1 Heating temperature and pressure test Thermistors are inexpensive, easily-obtainable temperature sensors. They are easy to use and adaptable. Circuits with thermistors can have reasonable outout voltages - not the millivolt outputs thermocouples have. Because of these qualities, thermistors are widely used for simple temperature measurements. Theyre not used for high temperatures, but in the temperature ranges where they work they are widely used. Thermistors are temperature sensitive resistors. All resistors vary with temperature, but thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature. However, unlike most other resistive devices, the resistance of a thermistor decreases with increasing temperature. Thats due to the properties of the semiconductor material that the thermistor is made from. For some, that may be counterintuitive, but it is correct. Here is a graph of resistance as a function of temperature for a typical thermistor. Notice how the resistance drops from 100 kW, to a very small value in a range around room temperature. Not only is the resistance change in the opposite direction from what you expect, but the magnitude of the percentage resistance change is substantial. Temperature Sensor - The Thermocouple You are at: Elements - Sensors - Thermocouples Return to Table of Contents A thermocouple is a junction formed from two dissimilar metals. Actually, it is a pair of junctions. One at a reference temperature (like 0 oC) and the other junction at the temperature to be measured. A temperature difference will cause a voltage to be developed that is temperature dependent. (That voltage is caused by something called the Seebeck effect.) Thermocouples are widely used for temperature measurement because they are inexpensive, rugged and reliable, and they can be used over a wide temperature range. 2 In particular, other temperature sensors (like thermistors and LM35 sensors) are useful around room temperature, but the thermocouple can The Thermocouple Why Use thermocouples To Measure Temperature? They are inexpensive. They are rugged and reliable. They can be used over a wide temperature range. What Does A Thermocouple Look Like? Here it is. Note the two wires (of two different metals) joined in the junction. What does a thermocouple do? How does it work? The junction of two dissimilar metals produces a temperature dependent voltage. For a better description of how it works, click here. How Do You Use A Thermocouple? You measure the voltage the thermocouple produces, and convert that voltage to a temperature reading. It may be best to do the conversion digitally because the conversion can be fairly nonlinear. Things You Need To Know About Thermocouples A junction between two dissimilar metals produces a voltage. In the thermocouple, the sensing junction - produces a voltage that depends upon temperature. Where the thermocouple connects to instrumentation - copper wires? - you have two more junctions and they also produce a temperature dependent voltage. Those junctions are shown inside the yellow oval. When you use a thermocouple, you need to ensure that the connections are at some standard temperature, or you need to use an electronically compensated system that takes those voltages into account. If your thermocouple is connected to a data acquisition system, then chances are good that you have an electronically compensated system. Once we obtain a reading from a voltmeter, the measured voltage has to be converted to temperature. The temperature is usually expressed as a polynomial function of the measured voltage. Sometimes it is possible to get a decent linear approximation over a limited temperature range. There are two ways to convert the measured voltage to a temperature reading. Measure the voltage and let the operator do the calculations. Use the measured voltage as an input to a conversion circuit - either analog or digital. Let us look at some 3 other types of base-metal thermocouples. Type T thermocouples are widely used as are type K and Type N. Type K (Ni-Cr/Ni-Al) thermocouples are also widely used in the industry. It has high thermopower and good resistance to oxidation. The operating temperature range of a Type K thermocouple is from -269 oC to +1260 oC. However, this thermocouple performs rather poorly in reducing atmospheres. Type T (Cu/Cu-Ni) thermocouples can be used in oxidizing of inert atmospheres over the temperature range of -250 oC to +850 oC. In reducing or mildly oxidizing environments, it is possible to use the thermocouple up to nearly +1000 oC. Type N (Nicrosil/Nisil) thermocouples are designed to be used in industrial environments of temperatures up to +1200 oC. A polynomial equation used to convert thermocouple voltage to temperature (oC) over a wide range of temperatures. We can write the polynomial as: The coefficients, an are tabulated in many places. Here are the NBS polynomial coefficients for a type K thermocouple. (Source: T. J. Quinn, Temperature , Academic Press Inc.,1990) Type K Polynomial Coefficients n an 0 0.226584602 1 24152.10900 2 67233.4248 3 2210340.682 4 -860963914.9 5 4.83506x1010 6 -1.18452x1012 7 1.38690x1013 8 -6.33708x1013 What If The Surrounding Temperature Exceeds Limits? There are really no thermocouples that can withstand oxidizing atmospheres for temperatures above the upper limit of the platinum-rhodium type thermocouples. We cannot, therefore, measure temperature in such high temperature conditions. Other options for measuring extremely high temperatures are radiation or the noise pyrometer. For non-oxidizing atmospheres, tungsten-rhenium based thermocouples shows good performance up to +2750 oC. They can be used, for a short period, in temperatures up to +3000 oC. The selection of the types of thermocouple used for low temperature sensing is primarily based on materials of a thermocouple. In addition, thermopower at low temperatue is 4 rather low, so measurement of EMF will be proportionally small as well. More Facts On Various Thermocouple Types A variety of thermocouples today cover a range of temperature from -250 oC to +3000 oC. The different types of thermocouple are given letter designations: B, E, J, K, R, S, T and N Types R,S and B are noble metal thermocouples that are used to measure high temperature. Within their temperature range, they can operate for a longer period of time under an oxidizing environment. Type S and type R thermocouples are made up of platinum (Pt) and rhodium (Rh) mixed in different ratios. A specific Pt/Rh ratio is used because it leads to more stable and reproducible measurements. Types S and R have an upper temperature limit of +1200 oC in oxidizing atmospheres, assuming a wire diameter of 0.5mm. Type S and type R thermocouples are made up of platinum (Pt) and rhodium (Rh) mixed in different ratios. A specific Pt/Rh ratio is used because it leads to more stable and reproducible measurements. Types S and R have an upper temperature limit of +1200 oC in oxidizing atmospheres, assuming a wire diameter of 0.5mm. Type B thermocouples have a different Pt/Rh ratio than Type S and R. It has an upper temperature limit of +1750 oC in oxidizing atmospheres. Due to an increased amount of rhodium content, type B thermocouples are no quite so stable as either the Type R or Type S. Types E, J, K, T, and N are base-metal thermocouples that are used for sensing lower temperatures. They cannot be used for sensing high temperatures because of their relatively low melting point and slower failure due to oxidation. Type B thermocouples have a different Pt/Rh ratio than Type S and R. It has an upper temperature limit of +1750 oC in oxidizing atmospheres. Due to an increased amount of rhodium content, type B thermocouples are no quite so stable as either the Type R or Type S. we will look into some differences between different base-metal thermocouples. Type E (Ni-Cr/Cu-Ni) thermocouples have an operating temperature range from -250 oC to +800 oC. Their use is less widespread than other base-metal 5 thermocouples due to its low operating temperature. However, measurements made by a Type E have a smaller margin of error. 1000 hours of operation in air of a Type E thermocouple at +760 oC, having 3mm wires, shold not lead to a change in EMF equivalent to more than +1 oC. Type J (Fe/Cu-Ni) thermocouples are widely used in industry due to their high thermopower and low cost. This type of thermocouple has an operating temperature range from 0 oC to +760 oC. Links to Related Lessons Temperature Sensors Thermistors Thermocouples LM35s Other Sensors Strain Gages Temperature Sensor Laboratories Return to Table of Contents Experiments With Temperature Sensors - Data Gathering Measuring temperature is the most common measurement task. There are numerous devices available for measuring temperature. Many of them are built using one of these common sensors. Thermistor Thermocouple LM35 Integrated Circuit Temperature Sensor You can get more information about these sensors by clicking the links above. Laboratory The purpose of this laboratory is to get time response data for the three sensors you were introduced to labs week. Here are links to LabVIEW programs you can use. NTempsHydra.vi - to measure temperature from the Hydra. NVoltsHydra.vi - to measure voltage from the Hydra. ResetHydra.vi - A sub-vi you need to reset the Hydra. 1Temp.vi - A sub-vi that will take one temperature measurement on the Hydra. 1VoltHydra.vi - A sub-vi that will take one voltage measurement on the Hydra. You should have all the files above on your desktop. You can click on each link and save to the desktop, or you can find the NMeas folder in my public space and copy the entire folder to the desktop (best). You only need to double click the NTemps or NVolts files to start and run them in LabVIEW - but they have to be taken out of the network folder! Once you have the files together in a single folder on 6 your desktop, Start NTempsHydra.vi to measure temperature using the thermocouple attached to terminals 21 (yellow lead) and 22 (red lead). Note that these terminals (21 and 22) are the connections for channel 1 for the Hydra. (For example, if you were doing a manual temperature reading using the front panel, you would need to set to channel 1.) You need to connect the yellow lead of the thermocouple to the top connector for Channel #1 (Terminal #21) and the red lead of the thermocouple to the bottom connector (ground?) for Channel #1 (Terminal #22). Both of those connections are made to the connector strip on the top of the Hydra Data Acquisition Unit. Start NVoltsHydra.vi to measure voltages using the LM35 and the voltage divider circuit for the thermistor. Both sets of measurements should be taken from the front panel connection points on the Hydra. For both the LM35 and the thermistor circuit, you need to supply 5v to the circuit board. In your lab notebook record any circuitry you use, and any pertinent points regarding the equipment you use. Note any other features of each sensor that will help you for your project or make things more difficult. Do the following: Connect each sensor. Here are links to using each sensor in a measurement. Thermocouples LM35s Thermistors For each sensor you need to get data in two situations: As the sensor heats up (rising time constant behavior) As the sensor cools down to ambient temperature (decaying time constant behavior) That data should be stored in a computer file. Use a different, understandable name for each file. The program will prompt you for a file name. Suggested file names are things like ThermistorUp.txt, etc. Before you leave lab be sure that you can bring your data up in Excel (to test that you have a good data file) and that you can plot the data to see that it looks like what you expect. Estimate the following for each sensor. The time it will take for the sensor to get within 1oC when the sensor is in good thermal contact with the temperature environment being measured and the temperature sensor starts at 25 oC and goes to 50 oC. (That means to 7 measure the time it takes to get to between 49 oC and 51 oC.) The time it will take for the sensor to get within 1oC of the final value when the sensor is in air at a constant temperature and the temperature sensor starts at 25oC and goes to 50oC. In other words, when will the temperature sensor reach 49oC? The time it will take for the sensor to get within 0.1oC for the two situations above. (i.e., between 49.9 oC and 50.1 oC.) The time it will take for the sensor to get within 1oC when the sensor is in good thermal contact with the temperature environment being measured and the temperature sensor starts at 50 oC and goes to 25 oC. Explain why there is a difference in the speed of the response in the various situations above. Your report should show calculations for the time constant(s) for each device, and should show the results using the three methods. Tabular presentation of the results is best. Finally, you should - as best possible - explain your results. Why would the time constant be different going up and going down. 8 供熱站溫度壓力實(shí)時(shí)檢測(cè) 熱敏電阻很便宜，易于得到的溫度傳感器。它們易于使用和適應(yīng)性。與熱敏電阻的電路可以有合理 outout電壓 - 而不是產(chǎn)出熱電偶毫伏的。 由于這些優(yōu)勢(shì)，熱敏電阻廣泛用于簡(jiǎn)單的溫度測(cè)量。 它們不是用于高溫，但在他們的工作溫度范圍在那里，他們被廣泛使用。 熱敏電阻溫度敏感電阻器。 所有的電阻隨溫度，但熱敏電阻的半導(dǎo)體材料建造，是一個(gè)電阻對(duì)溫度特別敏感。但是，與大多數(shù)其他電阻器，熱敏電阻的一個(gè)隨溫度降低。 這是由于半導(dǎo)體材料制成的熱敏電阻的特性。一些人認(rèn)為，這可能是違反直覺，但它是正 確的。 這里是一個(gè)電阻作為溫度函數(shù)的圖像為一典型的熱敏電阻。請(qǐng)注意阻力從 100千瓦下降到一個(gè)很小的值在 1左右房間溫度范圍。 不僅是從你期望相反的方向電阻的變化，但其電阻變化率的幅度是可觀的。 溫度傳感器 - 熱電偶您現(xiàn)在的位置：元素 - 傳感器 - 熱電偶返回目錄熱電偶表是由兩個(gè)不同的金屬組成的交界處。 其實(shí)，這是一個(gè)路口對(duì)。一位在參考溫度 0 oC的像（）和其他交界處的溫度進(jìn)行測(cè)量。 阿溫差會(huì)引起電壓要發(fā)展是溫度而定。 （該電壓是由一些被稱為塞貝克效應(yīng)引起的。）熱電偶廣泛用于溫度測(cè)量，因?yàn)樗鼈儍r(jià)格便宜，堅(jiān)固可靠 ，而且可以在很寬的溫度范圍內(nèi)使用。特別是，其他如熱敏電阻溫度傳感器和傳感器 LM35（）周圍室溫有用的，但為什么熱電偶熱電偶可以使用熱電偶測(cè)量溫度？ 他們是廉價(jià)的。他們是堅(jiān)固，可靠。它們可用于在很寬的溫度范圍。熱電偶是什么樣子？在這里。注意兩條線兩種不同的金屬（）參加了路口。熱電偶是什么呢？它是如何工作的？?jī)煞N不同的金屬交界處產(chǎn)生電壓的溫度依賴性。對(duì)于如何更有效的描述，請(qǐng)點(diǎn)擊這里。你是如何利用熱電偶？您測(cè)量熱電偶產(chǎn)生的電壓，并轉(zhuǎn)換成的電壓，溫度讀數(shù)。這可能是最好的進(jìn)行轉(zhuǎn)換，因?yàn)檗D(zhuǎn)換的數(shù)字可以相當(dāng)非線性的。事情你 需要知道關(guān)于熱電偶的兩種不同金屬之間的交界處產(chǎn)生電壓。在熱電偶，傳感交界處 - 產(chǎn)生的電壓時(shí)的溫度而定。凡熱電偶連接到儀器 - 銅導(dǎo)線？ - 你有兩個(gè)路口，同時(shí)也產(chǎn)生電壓的溫度依賴性。顯示這些路口內(nèi)的黃色橢圓形。當(dāng)您使用熱電偶時(shí)，您需要確保連接是在一些標(biāo)準(zhǔn)的溫度，或者你需要使用一個(gè)電子補(bǔ)償系統(tǒng)，考慮到這些電壓。如果您的熱電偶連接到數(shù)據(jù)采集系統(tǒng)，然后有很好的機(jī)會(huì)，你有一個(gè)電子補(bǔ)償制度。一旦我們獲得了由電壓表讀數(shù)，測(cè)得的電壓必須轉(zhuǎn)換為溫度。溫度通常表示為多項(xiàng)式函數(shù)的測(cè)量電壓。有時(shí)有可能獲得超過一有限的溫度范圍內(nèi) 像樣的線性近似。有兩種方法來測(cè)量電壓轉(zhuǎn)換為溫度讀數(shù)。測(cè)量電壓，讓經(jīng)營(yíng)者做計(jì)算。以此作為一個(gè)轉(zhuǎn)換電路 - 無論是模 9 擬或數(shù)字輸入電壓的測(cè)量。讓我們看看基本金屬熱電偶一些其他類型。 T型熱電偶被廣泛用作是 K型和 K型型北路（ Ni-Cr/Ni-Al）熱電偶也廣泛應(yīng)用在工業(yè)中使用。它具有較高的熱電勢(shì)和良好的抗氧化作用。一個(gè) K型熱電偶的工作溫度范圍為攝氏從 -269 1260 。然而，這種熱電偶表現(xiàn)不佳而在減少大氣。 T型（銅 /銅鎳）熱電偶可以在惰性氣氛氧化以上的 -250 oC的溫度范圍為 850 oC的使用。在減少或輕度氧 化環(huán)境中，可以使用熱電偶至接近 1000 。 N型（ Nicrosil / Nisil）熱電偶的設(shè)計(jì)可在溫度高達(dá) 1200 oC的工業(yè)環(huán)境下使用。一個(gè)多項(xiàng)式方程用于熱電偶電壓轉(zhuǎn)換溫度（ ）以上的溫度范圍。我們可以寫多項(xiàng)式為：系數(shù)，一個(gè)是在許多地方表列。下面是一個(gè) K型熱電偶國(guó)家統(tǒng)計(jì)局多項(xiàng)式系數(shù)。 （來源：勺奎恩，溫度，學(xué)術(shù)出版社有限公司， 1990年） K型 多項(xiàng)式系數(shù) 1 1 0 0.226584602 24152.10900 2 67233.4248 3 2210340.682 4 -860963914.9 5 4.83506x1010 6 1.18452x1012 7 1.38690x1013 8 6.33708x1013什么如果周邊溫度超過限制？真的有沒有能抵御氧化熱電偶以上的鉑銠熱電偶型大氣溫度的上限。我們不能，因此，在如此高的溫度測(cè)量環(huán)境溫度。測(cè)量溫度非常高輻射或其他選項(xiàng)的噪音高溫計(jì)。對(duì)于非氧化性氣氛，鎢錸熱電偶具有良好的性能高達(dá) 2750 。他們可以使用，很短的時(shí)間，在溫度高達(dá) 3000 。作者：熱電偶溫度傳感器采用低類型的選擇主要是基于一個(gè)熱電偶材料。此外，在低受熱熱電相當(dāng)?shù)?，所以?EMF測(cè)量比例太小了。更多的 事實(shí)在不同類型的熱電偶熱電偶品種涵蓋了從今天 -250 oC的溫度范圍為 3000 。不同類型的熱電偶，給出英文字母代號(hào)：二，電子，強(qiáng)，鉀，俄，西， T和 N型度 R， S和 B是貴金屬，用于測(cè)量高溫?zé)犭娕?。其溫度范圍?nèi)，他們可以經(jīng)營(yíng)了一段較長(zhǎng)時(shí)間的氧化環(huán)境下。 S型和 R型熱電偶是由白金注冊(cè)（鉑）和銠（銠）在不同的比例混合。一個(gè)具體的鉑 /銠比為使用，因?yàn)樗鼤?huì)導(dǎo)致更加穩(wěn)定和可重復(fù)性測(cè)量。 S型和 R在氧化氣氛的 1200 oC的溫度的上限，假設(shè)一個(gè)線徑為 0.5mm。 S型和 R型熱電偶是由白金注冊(cè)（鉑）和銠（銠）在不同的比例混合 。一個(gè)具體的鉑 /銠比為使用，因?yàn)樗鼤?huì)導(dǎo)致更加穩(wěn)定和可重復(fù)性測(cè)量。 S型和 R在氧化氣氛的 1200 oC的溫度的上限，假設(shè)一個(gè)線徑為 0.5mm。 B型熱電偶具有不同的 Pt / S型和 R比它在氧化氣氛的 1750 oC的溫度的上限 Rh的比例。由于對(duì)銠含量， B型熱電偶增加額沒有這么穩(wěn)定的任何類型的 R型或 E型南，強(qiáng)，鉀， T和 N是基本金屬可用于檢測(cè)用熱電偶溫度較低。它們不能用于檢測(cè)由于其相對(duì)較低的熔點(diǎn)和慢衰竭的高溫氧化。 B型熱電偶具有不同的 Pt / S型和 R比它在氧化氣氛的 1750 oC的溫度的上限 Rh的比例。由于對(duì)銠 含量， B型熱電偶增加額沒有這么穩(wěn)定的無論是 R型或類型號(hào)我們會(huì)研究不同基本金屬熱電偶存在一些分歧。 E型（ Ni-Cr/Cu-Ni）熱電偶從 -250 oC的工作溫度范圍 10 為 800 。它們的使用小于其他基本金屬熱電偶由于其較低的工作溫度普遍。不過，測(cè)量了由 E型有小幅度的誤差。 1000小時(shí)的行動(dòng)中 E型熱電偶在 760 oC的空氣，有 3毫米電線， shold不會(huì)導(dǎo)致在電磁場(chǎng)等效變化超過 1 。 J型（鐵 /銅鎳）熱電偶廣泛應(yīng)用于工業(yè)，由于其高熱電，成本低。這種類型的熱電偶從 0 到 +760 oC的工作溫度范圍。鏈接到溫度傳感器 熱敏電阻熱電偶相關(guān)教訓(xùn) LM35s應(yīng)變計(jì)溫度傳感器其他傳感器實(shí)驗(yàn)室返回目錄 隨著溫度傳感器實(shí)驗(yàn) - 數(shù)據(jù)采集測(cè)量溫度是最常見的測(cè)量任務(wù)。有許多設(shè)備可以測(cè)量溫度。他們中許多人都使用這些共同傳感器之一。熱敏電阻熱電偶溫度傳感器 LM35集成電路您可以通過點(diǎn)擊獲得更多關(guān)于上面的鏈接這些傳感器的信息。實(shí)驗(yàn)室這個(gè)實(shí)驗(yàn)室的目的是能為你介紹了三個(gè)星期，以實(shí)驗(yàn)室傳感器時(shí)間響應(yīng)數(shù)據(jù)。以下是鏈接到 LabVIEW的程式可以使用。 NTempsHydra.vi - 測(cè)量溫度從九頭蛇。 NVoltsHydra.vi - 衡量從九頭蛇 電壓。 ResetHydra.vi -