ENTC-370 PROF. ALVARADO 1

THERMODYNAMICS LAB:Temperature Measurement

ENTC - 370

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Temperature Sensors

• Contact Sensors:– Thermocouples (two wires made of different materials)– RTDs (Resistance Temperature Detector)– Thermistors (semiconductor)– Thermometers (mercury expansion)– ICs and Diode

• Non-Contact Sensors:– IR Thermometers (Infrared Temperature Probe)

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Thermocouples

A thermocouple consists of two wires of different materials joined together. The metals responded differently to the temperature difference, creating a current loop and a magnetic field. When the temperature at the junction is different from a reference temperature, the Seebeck effect is generated.

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Thermocouples Types• Thermocouples are classified based on the materials of the

wires. The most common thermocouple types are:– Type J: Iron-Constantan– Type T: Copper-Constantan– Type K: Chromel-Alumel– Type E: Chromel, Constantan– Type N: Nicosil, Nisil– Type R: Platinum-6% Rhodium, Platinum – 13%

Rhodium– Type S: Platinum, Platinum- 10% Rodium

• The first element is the positive element• The negative wire is color coded red.

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Thermocouples Types• Composition of Thermocouples alloys:

– Alumel: 94% nickel, 3% manganese, 2% aluminum, 1% silicon

– Chromel: 90% nickel, 10% chromium

– Constantan: 55% copper, 45% nickel

– Nicosil: 84% nickel, 14% chromium, 1.5% silicon

• Another classification is based on calibration accuracy:

– Precision

– Standard

– Lead-wire

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Thermocouples Limits

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Thermocouple ErrorsType Thermocouple

Error, Standard Grade

Error, Special Grade

E 1.7 C or 0.5% above 0 C

1.0 C or 0.4%

J 2.2 C or 0.75%above 0 C

1.1 C or 0.4%

K 2.2 C or 0.75%above 0 C

1.1 C or 0.4%

N 2.2 C or 0.75%above 0 C

1.1 C or 0.4%

R 1.5 C or 0.25% 0.6 C or 0.1%

S 1.5 C or 0.25% 0.6 C or 0.1%

T 1.0 C or 0.75% above 0 C

0.5 C or 0.4%

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Thermocouple Basic Circuit

•Ice Bath

•Electronically Controlled Reference

•Compensated Reference Temperature Systems

•Zone boxes

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References

• Notes on using thermocouples, Dr. Robert Moffat, Electronics – Cooling, Jan. 97.

• Experimental methods for engineers, Holman J., 7th. Edition, 2000