• In 1821 a German physicist named Seebeck discovered the thermoelectric effect which forms the basis of modern thermocouple technology. He observed that an electric current flows in a closed circuit of two dissimilar metals if their two junctions are at different temperatures.
• The thermoelectric voltage produced depends on the metals used and on the temperature relationship between the junctions.
• If the same temperature exists at the two junctions, the voltage produced at each junction cancel each other out and no current flows in the circuit.
• With different temperatures at each junction, different voltages are produced and current flows in the circuit.
• A thermocouple can therefore only measure temperature differences between the two junctions, a fact which dictates how a practical thermocouple can be utilized.

 

Table of Standard Thermocouple combinations and their operating temperatures in degrees C.

CODE	CONDUCTOR COMBINATION	TYPICAL OPERATING RANGE ºF
B	Platinum-30% Rhodium / Platinum-6% Rhodium	+2500 to +3100
C	Tungsten-5% Rhenium / Tungsten-26% Rhenium	+3000 to +4200
D	Tungsten-3% Rhenium / Tungsten-25% Rhenium	+2800 to +3800
E	Nickel Chromium / Constantan	0 to +1650
J	Iron / Constantan	+0 to +1400
K	Nickel Chromium / Nickel Aluminium	0 to +2300
N	Nickel-Chromium-Silicon / Nickel-Silicon-Magnesium	1200 to +2300
R	Platinum-13% Rhodium / Platinum	1600 to +2600
S	Platinum-10% Rhodium / Platinum	1800 to +2600
T	Copper / Constantan	-300 to +650

 

A graph of temperature vs. voltage shows thermocouple characteristics are not perfectly linear.

-Here is a graph of temperature versus voltage for several thermocouple conductor combinations.  Notice they are not perfectly linear.

-The voltages induced are in the range of a few “tens” of millivolts, roughly from about 0 to 80 millivolts.

Thermocouple Resolution

Temperature Change From 500 deg F to 510 deg F

TYPE	500 OF	510 OF	DIFF
C	4.140	4.248	0.108
E	17.945	18.371	0.426
J	14.110	14.418	0.308
K	10.561	10.789	0.228
R	2.017	2.070	0.053
S	1.962	2.012	0.050
T	12.574	12.887	0.313

 

Thermocouple Construction

Thermocouples are usually protected by placing them into a metal sheath, usually made of stainless steel.

-The conductors are then mineral insulated usually with Magnesium Oxide powder.

-The element is normally placed into a thermowell for protection.

-Elements are commonly 6 millimeters outside diameter.

-When required, the sheaths can be of special materials, like Inconel, Hastelloy or other materials.

-Duplex thermocouples have 2 elements inside one sheath.  This is used for the purposes of redundancy.

 

Thermocouple Tip Types

The different thermocouple tip types are Ungrounded, Grounded and Exposed.

–Ungrounded tips are used in corrosive and pressurized applications.  They have slow response times, but offers electrical isolation.

–Grounded tips are used in corrosive and pressurized applications too.  They have quicker response times than the ungrounded types due to improved heat transfer.

–Exposed tips are used in dry, non-corrosive, non-pressurized applications.  They offer the quickest response time of all three types.

Response time comparison among the different thermocouple tip types.

Here is a graph of probe diameter versus time.  This study was done using water.

-The exposed tip has the quickest response time.  But it has limited uses in our industry.  Grounded tip is next, then Ungrounded is the slowest.

-Notice the bigger the probe diameter the slower the response time for all tip types.

 

Resistance Temperature Detectors – RTDs

Resistance Temperature Detectors – RTDs operate under the principle that the electrical resistance of certain metals increases and decreases in a repeatable and predictable manner with a temperature change.

Resistance Temperature Detector Elements

Wire Wound Element
Precise lengths of wire are wrapped around a ceramic mandrel, then inserted inside a ceramic shell which acts to support and protect the wire windings.

Inner Coil Element
Wires are coiled then slid into the holes of a ceramic insulator.  Some manufacturers backfill the bores with ceramic powder after the coils are inserted.  This keeps the coils from shorting against each other.

Thin Film Element
Metallic ink is deposited onto a ceramic substrate. Lasers then etch the ink to provide a resistance path. The entire assembly is encapsulated in ceramic to support and protect.

Resistance Temperature Detector Lead Wire Configuration

• 2-wire: Should only be used with very short runs of lead wire.  No compensation for lead wire resistance.

• 3-wire: Most commonly used for industrial applications.  Lead wire compensation.

• 4-wire: Laboratory use historically, moving more into industrial applications.  Full compensation for lead wire resistance.

Wheatstone Bridge

The most common method for measuring the resistance of an RTD is to use a Wheatstone bridge circuit.  In a Wheatstone bridge, electrical excitation current is passed through the bridge, and the bridge output current is an indication of the RTD resistance.

 

• The most common material is Platinum.
• Its resistance is 100Ω at 0°Celsius.
  • Hence the term “PT100”
• Its resistance is 138.5Ω at 100°Celsius.
  • Hence the Fundamental Interval of 38.5Ω
  • Or 0.385Ω per 1°Celsius Rise in Temperature.
• There are other materials available for more unusual temperature ranges such as Germanium (e.g.10 to 100 °Kelvin).

RTDs and Thermocouples

Temperature Sensor Selection Guide
	RTD	Thermocouple
Temperature Range	-328°F to 1562°F	-310°F to 3308°F
Accuracy	±0.001°F to 0.1°F	±1°F to 10°F
Response Time	Moderate	Fast
Stability	Stable over long periods <0.1% error / 5 yr.	Not as stable 1°F error / 1yr.
Linearity	Best	Moderate
Sensitivity	High	Low
Vibration applications	Poor	Good

 

RTD vs Thermocouple Accuracy

RTD			Thermocouple Type J & K
Temp.°C	Grade B	Grade A	Standard	Premium
-200	±1.10°C	±0.47°C
-100	±0.67°C	±0.30°C
0	±0.25°C	±0.13°C	±2.2°C	±1.1°C
100	±0.67°C	±0.30°C	±2.2°C	±1.1°C
200	±1.10°C	±0.47°C	±2.2°C	±1.1°C
300	±1.50°C	±0.64°C	±2.3°C	±1.2°C
400	±1.90°C	±0.81°C	±3.0°C	±1.6°C
500	±2.40°C	±0.98°C	±3.8°C	±2.0°C

 

If you have any questions or need further information about RTDs and Thermocouples, feel free to reach out. We are here to help you harness the power of precise temperature measurement!

For more information on RTDs and other temperature sensors, visit our website or contact our team of experts. We are here to help you find the perfect solution for your temperature measurement needs.

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