TemperatureTemperature MeasurementMeasurement

• Thermocouple (T/C)• Resistance Temperature Detector (RTD)• Thermowell• Thermistor• Bi-metallic Thermometers• Filled Thermal Systems

Sequence will be change of

flow

Various Units of Temperature Measurement

• oC - degrees Celsius (or Centigrade)• oF – degrees Fahrenheit• K – Kelvin• R – Rankine

Relationship between different units• oC = (oF – 32)/1.8• oF = 1.8 x oC+ 32• K = oC + 273.15• R = oF + 459.67

Conversion tables or software can be utilized to

facilitate with converting between these units

Thermocouples (TC’s)Basic Theory• 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 junction 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 t each junction, different voltages are produces and current flows in the circuit.

• A thermocouple can therefore only measure temperature differences between the two junctions, a face which dictates how a practical thermocouple can be utilized.

Thermocouples (TC’s)

Cold Junction: Need to be held constant to give a fixed

reference. (early methods held cold junction at 0oC using ice or refrigeration unit).

Thermocouples (TC’s)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

1200To +2300

R Platinum -13% Rhodium / Platinum 1600 To +2600

S Platinum -10% Rhodium / Platinum 1800 to +2600

T Copper / Constantan -300 to +650

Thermocouples (TC’s)

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

Thermocouples (TC’s)

• Normally element is in a thermowell• Commonly element is 1/4” outside Diameter• Sheath material, normally Stainless steel but can be special

material such as Inconel, Incoloy, Hastelloy etc.• Duplex thermocouples have 2 elements inside one sheath.

Thermocouples (TC’s)

Capabilities and Limitations

• Capabilities: - Wide Range - Fast Response - Passive - Inexpensive

• Limitations: - CJC - Non-Linea

RTD’s

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

RTD 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.

RTD Elements

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.

RTD Element

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.

RTD Leadwire Configuration

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

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

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

RTDs

• The most common material is Platinum.• It’s resistance is 100Ω at 0o Celsius - Hence the term “PT100”• It’s resistance is 138.5Ω at 100o Celsius. - Hence the Fundamental Interval of 38.56 Ω - Or 0.385Ω per 1o Celsius Rise in

Temperature.• There are other material available for more

unusual temperature ranges such as Germanium ( e.g. 10 to 100 oKelvin)

Temperature Elements Assembly

Thermowells

Thermowells

Transmitters

• Single Conditioner• Low level inputs mV from themocouples Ω from RTD’s• High level output 4-20mA Current Digital (i.e. Fieldbus)

Transmitters• Thermistors are temperature sensing devices that are similar to

RTD’s in that their resistance changes as temperature changes.

• The major difference is that for most thermistors the resistance decreases as temperature increases.

• Thermistors are an inexpensive alternative to RTD’s when temperature ranges are below 150o C. Thermistors can be from temperature of 80o C to 300oC.

• Most thermistors have base resistance, which are much higher than RTD’s.

• One of the greatest advantages of using a thermistor is the large change in resistance to a relatively small change in temperature. This makes them very sensitive to small changes in temperature.

Bimetallic Thermometes

A Bimetallic Thermometer consists of an indicating or recording device, a sensing element and a means for connecting the two.

Bimetallic Thermometes

• Working principle ???

• What metals are used??• Construction details??

Filled Thermal System

Filled Thermal System

• Working principle??• Temperature gauges( mercury in

steel, vapor pressure)???• Construction details??• Calibration of Temperature

gauges???

Temperature Switch

• Working principle??• Temperature switches ( mercury in

steel, vapor pressure)???• Construction details??• Calibration of Temperature

switchses???

Thank You….