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?? 12-Nov-18

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 12-Nov-18

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 12-Nov-18

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 12-Nov-18

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 12-Nov-18

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 12-Nov-18

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. The voltage is TINY, usually 0.1-30 mV for 10-1000°C THI THI TLO TLO 12-Nov-18

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=? 12-Nov-18

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. 12-Nov-18

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’ 12-Nov-18

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’ 12-Nov-18

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. 12-Nov-18

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 20-40 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’ 12-Nov-18

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 12-Nov-18

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): 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Common TC types by letter, showing their metal combinations and voltage output with temperature. 12-Nov-18

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 12-Nov-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. 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples $ $$$ $ to $$ $ to $$ -50°C to 1200(*) °C –200 to +850 °C -50°C to 250 °C -55 to 150°C Comparison of the four Electronic means (*) Depends on Type 12-Nov-18

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. 12-Nov-18

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: 12-Nov-18

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 ??? 12-Nov-18

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) https://www.testequity.com/documents/pdf/inspecting-steam-systems.pdf http://www.flir.com/instruments/electrical/display/?id=44142 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Lastly- Thermal Imaging in the ChE 334 Lab! Thermal Property Measurements 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Lastly- Thermal Imaging in the ChE 334 Lab! Thermal Property Measurements 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Lastly- Thermal Imaging in the ChE 334 Lab! Thermal Property Measurements 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Lastly- Thermal Imaging in the ChE 334 Lab! Steam Heat Exchanger 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Lastly- Thermal Imaging in the ChE 334 Lab! Steam Heat Exchanger 12-Nov-18

ChE 334 S18 Temperature Measurement and Thermocouples Lastly- Thermal Imaging in the ChE 334 Lab! Thermal Property Measurements 12-Nov-18