The Iron - constantan “J” curve thermocouple with a Positive Iron wire and a negative Constantan wire is recommended for reducing atmospheres. The Operating range for this alloy combination is 870 º C for the largest wire sizes. Smaller size wires should operate in correspondingly lower temperatures.
Limited range (-40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that can not accept 'modern' thermocouples. J types should not be used above 760 °C as an abrupt magnetic transformation will cause permanent deceleration.
"K" type (Chromel - Alumel) The Chromel - Alumel “K” curve thermocouple with a Positive Chromel wire and a negative Alumel wire is recommended for use in clean oxidizing atmospheres.
This is a 'general purpose' thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. Thermocouples are available in the -200°C to +1200 °C range. Use type K unless you have a good reason not to.
Type N (Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy))
High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an 'improved' type K, it is becoming more popular.
“S, R” type (Platinum-Rhodium Alloys)
Two types of noble metal thermocouples are in common use. They are…
Type R (Platinum / Rhodium 13%)
A positive wire of 87% platinum and 13% rhodium used with a negative wire of pure platinum. They have high resistance to oxidization and corrosion, they recommended operating range for the platinum –rhodium alloys is 1600 º C. Low sensitivity and high cost makes them unsuitable for general purpose use.
Type S (Platinum / Rhodium 10%)
‘S’ type suited for high temperature measurement up to 1600 °C. Low sensitivity and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43 °C).
Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity they are usually only used for high temperature measurement (>300 °C).
T" type (Copper - Constantan) The Copper -constantan “T” curve thermocouple with a Positive Copper wire and a negative Constantan wire is recommended for use in mildly oxidizing and reducing atmospheres up to 400 º C, they are suitable for applications where moisture is present.
This alloy is recommended for low temperature work since the homogeneity of the component wires can be maintained better than other base metal wires. Therefore, errors due to in homogeneity of wires in zones of temperature gradients are greatly reduced.
A thermocouple best suited for measurements in the -200 to 0 degree Celsius range. The positive conductor is made of copper, and the negative conductor is made of constantan.
RESISTANCE TEMPERATURE DETECTORS (RTD'S)
Resistance Temperature Detectors are sensors that measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is typically relatively fragile, so it is generally installed inside a sheath to protect it. The RTD element is constructed from a pure material, the resistance of which, at various temperatures, has been documented by various international standards institutes, The material has a predictable change in resistance as the temperature varies; it is this Change that is used to determine temperature.
RTD's are generally considered to be among the most accurate temperature sensors available. In addition to providing very good accuracy, they feature excellent stability and repeatability. RTD's also provide high immunity to electrical noise and are, therefore, well suited for applications in process and industrial automation environments, especially around motors, generators and other high voltage equipment.
Platinum Resistance Thermometer sensor assemblies (including MI type) with high accuracy in Single, Duplex, Elements.
RTD TYPES:
1) RTD assemblies with explosion proof heads
2) Mineral Insulated Metal Sheathed RTD's
3) Heavy duty Industrial Metal Sheathed RTD's
4) Hand Held RTD's
Limits of thermocouple reference junctions:
Thermocouple Types
Temperature Range º C
Standard (which ever is greater)
T
0 to 350
± 1.0 º C or ± 0.75%
J
0 to 750
± 2.2 º C or ± 0.75%
E
0 to 900
± 1.7 º C or ± 0.5%
K
0 to 1250
± 2.2 º C or ± 0.75%
R or S
0 to 1450
± 1.5 º C or ± 0.25%
B
800 to 1700
± 0.5%
sub-zero range
T
-200 to 0
± 1.0 º C or ± 1.5%
E
-200 to 0
± 1.7 º C or ± 1.0%
K
-200 to 0
± 2.2 º C or ± 2.0%
Limits of error for PT-100 RTD:
Temp º C
Class “B” ±º C ± O
Class “A” ±º C ± O
0
0.3 0.12
0.15 0.06
100
0.8 0.30
0.35 0.13
200
1.3 0.48
0.55 0.20
300
1.8 0.64
0.75 0.27
400
2.3 0.79
0.95 0.33
500
2.8 0.93
1.15 0.38
SHEATH METERIALS
Sheathing
Melting
Usable
Characteristics
SS-304
1400
900
High resistance to heat and
Corrosion
SS-316
1400
900
Excellent resistance to Heat,Acids & Alkalies
SS-310
1410
1150
Good oxidation Resistance and
sculpture atmosphere at low
temperature
SS-321
1400
900
Excellent resistance
to corrosion
HRS-446
1400
1150
Excellent oxidation resistance
at elevated temperature
and sulphur atmosphere
Inconel
600/800
1410
1150
Excellent oxidation
resistance (do not use in
sulphur atmospheres)
High Alumina 710, recrystallised
1800
1500
Excellent thermal,
mechanical, electrical
(sintered Al ² O3,99.9%)
