1. ChE 334 S18 Temperature Measurement and Thermocouples
Why do ChE’s, BioE’s and EnvE’s need to measure Temperature?
To monitor/control critical/life processes
Chemical reaction rate?
Exothermic > may need cooling?
Environmental control/growth process
To evaluate thermal regimes
Needs more insulation?
Can save energy?
To make thermal decisions
Process too hot or cold?
Energy usage monitoring
>> Is my FOOD safely cooked yet??
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2. ChE 334 S18 Temperature Measurement and Thermocouples
How is temperature measured?
Thermometers work by detecting and displaying some physical change of a material.
Must be quantifiable (sensitive enough)
Must be repeatable (stable enough)
Must cover the range of interest
Three Categories of Thermometer:
Non-Electrical: Measure the thermal expansion/contraction of a material, OR some other visible change in property.
Electrical: Measure change in voltage, resistance, etc.
Non-Contact: Radiation (Infra-Red) Sense radiated heat, convert to numeric value/image
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3. ChE 334 S18 Temperature Measurement and Thermocouples
Old familiar methods of Temperature Measurement- mostly thermal expansion and contraction
Liquid-in-glass: a liquid expands within a capillary, the position of the meniscus shows T of the system
Liquid = alcohol or mercury
Relatively cheap
May be very accurate (not always)- best: +/- 0.2 degrees C
Breakable, Hg is toxic so we’re eliminating it
Usually must be totally immersed in the measured fluid
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4. ChE 334 S18 Temperature Measurement and Thermocouples
Old familiar methods of Temperature Measurement- mostly thermal expansion and contraction
Liquid-in-glass: a liquid expands within a capillary, the position of the meniscus shows T of the system
Liquid = alcohol or mercury
Relatively cheap
May be very accurate
Breakable, Hg is toxic so we’re eliminating it
Usually must be totally immersed in the measured fluid-
Or are calibrated to an immersion line
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5. ChE 334 S18 Temperature Measurement and Thermocouples
Old familiar methods of Temperature Measurement- mostly thermal expansion and contraction
Liquid-in-glass: a liquid expands within a capillary, the position of the meniscus shows T of the system
Liquid = alcohol or mercury
Relatively cheap
May be very accurate
Breakable, Hg is toxic so we’re eliminating it
Usually must be totally immersed in the measured fluid-
Freezer Thermometer
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6. ChE 334 S18 Temperature Measurement and Thermocouples
Old familiar methods to measure T using thermal expansion and contraction
Bi Metallic strip: differential expansion of two bonded metals
Usually the scale is on a round dial
Can be built so just the ‘stem’ needs to be immersed in the fluid
With a stem, better to measure temperature at a point
Durable
Usually not highly accurate- +/- 2 degrees C
Bi-metal strip inside the stem
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7. ChE 334 S18 Temperature Measurement and Thermocouples
What’s a Thermocouple? (TC)
How does it work?
A TC is a junction of two dissimilar metals.
A voltage appears when the junction is heated or cooled, compared to the other end of the wires (called the Seebeck effect, 1821).
The voltage is dependent on the type of metals, and also the difference of the T’s (THIGH – TLOW)
The voltage is repeatable.
THI
THI
TLO
TLO
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8. ChE 334 S18 Temperature Measurement and Thermocouples
What’s a Thermocouple? (TC)
How does it work?
A TC is a junction of two dissimilar metals.
A voltage appears when the junction is heated or cooled, compared to the other end of the wires (called the Seebeck effect, 1821).
The voltage is dependent on the type of metals, and also the difference of the T’s (THIGH – TLOW)
BUT… both Temps are still unknown:
TL=?
TL=?
TH=?
TH=?
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9. ChE 334 S18 Temperature Measurement and Thermocouples
What’s a Thermocouple? (TC)
How does it work?
Shown is the logical progression of using a Voltmeter to measure the Seebeck voltage.
The three circuits are functionally equal; we have just ‘inserted’ the Voltmeter into one of the ‘legs’ of the TC loop.
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10. ChE 334 S18 Temperature Measurement and Thermocouples
What’s a Thermocouple? (TC)
How does it work?
Shown is the logical progression of using a Voltmeter to measure the Seebeck voltage.
AHA! Now we’ve got the TLOW junction at a ‘reference’ (known) temperature.
An ice bath is (about) 0C, if carefully done.
This is called ‘Cold Junction Compensation’
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11. ChE 334 S18 Temperature Measurement and Thermocouples
What’s a Thermocouple? (TC)
How does it work?
Shown is the logical progression of using a Voltmeter to measure the Seebeck voltage.
But we can eliminate the ice bath in favor of convenience.
If the TLOW connections at the Voltmeter are all made on an isothermal block, whose temperature is known, then we have our built-in reference. Again, ‘Cold Junction Compensation’
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12. ChE 334 S18 Temperature Measurement and Thermocouples
So what is ‘Cold Junction Compensation’?
The Electronic Thermocouple Meter measures the temperature of the Input Terminals, and uses this for TLOW (shown below as TREF )
Then using the Seebeck voltage and TREF, the temperature at J1 can be calculated.
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13. ChE 334 S18 Temperature Measurement and Thermocouples
More about this ‘Seebeck’ voltage:
It varies directly with (T1-T2), the ‘cold junction’ – ‘hot junction’ temperature
It’s quite small about micro-volts per °C, so ~2-3 milliVolts for boiling H2O
Seebeck voltage vs. Temp is non-linear
>>Bottom Line: It takes some electronics to:
Amplify the tiny Seebeck voltage
Linearize the signal
Display in units of °C or °F
Do ‘Cold Junction Compensation’
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14. ChE 334 S18 Temperature Measurement and Thermocouples
What do Thermocouples look like?
Bare junction
Exposed to surroundings, easily contaminated
Quickest response to temperature change
Sheathed
Enclosed in metal tube
Usually stainless steel
Slower to respond
Much better protected
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15. ChE 334 S18 Temperature Measurement and Thermocouples
Why are they different colors?
There are many types of TC metal combinations. Common ones are: J, K, T, R, S and E. These fulfill a broad range of needs of range, sensitivity, and stability in differing environments.
Each has a defined Color in the ITS-90 system (that’s International Temperature Scale)
K and J are the most common in Chem. Eng. work.
T is best for ‘environmental’ temperatures- encountered on the earth’s surface, limited by one conductor being copper.
For others, please consult reference material. Here are the colors used in the USA (sleeve shows connector and jacket color):
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16. ChE 334 S18 Temperature Measurement and Thermocouples
Common TC types by letter, showing their metal combinations and voltage output with temperature.
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17. ChE 334 S18 Temperature Measurement and Thermocouples
How is the junction made?
Welding (discharge arc-welded)
Best for long-term stability and full temperature range
Best done with special equipment (should be inert-gas submerged arc to exclude O2 and N2, and other contaminants)
Crimping
Easiest to field-fabricate
Requires special tool and crimp lug
Soldering
High temperature limited by melting point of solder
Difficult to get good solder joint on some TC wire types
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18. ChE 334 S18 Temperature Measurement and Thermocouples
How about other Electronic Temperature measurement means?
RTD (Resistance Temperature Device)
Principle: the Resistance of metal has a positive temperature coefficient. Stable and repeatable, it covers a wide temperature range.
Material: usually platinum, engineered for temperature use
Thermistor
Principle: the Resistance R of a semiconductor has a negative temperature coefficient. ΔR/ΔT is much steeper for a metal. Thus, easier to measure small changes.
Material: silicon-based semiconductor, engineered for temperature use
Integrated Circuit temperature sensor
Principle: An IC is engineered to produce a linear output (either V or I) with temperature. Very simple to implement.
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19. ChE 334 S18 Temperature Measurement and Thermocouples$
$$$
$ to $$
$ to $$
Comparison of the four Electronic means
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20. ChE 334 S18 Temperature Measurement and Thermocouples
But wait! Another Thermometer you should know.
IR (Infrared) Temperature Sensing/Meters
Principle: A special element, sensitive to infrared radiation, registers the emitted heat from an object as its Surface Temperature
Surface temperature only, integrates over a certain area!!
Should be used with discretion: the indicated temperature depends on the emissivity of the surface of the object.
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21. ChE 334 S18 Temperature Measurement and Thermocouples
Beware… IR Thermometer Limitations
Gathers IR from a wide cone- sometimes adjustable
Here’s a typical sensing ‘gun’ pattern:
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22. ChE 334 S18 Temperature Measurement and Thermocouples
Beware… IR Thermometer Limitations
Very sensitive to Emissivity of the measured surface
Be sure you give our ChE 334 ‘tin can’ a try at the Thermal Property Measurements station
Cannot sense air temperature, OR liquid surface temperature reliably. e=
0.9
??? e=
0.1
???
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23. ChE 334 S18 Temperature Measurement and Thermocouples
Lastly- Thermal Imaging
IR Array sensor receives focused IR radiation
Processed to a visual image
Displayed showing comparative temperatures throughout an image
Highly sensitive to differences in surface-radiated energy
Can be calibrated to surface temp differences as little as 0.01C
Examples- Flir and Fluke (both NW corporations)
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24. ChE 334 S18 Temperature Measurement and Thermocouples
Lastly- Thermal Imaging in the ChE 334 Lab!
Steam Heat Exchanger
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25. ChE 334 S18 Temperature Measurement and Thermocouples
Lastly- Thermal Imaging in the ChE 334 Lab!
Steam Heat Exchanger
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26. ChE 334 S18 Temperature Measurement and Thermocouples
Lastly- Thermal Imaging in the ChE 334 Lab!
Thermal Property Measurements
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27. ChE 334 S18 Temperature Measurement and Thermocouples
Lastly- Thermal Imaging in the ChE 334 Lab!
Thermal Property Measurements
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28. ChE 334 S18 Temperature Measurement and Thermocouples
Lastly- Thermal Imaging in the ChE 334 Lab!
Thermal Property Measurements
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