted by a temperature controller. There are different types of thermocouples; each type has a different combination of metals that makes it suitable for a specific temperature range and environment. The most common are the J, K, T, E and N types. The section of a thermocouple that is used to monitor temperature is usually protected with a metallic shell. The wire junction is either exposed out of the shell-tip, internally attached to the sealed shell-tip (grounded), or isolated from the sealed metal casing (ungrounded). The metallic protective shell can be filled with compacted MgO. This construction (MI style) gives structural strength and durability. MI thermocouples can be welded or brazed to heating element sheathes and can also be field-bent to suit specialized mountings. Additionally, thermocouples can have a basic tube-and-wire construction (TW style) which is mostly used in the plastics industry. Many optional mounting arrangements are available with this style; these include bayonet mount, adjustable depth, nozzle melt, and ring or shim mounting.

RTD’s

Resistance Temperature Detectors (RTDs) are sensors that have resistors which change resistance with temperature variations. A controller monitors that change and converts it to temperature. Several different materials are used to make RTDs, the most common being platinum. RTDs are available in two basic constructions; the thin-film (-150°F to 1000°F) and the wire-wound (-250°F to 1100°F). RTDs can come with 2, 3 or 4 lead wires. The ones with 3 or 4 wires can have relatively long lead lengths without loss of temperature accuracy.

Thermocouples

The tables below show recommended temperatures, environments, accuracies as well as color codes. It’s good to know that maximum temperatures are defined not only by the type of a thermocouple but also by the gauge of its wire.

Type	Composition		Temperature Range ºF (Rec.)	Application Suitability (Bare Wire)
Type	Positive Leg	Negative Leg	Temperature Range ºF (Rec.)	Application Suitability (Bare Wire)
J	Fe	55% Cu 45% Ni	32 to 1400	Vacuum, reducing or inter atmospheres. Limited use in oxidizing atmospheres (heavy gauges(.
K	90% Ni 10% Cr	95% Ni, 2% Al 2% Mg, 1% Si	32 to 2300	Used mostly above 1000ºF. Oxidizing and inert atmospheres. Not used in vacuum or low-oxygen atmospheres.
T	Cu	55% Cu 45% Ni	-300 to 700	Recommended for moist atmospheres. Vacuum, oxidizing, reducing or inert atmospheres.
E	95% Ni, 2% Al 2% Mg, 1% Si	55% Cu 45% Ni	32 to 1500	High EMF output per degree. Suitable for oxidizing and inert atmospheres.
N	84.1% Ni, 14.4% Cr 14% Si, 0.1% Mg	95.5% Ni, 4.4% Si 0.1% Mg	32 to 2300	Alternative to "K" type thermocouples. High resistance to oxidation.

Type	Temperature ºF Versus Accuracy		Maximum Temperature (Rec.) for Thermocouple Probe Diameter & Wire Gauges				Negative Leg Color	Positive Leg Color	Jack/Leg Color
			0.062"	0.125"	0.188"	0.25"
			30 AWG	24 AWG	20 AWG	16AWG
J	32 to 520 530 to 1400	+/- 4ºF +/- 0.75%	600ºF	700ºF	800ºF	900ºF	Red	White	Black
K	32 to 559 560 to 2300	+/- 4ºF +/- 0.75%	1400ºF	1600ºF	1700ºF	1800ºF	Red	Yellow	Yellow
T	-300 to -90 -91 to 270 271 to 700	+/- 1.5% +/- 2ºF +/- 0.75%	300ºF	400ºF	500ºF	500ºF	Red	Blue	Blue
E	32 to 640 641 to 1500	+/- 4ºF +/- 0.5%	700ºF	800ºF	900ºF	1000ºF	Red	Purple	Purple
N	32 to 560 561 to 2300	+/- 4ºF +/- 0.75%	1600ºF	1700ºF	1700ºF	1700ºF	Red	Orange	Orange

RTDs

RTDs are specified through the following criteria:

Construction (thin film or wire wound)
Sensor material (Pt, Cu or Ni)
RTD resistance (100Ω/Platinum most common)
Accuracy (IEC751 standards are common)
Wire configuration (3 wire most common)
TCR value (0.00385Ω / Ω / °C most common)

IEC751 standards specify the accuracy for 100Ω platinum RTDs with 0.00385Ω / Ω / °C TCR as:

Class A: +/-0.15°C at 0°C, increasing linearly to +/-1.15°C at 500°C
Class B: +/-0.30°C at 0°C, increasing linearly to +/-2.8 °C at 500°C

Thermocouples Versus RTDs

Criteria	RTDs	Thermocouples
Recommended Temperature Range	-300°F to 1000°F	-300°F to 2300°F
Accuracy (Typical)	Excellent (+/- 0.3°F to +/- 2°F)	Medium (+/- 1°F to +/- 10°F)
Stability	Extremely Stables (0.1°F drift/year)	Variable
Response Time	Slow (1-50 seconds)	Fast (0.1-10 seconds)
Linearity	Almost Linear	Non-Linear
Mechanical Shock / Vibration	Not Suitable	Suitable
Electrical Noise Problems	Limited Susceptibility	Susceptible
Excitation	Yes	No
Cost	High	Low

Wiring Tips

To be cost-effective, thermocouple extension wires can be of a lower grade (have lower ambient temperature limits), but they should be of the same material as the thermocouple wires. For long distances between sensing points and instruments, (100 ft. or more), transmitters can be used.
Copper terminal blocks, as well as lugs or splices, can be used in a thermocouple circuit, provided that the negative and the positive legs are at the same temperature.
Standard copper wires are used as RTD extension wires. Two-wire RTDs should be used only for short runs. For long runs, it is preferable to use 3 or 4 wire RTDs. For distances longer than 100 ft, using transmitters could be a solution.
The insulation of thermocouple and RTD wires should be compatible with the environment in which they will be used.
As the electrical signals generated by thermocouples and RTDs are very low, it is recommended to shield their extension wires.
Thermocouple and RTD wires should not run in conduits that contain power lines.

Bucan Thermocouples can be made in the configurations shown below with basic Tube & Wire or Mineral Insulated designs. Most of the designs can be manufactured with the use of RTDs as well. The two first characters of each style part number shows the nature of the sensor. TW stands for (Tube and Wire thermocouple), MI for (Mineral insulated Thermocouple), RT for (tube and wire RTD) and RM for (Mineral insulated RTD). Standard Bucan RTD sensors use Class B platinum RTDs that have 0.00385 Ohm/Ohm/°C TCR (Temperature coefficient of resistance).