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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)
Positive Leg Negative Leg
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).

Ring Lug

TW01

Screw Tip

TW02

Shim Style

TW03

Spring Bayonet

TW04, MI04, RT04

Armor Bayonet

TW05, MI05, RT05

Basic Mini

MI06

Standard Probe

TW07, MI07, RT07, RM07

Fixed Bayonet

TW07S, MI07S, RT07S, RM07S

Compression Fitting

TW07C, MI07C, RT07C, RM07C

Standard 45°

TW08, MI08, RT08, RM08

Fixed Bayonet 45°

TW08S, MI08S, RT08S, RM08S

Compression Fitting 45°

TW08C, MI08C, RT08C, RM08C

Standard 90°

TW09, MI09, RT09, RM09

Fixed Bayonet 90°

TW09S, MI09S, RT09S, RM09S

Compression Fitting 90°

TW09C, MI09C, RT09C, RM09C

Rigid Probe Plug

TW10P, MI10P, RT10P, RM10P

Rigid Probe Jack

TW10J, MI10J, RT10J, RM10J

Fixed NPT

TW11, MI11, RT11, RM11

Spring Loaded NPT

TW12, MI12, RT12, RM12

Hose Clamp Style

TW13, MI13

Melt Bolt Style

TW14, MI14, RT14, RM14

Magnet Mount

TW15, MI15, RT15, RM15

Rigid Housing (Style 16)

TW16 / TW17, MI16 / MI17, RT16 / RT17, RM16 / RM17

Rigid Pipe Extension

TW018, MI018, RT018, RM018

Rigid Union

TW019, MI019, RT019, RM019

Protection Tube

TW20, MI20

For specifications, please check product catalogue

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Two, three and four wire configurations for RTDs