Thermometer liquid in glass

Liquid-in-glass thermometers measure the thermal expansion of a liquid, which is placed in a solid container, on a length scale. The mercury thermometer is one example of liquid thermometers. Alcohol is also used with this type of instrument. The temperature range is -80 to a-330 °C depending on the liquid. The quality, stability, and accuracy vary considerably. The advantages are a simple construction and low price. A disadvantage is that they are not compatible for connection to monitoring systems. 

Temperature (°C) 0.2 °C (thermistor thermometer) 0.5 °C (liquid-in-glass thermometer) Constant for 3 consecutive readings 0.2 °C... 

The invention of the thermometer is generally credited to Galileo. His instrument, built near the end of the sixteenth century, relied on the expansion of air with an increase of heat. Traditional liquid-in-glass thermometers were devised in the 1630s and are standard equipment today in research settings, medical practice, and meteorological measurement. 

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There are problems to be considered and avoided when using liquid-in-glass thermometers. One type of these is pressure errors. The change in height of the mercury column is a function of the volume of the bulb compared to the volume of the capillary. An external pressure (positive or negative) which tends to alter the bulb volume causes an error of indication, which may be small for normal barometric pressure variations but large when, for example, using the thermometer in an autoclave or pressure vessel. 

Glass-creep errors are also encountered. The liquid-in-glass thermometer should always be used to measure temperatures in ascending order. If the thermometer is stored at room temperature, a temporary ice point depression results, which maybe as much as 0.01 K for 10 K of temperature difference, when the thermometer is heated above room temperature. If the thermometer is used to measure a temperature and must then be used to measure a lower temperature, the thermometer should be stored at a still lower temperature for at least 3 days prior to use to assure recovery of bulb dimensions. 

Bimetal thermometers are made in ranges from + 1000°F (538°C) down to — 300°F (— 184°C) to and lower. However, at low temperatures, the rate of deflection drops off quite rapidly. Because of its long-term instability at high temperatures, the maximum temperature for continuous use is about 800°F (427°C). However, special bimetal thermometers can be obtained for continuous use up to 1200°F (649°C). Good bimetal thermometers retain their accuracy indefinitely. Usually industrial bimetal thermometers read with an accuracy of 1% at any point on the scale. The speed ot response of bimetal thermometers is generally about the same as that for liquid-in-glass thermometers in similar ranges. 

It is important to consider what happens when the substance or property employed is changed. For example, for a liquid in glass thermometer (using the Centigrade scale) ... 

Liquid-in-glass thermometers Bimetallic thermometers Filled-system thermometers Phase change... 

It is believed that Galileo invented the liquid-in-glass thermometer around 1592. Thomas Seebeck discovered the principle behind the TC—the existence of the thermoelectric current—in 1821. The same year Sir Humphry Davy noted the temperature dependence of metals, but C. H. Meyers did not build the RTD until 1932. Today, some 20 different types of temperature sensors are available, and Table 3.160 lists the temperature ranges and accuracies of a number of them. 

Liquid-in-glass thermometers used mercury or alcohol as the liquid that expands as it gets warmer. Most countries mandate the removal of any mercury-filled devices due to its extreme toxicity, but alcohol and other fillings are still used. The expansion rate is linear with temperature and can be accurately calibrated. Bimetallic thermometers bond two dissimilar metals with different coefficients of expansion to produce the bimetallic element in thermometers, temperature switches, and thermostats. Filled System Thermometers can be filled with either liquid or vapor. Liquid-filled units are the most popular although they require compensation for the weight of the liquid head and for capillary length. Vapor-filled elements cannot be used if the operating temperature crosses the vapor/liquid point. 

However, in addition to these primary reference points, a secondary series of reference points was established by the IPTS-68 (see Table 2.30). These secondary points can more easily be used (than the primary temperature points) for testing temperature equipment such as liquid-in-glass thermometers. They are useful because they require less equipment and are therefore easier to obtain. Remember that these points are secondary standards and should not be considered primary standards. 

Liquid-in-glass thermometer. Volume of liquid increases as heat... 

When you mention thermometers, volumetric expansion thermometers are what typically come to mind (see Fig. 2.30). The material that expands within a volumetric expansion thermometer is typically mercury or (ethyl) alcohol. Another name for a volumetric expansion thermometer is a liquid-in-glass thermometer. The parts of a standard liquid-in-glass laboratory thermometer are as follows ... 

Not all liquid-in-glass thermometers have all these parts. 

Fig. 2.30 The principle features of the solid-stem liquid-in-glass thermometer. From Figure 3 from the NBS Monograph 90, Calibration of Liquid-in-Glass Thermometers," by James F. Swindells, reprinted courtesy of the National Institute of Standards and Technology, Technology Adminstration, U.S. Department of Commerce. Not copyrightable in the United States.

It is possible to compensate on the calibration lines for these limitations to a certain degree. To make this compensation, three different types of liquid-in-glass thermometers have been designed with three different immersion requirements. They are ... 

NIST-calibrated thermometers are expensive, but very accurate tools. Unfortunately, they require special use and maintenance to maintain their integrity. Not only can abuse alter their calibration, but general use can as well. For example, if you are using an NIST-calibrated liquid-in-glass thermometer on a regular basis, an ice-point recalibration should be taken after each measurement. These variations should be added to the adjustments made to the corrected scale temperatures. 

Liquid-in-glass thermometers require a finite amount of time to achieve a final, equilibrium temperature. The time required can vary for individual thermometer types depending on the diameter of the thermometer, the size and volume of the bulb, the heat conductivity of the material into which the thermometer is placed, and the circulation rate of that material. 

By using two liquid-in-glass thermometers, you can verify the quality of both thermometers by their agreement in temperature readings. If the temperatures do not agree, one of the thermometers may have a bubble in the stem or some other defect. Unfortunately, this trick does not let you know which is the defective one, but it provides a clue to the problem. f Methanol is flammable and poisonous. 

Liquid-in-Glass Thermometry by J. A. Wise, U.S. Government Printing Office, Washington, D.C., 1976. An excellent NBS (now NIST) publication on the use of liquid-in-glass thermometers. 

The most common method of temperature measurement is with a liquid-in-glass thermometer. This method depends on the expansion of fluids when they are... 

Liquid in Glass Thermometers. Mercury-in-glass thermometers (or better yet, mercury-in quartz) function well between -25°C and 360°C their typical precision is 0.1 K. They must be corrected for (1) relatively small pressure effects and (2) a relatively large "exposed-stem correction," due to the different coefficients of thermal expansion of mercury and glass in the part of the thermometer not immersed in the system being measured. 

Alcohols, toluene, and pentane can be used as thermometric liquids in glass thermometers down to -100°C. 

Special Liquid Thermometers. For low temperatures, one can use several kinds of liquid-in-glass thermometers. Toluene thermometers may be used down to — 95°C, and pentane thermometers will operate as low as — 130°C. However, it is usually more convenient, as well as more accurate, to use thermometric devices of other types, especially thermocouples or resistance thermometers. 

Ionic liquids have been used to prepare liquid-in-glass thermometers. Traditional thermometers either contain liquid mercury, which is toxic, or ethanol, which has a boiling point of only 78 °C and so has a limited temperature range. Therefore, there is a need for an alternative if a suitable substance can be found. Two ionic liquids were chosen for study (Figure 6.11). 

Temperahire is commonly measured with liquid-in-glass thermometers, wherein the hquid expands when heated. Thus a uniform hibe, partially filled with mercury, alcohol, or some otherfluid, can indicate degree of "hotness" simply by the length of tire fluid colunm. However, numerical values are assigned to the various degrees of hotness by arbitrary definition. 

Rodriguez H, Williams M, Wilkes JS et al. (2008) Ionic liquids for liquid-in-glass thermometers. Green Chem 10 501-507... 

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