A thermometer is used to measure the temperature of solids, liquids, or gases. A thermometer contains a liquid (usually mercury or an alcohol solution) in a reservoir whose volume is linearly dependent on the temperature (as the temperature increases, the volume increases). When the liquid is heated it expands into a narrow tube that has been calibrated to indicate the temperature. Temperature can be recorded in Celsius, Farenheit, or Kelvin, therefore it is important to note which scale the thermometer is calibrated for. Show Corrections? Updates? Omissions? Let us know if you have suggestions to improve this article (requires login). Feedback Type Your Feedback Submit FeedbackThank you for your feedback Our editors will review what you’ve submitted and determine whether to revise the article. External Websites Share Share Share to social media Facebook Twitter URL https://www.britannica.com/technology/liquid-thermometer
Liquid thermometers were once the most common type in use. They were simple, inexpensive, long-lasting, and able to measure a wide temperature span. The liquid was almost always mercury or coloured alcohol, sealed in a glass tube with a gas like nitrogen or argon making… In liquid-in-glass thermometers, the thermal expansion of liquids is used for measuring the temperature!
Operating principleLiquid-in-glass thermometers are based on the principle of thermal expansion of substances. A liquid in a glass tube (called a capillary) expands when heated and contracts when cooled. A calibrated scale can then be used to read off the respective temperature that led to the corresponding thermal expansion. Such thermometers are also called capillary thermometers. Figure: Principle of temperature measurement with a liquid-in-glass-thermometerIn liquid-in-glass thermometers, the thermal expansion of liquids is used for measuring the temperature! Thermometric liquidsThe liquids used in such thermometers must have certain properties in order to be suitable for use. For example, they must not freeze at low temperatures, which is why water, for example, is not suitable. The liquids should also have a sufficiently high boiling point so that they do not vaporize at high temperatures. Furthermore, the liquid must expand evenly with the temperature in the measuring range used. Otherwise an uneven division of the scale on the thermometer would be necessary. Also for this reason water is not suitable, since water expands unevenly due to the density anomaly. However, liquids which have all the required properties and are therefore suitable for use in liquid-in-glass thermometers are also referred to as thermometric liquids. In the past, the highly toxic mercury was used, which has a solidification temperature of -39 °C and a boiling temperature of 357 °C. Nowadays, usually blue or red colored ethanol (alcohol) with a melting point of -115 °C and a boiling point of 78 °C is used instead of mercury. In this temperature range, the everyday temperatures in the range between -20 °C and +50 °C can be well covered. Measuring sensitivityThe measuring sensitivity of liquid-in-glass thermometers increases with the amount of liquid in the thermometer. The more liquid there is, the more liquid will expand and rise in the glass tube. For this reason, liquid thermometers have a reservoir to increase the amount of liquid in the thermometer. Too much liquid, however, would cause the thermometer to react very slow to temperature changes, as the heating of the liquid will take longer and the liquid will take longer to adapt to the outside temperatures to be displayed. NoteMercury was/is used not only because of the wide temperature range (theoretically applicable from -39 °C to 357 °C) but also because mercury hardly wets the glass tube. The capillarity is therefore lower and the temperature can be displayed and read off more accurately. I can’t help but think about what a critical role temperature plays in everything that we do. We use temperature to help us decide what to wear, how to prepare our food, diagnose illness, and determine where and when we take vacation. Of course, temperature measurement plays an important role in the laboratory too. The physical properties and characteristics of the materials that we test are influenced, as least in part, by temperature. Inarguably, accurate temperature measurement is one of the most critical components of laboratory testing. Measuring Temperature in the Laboratory Liquid-in-Glass Thermometers Dissecting the LiG Thermometer The Bulb Figure 1: Anatomy of a LiG Thermometer Stem Auxiliary Scale Contraction Chamber Expansion Chamber Mercury and Mercury-Thallium Thermometers In recent years, concerns over the toxicity of mercury have caused many states to prohibit or limit the use of mercury-containing devices. In fact, one of the world’s leading institutions on temperature measurement, the National Institute of Standards and Technology (NIST), recently announced that they will no longer provide calibration services for mercury thermometers. To know more about mercury reduction initiatives, please see my article, “Getting Rid of Mercury: A New Frontier for Temperature Measurement.” Nevertheless, few liquids have been found to mimic the thermometric properties of mercury in repeatability and accuracy of temperature measurement. Toxic though it may be, when it comes to LiG thermometers, mercury is still hard to beat. Spirit-Filled LiG Thermometers Organic liquids generally have inferior performance to mercury, and can leave a film on the glass as the liquid drains down the capillary wall. Separation of the fluid column is also known to be a common problem with spirit-filled thermometers. In addition, they tend to have a greater sensitivity to changes in stem temperature, which is a fundamental limitation in their use. These thermometers also have different capillary and bulb dimensions than mercury LiG thermometers, causing differences in response time and immersion characteristics. Spirit-filled thermometers are used in some low-temperature applications, as they can be used at temperatures as low as -200°C, which is well beyond the capabilities of mercury or mercury-thallium thermometers. ASTM E 1 covers specifics regarding spirit-filled LiG thermometers. Any thermometers described in ASTM E 1 which should contain toluene or other suitable liquids are specifically designated as such. At the time that this article was written, ASTM E 1 only contains specifications for two spirit-filled thermometers. These thermometers are specifically designed for use at extreme-cold temperatures at which it is not feasible to use mercury. Low-Hazard Precision LiG Thermometers The thermal expansion properties of the non-toxic fluids used in low-hazard precision LiG thermometers can be quite different from that of mercury. The bulb and capillary size required to achieve a similar movement along the thermometer scale can vary from that of its mercury counterpart. The surface tension of these liquids varies from mercury, causing differences in the meniscus. In addition, low-hazard precision liquids tend to react to change in temperature at a rate different from that of mercury, and should not be used when rate of rise or other time-temperature relationships are an important part of the test procedure. While these devices are great alternatives for some applications, their range in use is quite limited. Immersion Depth Figure 2: Immersion Depths for LiG Thermometers Total Immersion Thermometers Total immersion thermometers are commonly used in constant-temperature baths as a means of monitoring the bath’s temperature. For example, total immersion thermometers are used in kinematic and absolute viscosity baths. Partial Immersion Thermometers Partial immersion thermometers are commonly used in applications where total immersion thermometers are impractical or impossible to use. For instance, if the depth of a temperature bath is only 100 mm, a total immersion thermometer with a length of 300 mm is impossible to immerse correctly. In this case, a partial immersion thermometer with an immersion depth of 76 mm is a better choice. In addition, if a rapid, one-time temperature measurement is required, such as in soil specific gravity or hydrometer testing, a partial immersion thermometer works best. Complete Immersion Thermometers What’s the Big Deal? Immersion depth plays an important role in how the liquid inside of the device will react. If the portion of the thermometer containing mercury is meant to be submerged in the test medium (i.e. a total immersion thermometer), but is left exposed, the liquid will not behave as intended. The error incurred can vary greatly, and is dependent upon the temperature scale of the thermometer, the type of liquid used, and the emergent stem temperature. It is possible to obtain errors as large as several degrees by improper immersion of a LiG thermometer. These errors generally tend to be larger with spirit-filled devices than with mercury-filled ones. It is possible to apply a correction for 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 Method for Inspection and Verification of Thermometers, describes procedures that may be used to calculate these corrections. Corrections cannot be made for complete-immersion thermometers that are immersed improperly. What is the red liquid in thermometers?Mercury is a silver-white to gray substance. If your thermometer is filled with a red liquid, your thermometer contains red dyed alcohol or mineral spirits and not mercury. These are safer alternatives to mercury fever thermometers.
What are the 2 liquid in a thermometer?A liquid-in-glass thermometer comes in 2 basic types: mercury or alcohol.
Do they still use mercury in thermometers?The oldest thermometers used are mercury in glass. Newer thermometers include non-mercury liquids in glass and digital and electronic devices that use sensors to measure temperature. Thermometers that check body temperature in the ear, across the forehead, or have a digital display do not contain mercury.
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