Thermometers variations

Scanning thermal microscopy uses the world s smallest thermometer, actually a tiny thermocouple tip, to measure temperature variations as small as 10 microdegrees on a scale of <100 nm. 

There are problems to be considered and avoided when using Hquid-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. 

Bimetal thermometer A thermometer that uses two dissimilar bars of metals (with different rates of linear thermal expansion) riveted together. A variation in temperature produces a bending moment on the bar, which is magnified by a lever to record temperature on a dial. 

The exactness of a measurement is expressed by its precision. This concept can be explained with an example. Suppose three swimmers are discussing the temperature of a swimming pool. The first dips a finger in the water and says that the temperature is about 24 °C. The second examines an immersed pool thermometer and reports the temperature to be 26 °C. The third swimmer, who has been monitoring daily variations in the pool s temperature, uses a portable precision digital thermometer and reports, According to my precision thermometer, the pool temperature is 25.8 °C. ... 

A number of variations of the transient hot-wire method have been devised, and an optical method to detect the temperature rise has been used. A modified transient hot-wire technique using a mercury-incapillary probe was introduced by Nagashima et al., in which a thin mercury thread was used as a heater-thermometer and the capillary wall as an insulator. Using this method, they measured the thermal conductivity in mixture systems such as (Na, K)N03, (Li, Na)N03, and HTS(KN03-NaN03-NaN02, 44-7-49 mol.%). ... 

Most substances expand when heated and contract when cooled, but liquid mercury shows an especially large variation of volume with temperature. That is why it is so often used in thermometers and barometers. Mercury also mixes with a number of metals to form alloys called amalgams. Amalgam is a special name given to alloys of mercury. With silver it forms a silver amalgam, which quickly hardens. This is the silver filling used by dentists. 

The zero-th law, which justifies the existence of the thermometer, says that two bodies A and B which are in thermal equilibrium with a third body are in thermal equilibrium with each other. There is no heat flow from one to the other, and they are said to be at the same temperature. If A and B are not in thermal equilibrium, A is said to be at a higher temperature if the heat flows from A to B when they are placed in thermal contact. The changes in temperature usually produce changes in physical properties like dimension, electrical resistance and so on. Such property variations can be used to measure the temperature changes. 

Long term changes in precipitation, caused by changes in climatic temperature, are well documented in polar ice caps the heavier of the stable isotopes is depleted in ice laid down in the ice age by comparison with present day ice. In 1970 we extended this concept to trees, suggesting that they, also, are thermometers. Trees grow from water and atmospheric C02. In trees which grow on rain water, isotope variations in their rings should be climate indicators because the isotope composition in rain and C02 varies with temperature. 

If the stable isotope ratio of 13C/12C is to be further measured in tree rings and interpreted as an indicator of climate variation, (and we have barely begun to initiate its use as a thermometer in the present work, confining our measurements to the stable isotopes in water, because water is so abundant compared to carbon dioxide and because the dependence of its isotope ratios is relatively simple compared with those of carbon dioxide), some more sophisticated considerations must be given to the distribution of carbon dioxide among the reservoirs on the surface of the earth. 

The values of E(t) so computed are listed in Table 4. The correction for fractionation of carbon dioxide at the sea surface is a serious one. It makes the interpretation of 13C/12C variations in wood difficult and militates against the use of the isotope ratio of carbon as a thermometer. This correction, when applied to variations of carbon-14 in wood, is able to explain the Suess radiocarbon "wiggles" of about 100 years duration each, without the need to invoke changes in the neutron flux from the sun [54]. 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

The ideal (bio)chemical sensor should operate reversibly and respond like a physical sensor (e.g. a thermometer), i.e. it should be responsive to both high and low analyte concentrations and provide a nil response in its absence. One typical example is the pH electrode. In short, a reversible (bio)chemical sensor provides a response consistent with the actual variation in the analyte concentration in the sample and is not limited by any change or disruption in practical terms, responsiveness is inherent in reversibility. An irreversible-non-regenerable (bio)chemical sensor only responds to increases in the analyte concentration and can readily become saturated only those (bio)chemical sensors of this type intended for a single service (disposable or single-use sensors) are of practical interest. On the other hand, an irreversible-reusable sensor produces a response similar to that from an irreversible sensor but does not work in a continuous fashion as it requires two steps (measurement and renewal) to be rendered reusable. Figures 1.12 and 1.13 show the typical responses provided by this type of sensor. Note... 

Oxygen isotope fractionation has been applied as a thermometer and geospeedometer. Oxygen isotopic ratios vary slightly from one phase to another. The small variations are conventionally expressed by 8-notation defined as... 

However, it is clear that slight variations in vessel shape, etched markings, or external pressure can lead to disagreements as to which thermometer gives the true temperature. Moreover, the reference points chosen to standardize the readings between different thermometers could be subject to disagreements (see Sidebar 2.4), as could the choice of thermometric fluid (e.g., Hg vs. water, each of which has different values of aP in different temperature ranges). Under these circumstances, the choice of the true temperature scale may become subject to non-scientific influences. We therefore seek a universal standard that avoids such arbitrary choices. 

The Resistance Temperature Detector (RTD) or Resistance Thermometer The variation of resistance with temperature 0 for metals can be expressed by ... 

Temperature variations in the instrument are a source of error and electrical power dissipation is limited to avoid the effects of self-heating. This is achieved by means of the four lead system shown in Fig 6.25ft. This minimises any effects of variations in temperature on the resistance RCL of the connections between the RTD and the bridge and is used normally with digital thermometers and data acquisition systems where the sensor non-linearity is corrected within the computer software. 

When it is necessary to control the temperature of the liquid, the tube is opened at one end immediately after the reading and a thermometer inserted. Better still, in cases where variations of temperature exert a marked influence on the result, special tubes may be used which are furnished with a T-branch containing a stopper traversed by a thermometer and are surrounded by a metallic jacket through which water at constant temperature is circulated. 

Design temperatures, operator guesses, and actual operating temperature differentials almost never correspond. Representatives should always carry a pocket thermometer with them (accurate to 0.2°F/0.rC) and carry out an actual measurement during the survey. A difference of 1.0°C between actual and estimated temperatures can eventually produce a considerable difference in estimated annual chemical costs. Again, it is necessary to qualify the temperature differentials for seasonal variations. 

The Beckmann thermometer used with the bomb calorimeter should be calibrated for the normal depth of immersion with which it is used. To cover the normal range of laboratory temperatures, this calibration should be obtained for three settings of the zero on the scale, convenient values being 10, 15, and 20°C. Such a series of calibrations allows automatically for emergent stem corrections and variations in the value of the degree on the thermometer scale with different quantities of mercury in the bulb, in addition to those arising from inherent variations in the diameter of the capillary bore. 

A 50 ml flask equipped with a reflux condenser and a thermometer was charged with thiophenol (10 mmol), the iodonium salt (20 mmol), tributylamine (2.22 g, 10 mmol) and cupric benzoate (0.2 g). The mixture was stirred and heated at 120-125°C for 3 h under nitrogen. On cooling, the reaction mixture was washed with anhydrous ether (3 x 50 ml) to remove the iodoarenes. The residue was recrystallized from ethanol to give triarylsulphonium salts (48-87%). Similar conditions with slight variations were used when sulphides were the starting material. 

Although there is no record that he ever calibrated the tube, he used it in temperature study. Galileo s thermometer was impossible to calibrate even if he had decided on fixed points with which to establish specific temperatures because it was exposed to the atmosphere and subject to variations in atmospheric pressure. By 1640, it was realized that the air thermometer was subject to variations of barometric pressure and the sealed thermometer was created. However, the need to establish fixed points of reference had still not been addressed. 

If you are concerned about accurately calibrating the entire scale of a particular thermometer, it may be sent to the NIST for calibration for a fee. You will need to contact the NIST for more information on that service. Once calibrated, any variations in the zero point should not significantly alter the corrected calibrations along the scale. 

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