Thermometers high-precision

For measurement of the oaks we used, per force, a mass spectrometer of somewhat low accuracy, and achieved the accuracy to demonstrate that trees are thermometers by making many measurements on each sample. On the tree sequences which we measured later, we used high precision spectrometers with accuracies of 0.1 parts per thousand (ppt) for 180/160 and 13C/12C, and 2 ppt for D/H. The measurements are expressed in terms of 8D and 6 g. 

Figure 3.1 Analytical working curve for a self-indexed luminescent thermometer based on the ratio between the measured excimer (E, 475 nm) and monomer (M, 375 nm) emission bands of l,3-b/s(l-pyrenyl)propane in [C4Cjpyr][Tf2Nj. The optical thermometer is perfectly reversible in the temperature range shown and highly precise, with the measured uncertainties in the ratio (1 /1 ) falling well within the symbol dimensions. The dashed curve represents the temperature uncertainty predicted from explicit differentiation of a sigmoidal fit to the calibration profile 5T = 0T/0R 5R where R = I /Iu- (Reprinted from Baker, G.A., Baker, S.N., and McCleskey, T.M., Chem. Commun., 2932-2933, 2003. Copyright 2003 Royal Society of Chemistry. With permission.)...

Electrolytic type sensors Uxt thick film techniques, e.g. capacitor coated in gl bonded on to a ceramic disc mounted on a thermoelectric (Peltier effect) cooler. Control is by a platinum resistance thermometer which adjusts the temperature of the cooler to regain equilibrium after a change in capacitance due to moisture deposit. Range depends on technique. Capable of high precision. Limitations are similar to those for AIjO) sensor. Capable of being direct mounted. Relatively cheap. Suitable for on-line use. 

The size of the kelvin, the SI temperature unit with symbol K, is defined by the statement that the triple point of pure water is exactly 273.16 K. The practical usefulness of the thermodynamic scale suffers from the lack of convenient instruments with which to measure absolute temperatures routinely to high precision. Absolute temperatures can be measured over a wide range with the helium-gas thermometer (appropriate corrections being made for gas imperfections), but the apparatus is much too complex and the procedure much too cumbersome to be practical for routine use. 

Mercury Thermometers. In the past, mercury thermometers were by far the most common type of laboratory thermometer. Concerns about health hazards associated with mercury have now reduced their role. However, when high precision is required (calorimetry and freezing-point depressions, for example), oil-in-glass thermometers are not suitable since the use of fine capillaries is not feasible with oil. Furthermore, the safety hazards involved in the laboratory use of mercury thermometers are quite low, as discussed at the end of this section. For these reasons, a description of the use of mercury thermometers is still pertinent. 

For a PRT with R, = 100 O, a temperature resolution of 1 mK requires that Rt be measured with a precision of 4 X lO"" fl i.e., a random error <4 X 10 percent. Thus the measurement bridge and digital voltmeter must be high-precision instalments. In the case of the DVM, one needs at least six-digit resolution. It should be noted that, owing to decreasing sensitivity as T approaches 0 K, PRTs do not make attractive practical thermometers at very low temperatures. 

Presently, resistance thermometers are the most suitable temperature meters because of their high precision and stability. Mainly, they are used when resistance elements are wound directly on the surface of the calorimetric vessel and cover. Change of resistance with temperature can be in the current range of the temperature change of the calorimeter (less than 3 K) regarded as linear. 

In the end he was forced to write his own version and this is how his famous textbook Elementa Chemiae ( Elements of Chemistry ) came to be published in 1732. He laid great weight on exact methods and experimental work rather than fanciful theories and introduced the use of thermometers and precision balances in chemical work. At the same time, he did not completely reject the claim by the alchemists to have transformed base metals into gold. His own extensive experiments with mercury, however, had not produced any gold even if they yielded mercury preparations of high purity as a spin-off. Boerhaave s wariness of indulging in speculative chemical theories could explain why in his textbook he does not even mention Stahl s celebrated phlogiston theory. 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

Fixed-point calibration has the highest accuracy, but to achieve such accuracy requires great precautions and is very time-consuming. Most freezing- and triple-point cells are difficult to maintain and usually cannot accommodate more than one sensor at a time. Thus, fixed-point calibrations are usually carried out only for high-precision thermometers and usually only at national laboratories. However, easily maintained water triple-point (or ice-point) systems are used in many laboratories to correct thermometer drift, particularly for SPRTs. 

DS18S20 High-Precision 1-Wire Digital Thermometer... 

Nuclear orientation thermometry depends on the anisotropic emission of y-rays from polarized radioactive nuclei. The extent of the anisotropy is detemined by the degree of polarization which, in turn, varies inversely with the absolute temperature. Since the nuclear hyperfine splittings are usually known to high precision, the thermometer is in principle absolute and does not require calibration. The useful temperature range is relatively narrow, however, being limited by radioactive self-heating at the lower end and insensitivity at the upper end. 

High precision instruments are required to detect density, temperature, and composition changes of the order of 1 %, while location of the liquid-liquid interface requires multi-point measuring heads, or a vertically traversing head, all measuring to 0.1 % precision. This level of precision is a challenge. Measurement of temperature to 0.1 % can be achieved in the laboratory, but considerable effort is required to meet this level of precision under industrial conditions. The precision of normal industrial thermometers is in the range of 1-2 %. 

A calorimeter measures heat and it is therefore important that the calorimeter is placed in a constant-temperature environment, usually a high-precision thermostat. It is important to check that the temperature of the thermostat is accurate. For this, one needs a calibrated thermometer that can be inserted into the calorimeter thermostat. 

High purity platinum wire is used in resistance thermometers because the temperature coefficient of resistance of pure platinum is linear over a wide temperature range. The platinum resistance thermometer is the recognized instrument for the interpolation of the international practical temperature scale from—259.35 to 630.74°C. Whereas such precision measurements require very high purity platinum, for most routine industrial measurements lower purity metal can be tolerated. Conventional wire-wound devices are quite fragile and this disadvantage has led to the introduction of printed resistance thermometers, which are cheap to produce and much more durable. They can be used as an inexpensive replacement for thermocouple applications in intermediate temperature applications. 

Bomb calorimeter (Parr design), shown with an adiabatic jacket, which may also be used empty as an insulating air jacket. The precision mercury thermometer can be replaced by a high-resolution resistance thermometer or a calibrated thermistor. 

The precision mercury thermometer can he replaced by a high-resolution resistance thermometer or a calibrated thermistor. 

Since the triple point of water is defined to be exactly 273.16 K on the thermodynamic temperature scale, this is an especially important fixed point. It is also a point that can be reproduced with exceptionally high accuracy. If the procedure of inner melting (described below) is used, the temperature of the triple point is reproducible within the accuracy of current techniques (about 0.00008 K). This precision is achieved by using the triple-point cellshov n in Fig. 1. This cell, which is about 7.5 cm in outer diameter and 40 cm in overall length, has a well of sufficient size to hold all thermometers that are likely to be calibrated. ... 

Although the calorimeters used for highly accurate work are precision instruments, a very simple calorimeter can be used to examine the fundamentals of calorimetry. All we need are two nested Styrofoam cups with a Styrofoam cover through which a stirrer and thermometer can be inserted, as shown in Fig. 9.7. This device is called a coffee cup calorimeter. The outer cup is used to provide extra insulation. The inner cup holds the solution in which the reaction occurs. 

Traditional bucket samples generally are inadequate for the calibration of in situ temperature because preferential near-surface absorption of solar radiation occurs in the upper few meters of the water column 12). Hence, uncertainty in the depth of the bucket sample can introduce significant errors in the in situ temperature measurement. If the flow rate to the ship s laboratory is sufficiently high (>15-20 L/min), then discrete temperature measurements can be made with a laboratory-grade thermometer inserted in the flow prior to the debubbler. The combination of discrete temperature measurements with a precise thermal-bath calibration of the in situ temperature sensor prior to use at sea produces high-quality underway temperature data to within 0.03 °C. 

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