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ByMaria Knake, activities Assessment routine Manager

Posted: April 2011

Temperature Measurement: A fact of LifeSome of mine fondest memories from childhood room of working in the kitchen v my mother—canning tomatoes, making candy apples, and baking bread. I still remember just how my mom taught me come carefully check the liquid thermometer together we make the caramel coating, and how i learned to collection the cooktop to simply the ideal temperature so the the bread would certainly rise and bake correctly. I never thought about it as a child, but temperature measurement played critical role in all of the cooking and also baking jobs that mine mother and I shared.

You are watching: How do thermometers with liquid in glass measure temperature

I can’t aid but think about what a crucial role temperature dram ineverythingthat we do. We use temperature to help us decision what to wear, just how to prepare our food, diagnose illness, and determine where and when us take vacation. That course, temperature measure plays an essential role in the laboratory too. The physics properties and also characteristics of the materials that us test space influenced, as least in part, through temperature. Inarguably, accurate temperature measurement is one of the most crucial components of activities testing.

Measuring Temperature in the LaboratorySo, exactly how do us measure temperature in the laboratory? Certainly, the liquid thermometer the my mother taught me to use as a kid would not be enough for the critical temperature measurements that are forced for many laboratory tests. Yet there is a myriad of temperature measuring products available—liquid-in-glass thermometers, resistive detectors, thermocouples, thermistors, dial-stem thermometers, infrared thermometers… the list goes on and also on. Which of these tools should be used, and also when? What kind of readability, accuracy, and also uncertainty execute these instruments provide? With all of the devices easily accessible for measuring temperature, it’s no wonder thermometry is such a confound topic. In upcoming posts,I will shot to describe some of these different types of thermometers, how and when to use them, and the various techniques of calibration.

Liquid-in-Glass ThermometersLet’s start our story with among the most usual thermometers supplied today, the liquid-in-glass (LiG) thermometer. A LiG thermometer, through definition, is a glass capillary tube through a liquid-filled bulb at one end. As the temperature of the fluid in the reservoir increases, the expands and also rises right into the capillary tube. The level of the fluid in the column synchronizes to a specific temperature, i beg your pardon is marked on the outside of the glass. The liquid that is consisted of within the thermometer may be one of countless different substances, yet the most common are mercury, toluene (or a comparable organic substance), and low-hazard biodegradable liquids.

Dissecting the LiG ThermometerOk, so perhaps you knew every one of that. But, our story doesn’t finish there. To truly recognize these precision instruments, we an initial must know a little bit an ext about how they work. The glass, materials, and also dimensions the a certain LiG thermometer are carefully engineered to carry out us v the accurate temperature measurements that we count upon. Let’s take it a closer look.

The BulbAs portrayed inFigure 1, the pear of the thermometer is the slim glass reservoir the holds the liquid. The pear is carefully designed to contain a calculation volume of liquid, based upon the length and also diameter that the capillary (or stem), as well as the thermal growth coefficient of the liquid.

Figure 1: Anatomy of a LiG Thermometer

StemThe stem, or capillary, of a LiG thermometer is made of annealed glass. The kind of glass offered is preferred based ~ above the temperature range of the an equipment so regarding minimize the results of expansion and also contraction of the tube. The section of the capillary above the fluid level is often times filled v an inert gas, such together nitrogen, to avoid separation of the liquid obelisk or vaporization that the fluid at the optimal of the column.

Auxiliary ScaleSome thermometers, however not all, room equipped with an auxiliary scale, i m sorry is located well listed below the main scale the is employed during normal use. Often times, this scale contains an ice point reference that deserve to be supplied for calibration purposes if this temperature is not had within the variety of the main scale.

Contraction ChamberOccasionally, a LiG thermometer will have a convulsion chamber that is located just listed below the main scale that the device. The purpose of this chamber is come shorten the full stem size needed to with the main scale.

Expansion ChamberAn expansion chamber is provided at the finish of LiG thermometers, and also is used to stop the buildup of press if the temperature of the liquid rises past the top of the scale. Again, the volume of this chamber is carefully designed come contain a specific volume of liquid.

Mercury and Mercury-Thallium ThermometersFor years mercury thermometers were a mainstay in countless testing laboratories. If offered properly and calibrated correctly, certain types of mercury thermometers can be very accurate. Mercury thermometers deserve to be used in temperatures varying from around -38 to 350°C. The use of a mercury-thallium mixture can prolong the low-temperature functionality of mercury thermometers come -56°C. Timeless mercury LiG thermometers are identified in detail in ASTM Specification E 1,Specification because that ASTM Liquid-in-Glass Thermometers.

In current years, pertains to over the toxicity the mercury have actually caused many states to prohibit or limit the usage of mercury-containing devices. In fact, among the world’s leading institutions on temperature measurement, the nationwide Institute of criter and technology (NIST), freshly announced that they will certainly no longer administer calibration solutions for mercury thermometers. To know an ext about mercury palliation initiatives, please view my article,“Getting Rid that Mercury: A new Frontier because that Temperature Measurement.”

Nevertheless, couple of liquids have actually been found to mimic the thermometric nature of mercury in repeatability and accuracy of temperature measurement. Toxicity though it may be, as soon as it pertains to LiG thermometers, mercury is still difficult to beat.

Spirit-Filled LiG ThermometersSpirit-filled thermometers save toluene, alcohol, butane, or other similar organic liquids colored through a red dye. These gadgets are not frequently used for laboratory testing and other precision applications. When the substances included in these varieties of LiG thermometers are fairly harmless and also safe for laboratory use, they suffer from troubles with accuracy and reliability. The short surface stress and anxiety of these liquids, as well as their propensity because that vaporization, makes them improbable candidates for general laboratory use.

Organic liquids generally have inferior power to mercury, and also can leaving a film on the glass together the fluid drains down the capillary wall. Separation that the liquid column is also known to it is in a typical problem v spirit-filled thermometers.In addition, they often tend to have actually a better sensitivity to transforms in stem temperature, i beg your pardon is a basic limitation in your use.These thermometers also have various capillary and bulb dimensions than mercury LiG thermometers, causing differences in solution time and also immersion characteristics.

Spirit-filled thermometers are supplied in some low-temperature applications, as they can be used at temperatures as low together -200°C, i beg your pardon is well beyond the capability of mercury or mercury-thallium thermometers. ASTM E 1 covers specifics concerning spirit-filled LiG thermometers. Any type of thermometers described in ASTM E 1 which have to contain toluene or other perfect liquids space specifically designated as such. At the moment that this post was written, ASTM E 1 only includes specifications for 2 spirit-filled thermometers. These thermometers room specifically designed for usage at extreme-cold temperature at which it is no feasible to usage mercury.

Low-Hazard Precision LiG ThermometersLow-hazard precision thermometers have actually been emerged in current years together an alternative to mercury LiG thermometers. They contain non-toxic, biodegradable fluids of i m sorry the contents and also chemical makeup are typically proprietary and also undisclosed by their manufacturers. Ideally, they can be used as straight replacements for mercury-containing ASTM thermometers. However, these types of thermometers have some serious restrictions that need to be considered before use as a direct replacement for mercury LiG thermometers, such together those explained in ASTM E 1. ASTM has developed a specification because that low-hazard precision LiG thermometers, ASTM E 2251,Specification for Liquid-in-Glass ASTM Thermometers v Low-Hazard Precision Liquids. This standard provides certain details on proper use of this devices, the repeatability of your measurements, and also other limitations.

The thermal growth properties of the non-toxic fluids used in low-hazard precision LiG thermometers can be quite various from the of mercury. The bulb and capillary size required to achieve a comparable movement follow me the thermometer scale deserve to vary from the of its mercury counterpart. The surface stress of this liquids different from mercury, causing distinctions in the meniscus. In addition, low-hazard precision liquids tend to reaction to readjust in temperature in ~ a rate various from that of mercury, and should no be provided when rate of increase or various other time-temperature relationship are crucial part the the test procedure. When these gadgets are an excellent alternatives for part applications, their variety in use is rather limited.

Immersion DepthAs we’ve learned, LiG thermometers are as complicated and elaborate as the tests the we use them for. But, the intricacies don’t finish there. Within the category of LiG, there are three types of thermometers typically used in laboratory testing—partial immersion, complete immersion, and complete immersion. Each of these species of thermometer is calibrated differently and is plan for various uses in laboratory testing. See figure 2 because that a visual explanation of each type of thermometer.


Figure 2: Immersion depth for LiG Thermometers

Total Immersion ThermometersTotal immersion thermometers room designed to read correctly when the bulb and also the part of the stem that the machine filled with liquid are immersed in the tool to be measured. In various other words, this form of thermometer need to be immersed to the test temperature. The section of the stem containing the meniscus should remain outside of the check medium. Immersion the the meniscus can cause an overabundance of gas push that may damage the device, or cause distillation of the liquid, i beg your pardon may reason inaccurate readings (in addition to make the thermometer difficult to read). Once using full immersion LiG thermometers, that is acceptable to leave around 1 centimeter of the liquid obelisk exposed.

Total immersion thermometers are frequently used in constant-temperature baths as a method of surveillance the bath’s temperature. For example, complete immersion thermometers are offered in kinematic and absolute viscosity baths.

Partial Immersion ThermometersPartial immersion thermometers space designed come read correctly if the stem of the thermometer is immersed to a specific depth. This depth is usually marked on the device. The component of the stem that is not exposed come the test medium, commonly referred to together the emergent stem, is not kept in a temperature-controlled environment. Therefore, thermal development of the fluid in the emergent stem is extremely unpredictable, and may lead to inaccuracies in the temperature measurement. Consequently, partial immersion thermometers often tend to have actually a higher calibration uncertainty than your total and complete immersion counterparts.

Partial immersion thermometers are typically used in applications where complete immersion thermometers are impractical or difficult to use. Because that instance, if the depth of a temperature bath is only 100 mm, a total immersion thermometer with a size of 300 mm is difficult to immerse correctly. In this case, a partial immersion thermometer with an immersion depth of 76 mm is a much better choice. In addition, if a rapid, one-time temperature measure is required, such as in soil specific gravity or hydrometer testing, a partial immersion thermometer functions best.

Complete Immersion ThermometersComplete immersion thermometers space designed to check out correctly once the entire maker is immersed in the check medium. The use of complete immersion thermometers in the United claims is fairly uncommon. There room no complete immersion thermometers defined in ASTM E 1 or ASTM E 2251.

What’s the huge Deal?Perhaps you’ve currently realized that you’ve been using one or much more of your LiG thermometers improperly. Maybe you room thinking to yourself “No big deal, how much error might this add to mine measurements, anyway? It more than likely doesn’t even make a difference.” Think again. Civilization are frequently quite surprised when they discover out just exactly how much error improper usage of a LiG thermometer imposes on your measurements. Let me explain.

Immersion depth plays vital role in how the liquid within of the an equipment will react. If the section of the thermometer containing mercury is intended to it is in submerged in the test tool (i.e. A complete immersion thermometer), but is left exposed, the liquid will certainly not behave as intended. The error incurred deserve to vary greatly, and is dependent top top the temperature scale of the thermometer, the form of liquid used, and also the emergent stem temperature. It is possible to acquire errors as huge as several degrees by not correct immersion of a LiG thermometer. This errors usually tend come be bigger with spirit-filled devices than v mercury-filled ones.

It is possible to apply a correction because that intentional immersion of either a total or partial immersion thermometer at a point other than that which it was designed for. ASTM E 77,Test technique for Inspection and Verification the Thermometers, explains procedures that might be provided to calculation these corrections. Correction cannot be made for complete-immersion thermometers that space immersed improperly.

Sometimes being Wrong is RightTo confused the problem further, there are numerous test requirements which contact for the usage of a liquid-in-glass thermometer to be provided incorrectly. In these cases, that is important to use the thermometer as described in the test procedure, also if it is technically incorrect. If the thermometer is being supplied incorrectly, it’s essential that it is provided the same method by everybody that runs the test. In other words, everyone should use the device wrong in the appropriate way. Table 1 shows a perform of typical ASTM and AASHTO test methods that call for the use of liquid-in-glass thermometers.

ParallaxYet an additional cause the error in taking dimensions from LiG thermometers is due to the impacts of parallax. Parallax is a phenomenon that occurs as soon as the thermometer is not viewed with the eye level with optimal of the mercury column. Differences in the angle from i beg your pardon the top of the tower is viewed can reason the mercury obelisk to appear either higher or reduced in the capillary than it actually is (See figure 3). To prevent parallax, constantly keep your eyes at the same level as the mercury column. If a thermometer is particularly challenging to read, a magnifying glass, telescope, or comparable optical machine can aid in staying clear of the affect of parallax top top temperature measurements.


Figure 3: The impacts of Parallax

In ConclusionFrom pear to development chamber, LiG thermometers are intricately-made gadgets that can create accurate and effective temperature measurements. I hope that you’ve learned a little about the anatomy of these amazing instruments, as well as how to usage them for precise temperature measurement. In my next post, I will “dissect” some other species of temperature measuring devices and discuss their use in activities testing.

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What a Minute… What about Calibration???I knew you to be going come ask that question. You’re for sure right— calibration is a an important component of systematic temperature measurement. However, i cannot execute the subject of thermometer calibration justice within the boundaries of this article. Such crucial topic deserves an short article all that is own. I’ll cover this topic in a separate, future post, so stay tuned!


ASTM International, “ASTM E 1, traditional Specification for ASTM Liquid-in-Glass Thermometers,”Book of Standards,Volume 14.03, 2007. ASTM International, “ASTM E 77, typical Test method for Inspection and also Verification that Thermometers,”Book the Standards,Volume 14.03, 2007. ASTM International, “ASTM E 2251, typical Specification for Liquid-in-Glass ASTM Thermometers through Low-Hazard Precision Liquids,”Book the Standards,Volume 14.03, 2010. Ripple, Dean and also Gregory Strouse, “Selection of choices to Liquid-in-Glass Thermometers,”Journal of ASTM International, Vol. 2, worry 9, October 2005. Webster, john (ed.),The Measurement, Instrumentation, and also Sensors Handbook,”CRC press LLC, Boca Raton, Florida, 1999. Wise, Jacquelyn, “NIST measure Services: Liquid-In-Glass Thermometer Calibration Service,” unique Publication 250-23, nationwide Institute of Standards and also Technology, Gaithersburg, Maryland, 1988. Wise, Jacquelyn, “A Procedure for the efficient Recalibration that Liquid-in-Glass Thermometers,” one-of-a-kind Publication 819, nationwide Institute the Standards and Technology, Gaithersburg, Maryland, 1991.

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