Thermometer other devices

Experimental measuring equipment developed for drying by convection is shown in Figure 2.5. The equipment is in fact a controlled climate air duct. The wet material is placed on a balance in the measuring space. The temperature and moisture content of the drying air is measured by wet and dry bulb thermometers. Other online humidity measuring devices may be used for convenience. 

Mercury has been known for a long time because it was found in Egyptian tombs that date back to 1500 B.C. [18]. It has had applications in electronics, amalgams, thermometers, barometers, and other devices. This element s use has decreased over the years because exposure to it can result in serious health problems. 

Secondary standards are liquid-in-glass thermometers and base-metal thermocouples. They are calibrated by comparing them with primary-standard platinum-resistance thermometers or standard platinum-rhodium versus platinum thermocouples at temperatures generated in comparators. These secondary standards are used in turn for the calibration of other devices, such as liquid-in-glass thermometers, bimetallic thermometers, filled-system thermometers, and base-metal thermocouples, in which the highest degree of accuracy is not required. Optical pyrometers as secondary standards are compared with primary-standard optical pyrometers, and they are then used for calibration of r ular test pyrometers. 

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

It is not necessary to calibrate all test and measuring equipment. Some equipment may be used solely as an indicator, such as a thermometer, a clock, or a tachometer other equipment may be used for diagnostic purposes, to indicate if a fault exists. If such devices are not used for determining the acceptability of products and services or process parameters, their calibration is not essential. However, you should identify such devices as for Indication Purposes Only if their use for measurement is possible. You don t need to identify all clocks and thermometers fixed to walls unless they are used for measurement. Having observed that you record the time when observations were made, a zealous assessor may suggest that the clock be calibrated. If the time is not critical to product or process acceptability, calibration is unnecessary. 

The impedance of the transducer is important if it provides an output signal to an electronic device (an amplifier, for example) and the impedance of the two must be matched for accurate measurement. Some transducers (thermocouples, for example) generate their output by internal mechanisms (i.e. they are self-excited). Others such as resistance thermometers need an external source and an appropriate type must be available. Transducers used in the measurement of the more common physical quantities are discussed below. 

The ability to measure temperature and temperature differences accurately and reproducibly is essential to the experimental study of thermodynamics. A thermometer constructed with an ideal gas as its working fluid yields temperatures that correspond to the fundamental thermodynamic temperature scale. However, such thermometers are extremely difficult to use, are not amenable to miniaturization, and are very expensive. Therefore, other means to measure temperatures that reproduce the ideal gas or thermodynamic temperature scale (Kelvin) have had to be developed. The international temperature scale represents a method to determine temperatures over a wide range with measuring devices that are easier to use than the ideal gas thermometer. The goal is to make temperature measurements that correspond to the thermodynamic temperature as accurately as possible. 

A thermometer is a device by which we can measure a property of matter function of temperature. If a relation, based on fundamental laws of physics, between such property and the thermodynamic temperature is considered reliable, the thermometer does not need a calibration and is called primary thermometer. In the other cases, the thermometer needs a calibration and is called secondary. Examples of primary thermometers are gas thermometers and noise thermometers. 

Filled-System Thermometers The filled-system thermometer is designed to provide an indication of temperature some distance removed from the point of measurement. The measuring element (bulb) contains a gas or liquid that changes in volume, pressure, or vapor pressure with temperature. This change is communicated through a capillary tube to a Bourdon tube or other pressure- or volume-sensitive device. The Bourdon tube responds so as to provide a motion related to the bulb temperature. Those systems that respond to volume changes are completely filled with a liquid. Systems that respond to... 

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. 

Gas thermometers tend to be awkward in use. The general practice, therefore, is to employ, where possible, other thermometers that exhibit a near-linear variation of some other property x, with the empirical temperature r. A common device for this purpose is the mercury or alcohol thermometer which expands in volume as r rises. For many liquids the quantity a (1/V) (3V/dT)P remains approximately constant. Thus (3V/dT) >... 

Boiling-point apparatus. The mercury thermometer can be replaced by a resistance thermometer or other direct-reading thermometric device. 

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. 

Other Thermometric Devices. The vapor pressure of a pure liquid or solid is a physical property sensitive to temperature and thus suitable for use as a thermometer. The use of a liquid-nitrogen vapor-pressure thermometer is suggested for the range 64 to 78 K in Exp. 47. At very low temperatures (1 to 4.2 K), the vapor pressure of liquid helium can be used. 

Mercury-in-glass laboratory thermometers are convenient for many applications and are still frequently used. The only safety concern is the possibility of a broken thermometer bulb. If breakage occurs, the thermometer must be given to the laboratory staff for disposal and any mercury spillage must be promptly cleaned up. Safety concerns about mercury should be kept in perspective. There is no appreciable danger from the normal use of mercury thermometers or other laboratory devices containing mercury (such as manometers or barometers) if they are used properly. 

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