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Overview of Temperature Measurement Figures are from www. omega. com “Practical Guidelines for Temperature Measurement” unless otherwise noted
Outline n Thermocouples – overview, reference junction, proper connections, types, special limits of error wire, time constants, sheathing, potential problems, DAQ setup n RTDs – overview, bridges, calibration, accuracy, response time, potentail problems Thermistors n Infrared Thermometry n – fundamentals, emissivity determination, field of view n Other – Non-electronic measurement, thin-film heat flux gauge Temperature Controllers n How to Choose n – Standards, cost, accuracy, stability, sensitivity, size, contact/noncontact, temperature range, fluid type
Thermocouples n Seebeck effect – If two wires of dissimilar metals are joined at both ends and one end is heated, current will flow. – If the circuit is broken, there will be an open circuit voltage across the wires. – Voltage is a function of temperature and metal types. – For small DT’s, the relationship with temperature is linear – For larger DT’s, non-linearities may occur.
Measuring the Thermocouple Voltage n If you attach thermocouple directly to a voltmeter, you will have problems. n You have just created another junction! Your displayed voltage will be proportional to the difference between J 1 and J 2 (and hence T 1 and T 2). Note that this is “Type T” thermocouple.
External Reference Junction n A solution is to put J 2 in an ice-bath; then you know T 2, and your output voltage will be proportional to T 1 -T 2.
Other types of thermocouples n Many thermocouples don’t have one copper wire. Shown below is a “Type J” thermocouple. n If the two terminals aren’t at the same temperature, this also creates an error.
Isothermal Block n The block is an electrical insulator but good heat conductor. This way the voltages for J 3 and J 4 cancel out. Thermocouple data acquisition set-ups include these isothermal blocks. n If we eliminate the ice-bath, then the isothermal block temperature is our reference temperature
Software Compensation How can we find the temperature of the block? Use a thermister or RTD. n Once the temperature is known, the voltage associated with that temperature can be subtracted off. n Then why use thermocouples at all? n – Thermocouples are cheaper, smaller, more flexible and rugged, and operate over a wider temperature range. n Most data acquisition systems have software compensation built in. To use Labview, you’ll need to know if you have a thermister or RTD.
Hardware Compensation With hardware compensation, the temperature of the isothermal block again is measured, and then a battery is used to cancel out the voltage of the reference junction. n This is also called an “electronic ice point reference”. With this reference, you can use a normal voltmeter instead of a thermocouple reader. You need a separate ice-point reference for every type of thermocouple. n
Making Thermocouple Beads Soldering, silver-soldering, butt or spot or beaded gas welding, crimping, and twisting are all OK. n The third metal introduced doesn’t effect results as long as the temperature everywhere in the bead is the same. n Welding should be done carefully so as to not degrade the metals. n If you consistently will need a lot of thermocouples, you can buy a thermocouple welder; you stick the two ends into a hole, hit a button, and the welding is done. n
Time Constant vs. Wire Diameter
Time Constant vs. Wire Diameter, cont.
Thermocouple Types If you do your own calibration, you can usually improve on the listed uncertainties.
Thermocouple Types, cont. n n n Type B – very poor below 50ºC; reference junction temperature not important since voltage output is about the same from 0 to 42 ºC Type E – good for low temperatures since d. V/d. T (a) is high for low temperatures Type J – cheap because one wire is iron; high sensitivity but also high uncertainty (iron impurities cause inaccuracy) Type T – good accuracy but low max temperature (400 ºC); one lead is copper, making connections easier; watch for heat being conducted along the copper wire, changing your surface temp Type K – popular type since it has decent accuracy and a wide temperature range; some instability (drift) over time Type N – most stable over time when exposed to elevated temperatures for long periods
Sheathing and SLE n n n “Special Limits of Error” wire can be used to improve accuracy. Sheathing of wires protects them from the environment (fracture, oxidation, etc. ) and shields them from electrical interference. The sheath should extend completely through the medium of interest. Outside the medium of interest it can be reduced. Sometimes the bead is exposed and only the wire is covered by the sheath. In harsher environments, the bead is also covered. This will increase the time constant. Platinum wires should be sheathed in non-metallic sheaths since they have a problem with metallic vapor diffusion at high temperatures.
Sheathing, cont. n From J. Nicholas & D. White, 2001, Traceable Temperatures: An Introduction to Temperature Measurement and Calibration, 2 nd ed. John Wiley & Sons.
Potential Problems n Poor bead construction – Weld changed material characteristics because the weld temp. was too high. – Large solder bead with temperature gradient across it n Decalibration – If thermocouples are used for very high or cold temperatures, wire properties can change due to diffusion of insulation or atmosphere particles into the wire, coldworking, or annealing. – Inhomogeneities in the wire; these are especially bad in areas with large temperature gradients; esp. common in iron. Metallic sleeving can help reduce their effect on the final temperature reading.
Potential Problems, cont. n Shunt impedence – As temperature goes up, the resistance of many insulation types goes down. At high enough temperatures, this creates a “virtual junction”. This is especially problematic for small diameter wires. n Galvanic Action – The dyes in some insulations form an electrolyte in the water. This creates a galvanic action with a resulting emf potentially many times that of thermocouple. Use an appropriate shield for a wet environment. “T Type” thermocouples have less of a problem with this.
Potential Problems, cont. n Thermal shunting – It takes energy to heat thermocouple, which results in a small decrease in the surroundings’ temperature. For tiny spaces, this may be a problem. – Use small wire (with a small thermal mass) to help alleviate this problem. Small-diameter wire is more susceptible to decalibration and shunt impedence problems. Extension wire helps alleviate this problem. Have short leads on thermocouple, and connect them to the same type of extension wire which is larger. Extension wire has a smaller temperature range than normal wire. n Noise – Several types of circuit set-ups help reduce line-related noise. You can set your data acquisition system up with a filter, too. – Small-diameter wires have more of a problem with noise.
Potential Problems n Conduction along thermocouple wire – In areas of large temperature gradient, heat can be conducted along thermocouple wire, changing the bead temperature. – Small diameter wires conduct less of this heat. – T-type thermocouples have more of a problem with this than most other types since one of the leads is made of copper which has a high thermal conductivity. n Inaccurate ice-point
Data Acquisition Systems for Thermocouples Agilent, HP, and National Instruments are probably the most popular DAQ systems n Example National Instruments DAQ setup for thermocouples and costs n
Things to Note During System Assembly n n n n n Make sure materials are clean, esp. for high temperatures. Check the temperature range of materials. Materials may degrade significantly before the highest temperature listed. Make sure you have a good isothermal junction. Use enough wire that there are no temperature gradients where it’s connected to your DAQ system. If you’re using thermocouple connectors, use the right type for your wire. If you’re using a DAQ system, use the right set-up for thermocouples. Check the ice-point reference. Provide proper insulation for harsh environments. Pass a hair-dryer over the wire. The temperature reading should only change when you pass it over the bead. Mount a thermocouple only on a surface that is not electrically live (watch for this when measuring temperatures of electronics).
RTDs (Resistance Temperature Detectors) n n n Resistivity of metals is a function of temperature. Platinum often used since it can be used for a wide temperature range and has excellent stability. Nickel or nickel alloys are used as well, but they aren’t as accurate. In several common configurations, the platinum wire is exposed directly to air (called a bird-cage element), wound around a bobbin and then sealed in molten glass, or threaded through a ceramic cylinder. Metal film RTDs are new. To make these, a platinum or metalglass slurry film is deposited onto a ceramic substrate. The substrate is then etched with a laser. These RTDs are very small but aren’t as stable (and hence accurate). RTDs are more accurate but also larger and more expensive than thermocouples.
RTD geometry From Nicholas & White, Traceable Temperatures. Sheathing: stainless steel or iconel, glass, alumina, quartz n Metal sheath can cause contamination at high temperatures and are best below 250ºC. n At very high temperatures, quartz and high-purity alumina are best to prevent contamination. n
Resistance Measurement n Several different bridge circuits are used to determine the resistance. Bridge circuits help improve the accuracy of the measurements significantly. Bridge output voltage is a function of the RTD resistance.
Resistance/Temperature Conversion Published equations relating bridge voltage to temperature can be used. n For very accurate results, do your own calibration. n – Several electronic calibrators are available. – The most accurate calibration that you can do easily yourself is to use a constant temperature bath and NIST-traceable thermometers. You then can make your own calibration curve correlating temperature and voltage.
Accuracy and Response Time n Response time is longer than thermocouples; for a ¼ sheath, response time can easily be 10 s.
Potential Problems RTDs are more fragile than thermocouples. n An external current must be supplied to the RTD. This current can heat the RTD, altering the results. For situations with high heat transfer coefficients, this error is small since the heat is dissipated to air. For small diameter thermocouples and still air this error is the largest. Use the largest RTD possible and smallest external current possible to minimize this error. n Be careful about the way you set up your measurement device. Attaching it can change the voltage. n When the platinum is connected to copper connectors, a voltage difference will occur (as in thermocouples). This voltage must be subtracted off. n
Thermistors n n n Thermistors also measure the change in resistance with temperature. Thermistors are very sensitive (up to 100 times more than RTDs and 1000 times more than thermocouples) and can detect very small changes in temperature. They are also very fast. Due to their speed, they are used for precision temperature control and any time very small temperature differences must be detected. They are made of ceramic semiconductor material (metal oxides). The change in thermistor resistance with temperature is very non-linear.
Resistance/Temperature Conversion Standard thermistors curves are not provided as much as with thermocouples or RTDs. You often need a curve for a specific batch of thermistors. n No 4 -wire bridge is required as with an RTD. n DAQ systems can handle the non-linear curve fit easily. n Thermistors do not do well at high temperatures and show instability with time (but for the best ones, this instability is only a few millikelvin per year) n
Infrared Thermometry n n n Infrared thermometers measure the amount of radiation emitted by an object. Peak magnitude is often in the infrared region. Surface emissivity must be known. This can add a lot of error. Reflection from other objects can introduce error as well. Surface whose temp you’re measuring must fill the field of view of your camera.
Benefits of Infrared Thermometry n Can be used for – Moving objects – Non-contact applications where sensors would affect results or be difficult to insert or conditions are hazardous – Large distances – Very high temperatures
Field of View n On some infrared thermometers, FOV is adjustable.
Emissivity To back out temperature, surface emissivity must be known. n You can look up emissivities, but it’s not easy to get an accurate number, esp. if surface condition is uncertain (for example, degree of oxidation). n Highly reflective surfaces introduce a lot of error. n Narrow-band spectral filtering results in a more accurate emissivity value. n
Ways to Determine Emissivity 1. 2. 3. 4. 5. Measure the temperature with a thermocouple and an infrared thermometer. Back out the emissivity. This method works well if emissivity doesn’t change much with temperature or you’re not dealing with a large temperature range. For temperatures below 500°F, place an object covered with masking tape (which has e=0. 95) in the same atmosphere. Both objects will be at the same temperature. Back out the unknown emissivity of the surface. Drill a long hole in the object. The hole acts like a blackbody with e=1. 0. Measure the temperature of the hole, and find the surface emissivity that gives the same temperature. Coat all or part of the surface with dull black paint which has e=1. 0. For a standard material with known surface condition, look up e.
Spectral Effects Use a filter to eliminate longer-wavelength atmospheric radiation (since your surface will often have a much higher temperature than the atmosphere). n If you know the range of temperatures that you’ll be measuring, you can filter out both smaller and larger wavelength radiation. Filtering out small wavelengths eliminates the effects of flames or other hot spots. n If you’re measuring through glass-type surfaces, make sure that the glass is transparent for the wavelengths you care about. Otherwise the temperature you read will be a sort of average of your desired surface and glass temperatures. n
Price and Accuracy Prices range from $500 (for a cheap handheld) to $6000 (for a highly accurate computer-controlled model). n Accuracy is often in the 0. 5 -1% of full range. Uncertainties of 10°F are common, but at temperatures of several hundred degrees, this is small. n
Non-Electronic Temperature Gages Crayons – You can buy crayons with specified melting temperatures. Mark the surface, and when the mark melts, you know the temperature at that time. n Lacquers – Special lacquers are available that change from dull to glossy and transparent at a specified temperature. This is a type of phase change. n Pellets – These change phase like crayons and lacquers but are larger. If the heating time is long, oxidation may obscure crayon marks. Pellets are also used as thermal fuses; they can be placed so that when they melt, they release a circuit breaker. n Temperature sensitive labels – These are nice because you can peel them off when finished and place them in a log book. n
Non-Electronic Temperature Gages, cont. Liquid crystals – They change color with temperature. If the calibration is know, color can be determined very accurately using a digital camera and appropriate image analysis software. This is used a fair amount for research. n Naphthalene sublimation (to find h, not T)– Make samples out of naphthalene and measure their mass change over a specified time period. Use the heat and mass transfer analogy to back out h. n
Thin-Film Heat Flux Gauge Temperature difference across a narrow gap of known material is measured using a thermopile. n A thermopile is a group of thermocouples combined in series to reduce uncertainty and measure a temperature difference. n From Nicholas & White, Traceable Temperatures.
Thin-Film Heat Flux Gauge, cont. n Fig pg a-26
Thin-Film Heat Flux Gauge, cont. n Difficulties with these gauges – The distance between the two sides is very small, so the temperature difference is small. The uncertainty in the temperature difference measurement can be large. – Watch where you place them. If the effective conductivity of the gauges is different than the conductivity of the material surrounding it, it will be either easier or harder for heat to pass through it. Heat will take the path of least resistance, so if you don’t position the gauge carefully, you may not be measuring the actual heat flux.
Temperature Controllers n Consider the following when choosing a controller – Type of temperature sensor (thermocouples and RTDs are common) – Number and type of outputs required (for example, turn on a heater, turn off a cooling system, sound an alarm) – Type of control algarithm (on/off, proportional, PID) n On/off controllers – – – These are the simplest controllers. On above a certain setpoint, and off below a certain setpoint On/off differential used to prevent continuous cycling on and off. This type of controller can’t be used for precise temperature control. Often used for systems with a large thermal mass (where temperatures take a long time to change) and for alarms.
Proportional controllers n Proportional controllers – Power can be varied. For example, in a heating unit the average power supplied will decrease the closer one gets to the set point. – Power is often varied by turning the controller on and off very quickly rather than using a VFD – Some proportional controllers use proportional analog outputs where the output level is varied rather than turning the controller on and off.
PID Combines proportional with integral and derivative control. n With proportional control, the temperature usually stabilizes a certain amount above or below the setpoint. This difference is called offset. n n With integral and derivative control, this offset is compensated for so that you end up at the setpoint. This provides very accurate temperature control, even for systems where the temp. is changing rapidly.
How to Choose a Temperature Control Device or System n Things to take into account – – – – – Standards Cost Accuracy Stability over time (esp. for high temperatures) Sensitivity Size Contact/non-contact Temperature range Fluid
International Standards n North America – NEMA (National Electrical Manufacturers Association), UL (Underwriters Laboratories), CSA (Canadian Standards Association
Enclosure Ratings n n n n Type 1 – general purpose indoor enclosure to prevent accidental contact Type 2 – indoor use, provides limited protection from dirt and dripping water Type 3 – outdoor use to protect against wind-blown dust, sleet, rain, but no ice formation Type 3 R – outdoor use to protect against falling rain but no ice formation Type 4 – add splashing or hose-directed water to 3 Type 4 x – add corrosion Type 6 – add occasional submersion to 4 x etc.
Choice Between RTDs, Thermocouples, Thermisters n n n n Cost – thermocouples are cheapest by far, followed by RTDs Accuracy – RTDs or thermisters Sensitivity – thermisters Speed - thermisters Stability at high temperatures – not thermisters Size – thermocouples and thermisters can be made quite small Temperature range – thermocouples have the highest range, followed by RTDs Ruggedness – thermocouples are best if your system will be taking a lot of abuse
Simplified Uncertainty Analysis for Lab 1 n Random (precision) error – For temperature measurements, this typically includes fluctuations in the electronics of the data acquisition units as well as fluctuations in the quantities measured n Bias (fixed) error – For temperature measurements, this typically includes the finite resolution of the A/D card (if one is used), the use of a curve fit for thermocouples, reading of calibration thermometers, and conduction and radiation errors. n Total uncertainty is found using the root mean square of these two errors
Random Error n 95% confidence interval – 95% of temperature readings will fall in this range – =+/- 2 standard deviations – For your lab, during calibration, take at least 35 data points (N=35) at one temperature. Then calculate the average and standard deviation using the equations below. – Excel can also be used.
Bias Error Conduction and radiation errors should be negligible. n For our lab, we will do a simplified analysis. n Once you have a calibration curve fit, find the deviation between the curve fit and each data point. Use the magnitude of the maximum deviation as your bias error. n In ME 120 you’ll learn a lot more about calculating uncertainties! n