Temperature calibration 电路图.pdf

Application Note Temperature measurement and calibration: What every instrument technician should know F ro m t h e F l u k e D i g i t a l L i b r a r y w w w. f l u k e . c o m / l i b r a r y Introduction Temperature may be the most commonly measured physical parameter. Yet there have never been so many ways to measure it as there are today. With so many options its natural to have a few questions. How do I measure temperature? How accurate is my measurement? What temperature range is required? What type of device best measures tempera- ture? Does my instrument require certification? These are very common ques- tions when confronted with the need to measure temperature. A variety of measurement devices may be used for temperature; liquid-in-glass thermometers (LIG), thermocouples (TCs), thermistors, resistance tempera- ture detectors (RTDs), platinum resistance thermometers (PRTs) and standard platinum resistance thermometers (SPRTs). This appli- cation note focuses on electronic temperature measurements, and helps clarify the answers to some of these nagging questions. More information on these subjects is available at . How do I measure temperature? After inserting a temperature sensor into the area to be measured, it takes time before the temperature reading is stabilized. For the thermom- eter to stabilize at the correct temperature, the probe must be sufficiently immersed. Some thermometers require more immersion depth than others. Most precision thermometers require four to six inches when inserted into a liquid or snug fitting well, depending on the diameter of the probe. Best results in terms of accuracy and stabilization time occur when the probe can be inserted into a stirred liquid. Air pockets between probes and solid sur- faces lead to longer stabilization times and require more immer- sion than would be required in a liquid. Specialized thermom- eters are needed for measuring temperatures on surfaces and for situations where the probe cable will be exposed to extreme temperatures. Often devices that measure and display temperature need to be verified or calibrated against a reference thermom- eter. Accuracy is improved when the distance between the two thermometers is reduced. A best practice is to align the centers of the sensing elements of the reference thermometer and the device under test. Be aware that the location of the center of the sensor depends on the sensor type and model (i.e. PRT, ther- mocouple, bimetallic). A common method of calibrat- ing temperature sensors is to remove them from where they are installed and place them in a dry-well calibrator or a Micro- Bath. These calibrators provide a stable temperature environment over a range of temperatures to compare the thermometer under test to the calibrator display or to a reference thermometer for more accuracy. Alternatively, temperature sensors may be calibrated or verified without RadioFans.CN 收音机爱 好者资料库 2 Fluke Calibration Temperature measurement and calibration: What every instrument technician should know removing them from their installed location. Usually this is done by inserting a reference thermometer into a thermowell, immersion well, or thermometer pocket installed next to the ther- mometer to be tested. In other cases the sensing element of the reference thermometer must be placed inside of the freezer, oven, or environmental cham- ber being verified, calibrated or adjusted. In these cases it is often necessary to record data over a period of time such as a few hours to verify performance. Statistics such as average value, maximum and minimum, or standard deviation are some- times recorded. Testing the energy perfor- mance of steam systems, cooling towers, heat exchangers, and refrigeration systems, turbines, and internal and external combustion engines requires measuring differences between inlet and outlet temperatures. Sometimes these measurements have to be made from outside the pipe using thermocouples, thin-film sensors, or infrared temperature measurements. However, the best accuracy will be achieved when a thermowell has been properly installed in both the inlet and outlet pipes so that a probe can be inserted and sufficiently immersed. Because pipe diameters are sometimes a limiting factor for immersion, the best location for a thermowell is at an elbow in the piping so that the probe can be inserted parallel to fluid flow with as much immer- sion depth as needed. How much accuracy is needed? Decisions about accuracy should be made carefully. Inaccuracy leads to mistakes and mistakes cost money. Mistakes may lead to down time, excessive energy costs, high product defect rates, safety hazards, and public health threats. Thermometers are specified by design engineers for tempera- ture monitoring or control. These specifications should include the accuracy of the thermom- eters. A design engineer, quality engineer or metrologist should also specify the calibration requirements. However, it is not uncommon for instrument tech- nicians to receive a calibration job and little or no information about calibration requirements. A common calibration strategy is to reduce mistakes by keeping the uncertainty of the calibration standards to a low percentage of the accuracy of the thermom- eter under test. This percentage is usually described as a Test Uncertainty Ratio (TUR). For example, the 4:1 TUR used by the military and other industries keeps the collective uncertainty of the calibration standards to 25 % of the thermometer under test accuracy. For comparison, a TUR of 2:1 means that the uncertainty is 50 % of the thermometer accuracy, and if the reference thermometer has the same accuracy as the thermom- eter under test, then the TUR is 1:1. The latter TUR is never recommended for calibration and would produce unreliable results. With a more accurate calibra- tion standard you can identify more actual out-of-tolerance field devices. Table 1 illus- trates the expected frequency of making mistakes at various TURs. Table 1 is based on a scenario where 950 of 1,000 instruments are truly in toler- ance. For example, if all 1,000 are calibrated with a 2:1 TUR then we expect that 926 will be found in tolerance (accepted), 12 of which are truly out of toler- ance (false accept). Of the 74 expected to be rejected, 41 are expected to be truly in tolerance (false reject). The cost incurred for each of those falsely rejected instruments could range from $50/each for a calibration house to $10,000/each in down time in the chemical process industry. Temperature rangeAccuracyCost Noble-metal thermocouples (Special tolerances) R, S: 50 C to 1760 C 0.6 CMed Base-metal thermocouples (Special tolerances) B: 0 C to 1820 C E: 270C to 1000 C J: 210 C to 1200 C K: 270 C to 1370 C N: 270 C to 1300 C T: 270 C to 400 C 0.25 % 1 C 1.1 C 1.1 C 1.1 C 0.5 C Low Low Low Low Low Low PRTs and SPRTsIndustrial: 80 C to 480 C Reference: 200 C to 660 C High temp: 0C to 1000 C 0.05 - 0.1 C 0.001 0.02 C 0.01 0.02 C Low - Med Med - High Med High Precision thermistors0 C to 100 C0.002 CMed Table 2. Temperature sensor tradeoffs among temperature range, accuracy and cost. The most accurate sensors are the most expensive. Often accuracy is sacrificed for wider temperature range. TURAcceptedFalse AcceptRejectedFalse Reject 1:184317157128 2:1925127541 3:194195922 4:194785315 Table 1. What-if table summarizing false accept and false reject risk for a hypothetical scenario of 1000 instruments that are truly 95 % in tolerance. A normal distribution without guard-banding is assumed. RadioFans.CN 收音机爱 好者资料库 3 Fluke Calibration Temperature measurement and calibration: What every instrument technician should know Thermometer probe types There have never been as many temperature sensor (probe type) choices available for your mea- surements as there are today. With so many choices, the task can become time consuming and difficult without some help. The most important factors are tem- perature range, accuracy and cost. Table 2, on the previous page, illustrates the tradeoffs among these factors for several thermom- eter types. Thermocouples (TCs) Thermocouples are temperature sensors that measure temperature by generating a small voltage signal proportional to the tem- perature difference between the junctions of two dissimilar metals. One junction (the measurement junction) is typically encased in a sensor probe at the point of measurement; the other junction (the reference junction) is typi- cally connected to the measuring instrument. The measurement instrument measures two things: the voltage signal and the refer- ence junction temperature. From those two things the instrument computes the temperature at the measuring end of the probe. It is important to note that the voltage generated by the sensor is not based on the absolute temperature of the measurement junction, but rather a temperature difference between the measure- ment junction and the reference junction. Thermocouple types are dis- tinguished by the metals used in each leg of the thermocouple. Noble metal thermocouples all contain platinum in one leg of the thermocouple and include Type S, Type R, Au/Pt, and Pt/ Pd. Base metal thermocouples include Type B, Type E, Type J, Type K, Type N, and Type T. These thermocouples come in two accuracy classes: standard limits of error and special limits of error. The special limits of error thermocouples are the most accurate. Letter designated ther- mocouple tables are available from NIST on the web or in NIST monograph 175. A thermocouple voltage and sensitivity calculator is also available on the web at . Reference junction com- pensation is one of the most significant contributors to the accuracy of a thermocouple measurement. Thermocouple tables like those in NIST mono- graph 175 are based on a reference junction temperature of 0 C. Although external refer- ence junctions can be used to achieve this with an ice bath, thermocouple wire is usually connected directly to the ther- mocouple readout binding posts at room temperature. Automatic reference junction compensation is needed to compensate for the deviation from 0 C. A therm- istor bead is usually used to measure the temperature of the junction. The readout measures the resistance of the thermistor and calculates a correction for the thermocouple temperature. In Figure 2, thermocouple wire meets with copper wire at the binding posts of the meter form- ing the reference junction (J). The temperature in the region surrounding the binding posts (TJ ) is usually measured by a thermistor. Automatic refer- ence junction compensation is accomplished by measuring the difference from 0 C at the bind- ing posts (TJ) and compensating for it digitally. The accuracy of this measurement has a signifi- cant impact on the accuracy of the overall temperature mea- surement. Resistance based tem- perature measurement An RTD is a temperature sensing element that relates tempera- ture to its own resistance. There are several kinds of RTDs. RTD sensing elements include coils of Figure 1. Model of a thermocouple circuit where A and B are dissimilar thermocouple wire, T1 represents the temperature at the measurement junction, and T2 repre- sents the temperature at the reference junction. The absolute temperature at T1 does not produce the voltage measured at V; rather, the temperature difference between T1 and T2 produces the measured voltage. A BC C T2 T1 V RadioFans.CN 收音机爱 好者资料库 4 Fluke Calibration Temperature measurement and calibration: What every instrument technician should know + - + - V (T )TCTC V (T )J1J V (T )J2J TJ V =V (T )+V (T )+V (T )oTCTCJ1JJ2J platinum wire (PRT), nickel wire, copper wire, thin films and more. Another resistance based sensor is the thermistor which is made of semi-conducting material. Figure 3 illustrates a simple 2-wire measurement circuit. The sens- ing element is labeled RT. The lead-wires have finite resistances labeled RL1 and RL2. When current passes through the sensor, the environment is going to get a little warmer because of power dissipation. The more resistance or current there is, the more power gets dissipated (P=I2R). The self- heating will be higher in air because the heat will not flow away as efficiently as it would in a stirred fluid. Self-heating errors can be minimized by using the same level of current used during calibration. Using the correct current is particu- larly important in thermistors because they can have very large resistances causing greater self-heating. Current reversal is a very effective technique used in resistance measurements to eliminate errors associated with thermal EMFs. Thermal EMFs are unwanted voltages in a resistance measurement circuit caused by the same principle that produces a voltage in ther- mocouples. The measurement is made with the current flowing in one direction and then again with the current flowing in the other direction. Thermoelectric EMFs are removed by averaging the results of both sets of measurements. This technique used by many modern instru- ments improves measurement stability and reduces significant errors that are common in other instruments. Platinum resistance thermometers A platinum resistance thermom- eter (PRT) element contains coils of highly-pure platinum wire. The resistance of a PRT element varies more linearly with temperature than any other temperature sensor. A Standard Platinum Resistance Thermometer (SPRT) is the most accurate temperature sensor available and is used in National Standards Laboratories and in industry for traceability to the International Temperature Scale of 1990 (ITS-90). The full text of the ITS-90 is available at www.bipm.org. Temperature measurement with a PRT requires correlating the resistance of the sens- ing element with temperature using the correct equations and coefficients. Fortunately, most thermometer readout devices have support for these equa- tions, so the calculations are handled automatically. Examples include ITS-90 equations, Figure 2. Reference junction compensation is one of the most significant contributors to the accuracy of a thermocouple measurement. Caution: some manufacturers may not advertize this important part of their accuracy. RT RL1 RL2 + - Figure 3. Current is passed through the sensing element to produce a voltage measured by a meter. Lead-wire resistance in two-wire measurements causes potentially large temperature measurement errors. Other types of resistance measurements include three-wire and four-wire resistance measurements. Four-wire measurements are preferred in temperature applications because they eliminate lead wire resistance from the measurement. Figure 4. Current is required to measure resistance. Current that passes through a resistance dissipates power and generates heat, causing temperature errors. 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