Thermometer, accuracy indicating

The terms reproducibility and accuracy require some explanation. Reproducibility is defined as the variability observed in repeating a given measurement using the best present-day laboratory techniques. Changes produced on thermal cycling of the thermometer to and from ambient are included in this parameter. Accuracy indicates the significance with which the thermometer can indicate the absolute thermodynamic temperature. This... 

In filled thermometers the thermal expansion of a gas or a liquid is transmitted through a thin capillary tube to a bellows or helix, where the deformation indicates the temperature. The temperature range of filled thermometers is very wide, approximately -200 to +700 °C. They are extremely robust but are not very high in accuracy. The application is mainly for process instrumentation and as stand-alone control devices. 

A schematic view of a microdielectrometer sensor is shown in Fig. 8 and illustrates the electrode array, the field-effect transistors and a silicon diode temperature indicator 15) which functions as a moderate accuracy ( 2 °C) thermometer between room temperature and 250 °C. The sensor is used either by placing a small sample of resin over the electrodes, or by embedding the sensor in a reaction vessel or laminate. Since all dielectric and conductivity properties are temperature dependent, the ability to make a temperature measurement at the same point as the dielectric measurement is a useful feature of this technique. 

The maximum surface temperature that may occur under total incident radiation can be measured by use of a black standard thermometer. Alternatively, a white standard thermometer can be used to predict the surface temperature of a reflecting surface, e.g., white tablets. In our laboratory, we have found the use of a temperature level strip (Thermax, Thermographic Measurements Ltd., South Wirral, U.K.) quite convenient to predict the temperature of the exposed sample. The strip can be placed on the exposed surface (e.g., surface of an infusion bag) or inside a container (e.g., inside a petri dish). A color change indicates that the rating quoted on the indicator has been reached. The maximum temperature reached during the exposure is thereby registered with good accuracy ( 1%). 

On account of the mechanical construction of pressure thermometers, both as regards the mechanism for indicating and recording and the necessity for using comparatively large and heavy bulbs, the accuracy that can be secured with this type... 

The value of 1/a for nitrogen at 1 atm is 272. Experiments with other gases indicate that the ice point, 0°C, is equivalent to a value of near 273 K. The Kelvin scale is now defined with high accuracy such that the triple point of water (where ice, water and water vapour are all in equilibrium, at 0.01°C) has the temperature 273.1600° on the Kelvin scale. The triple point is more accurately defined than the ice point. On this basis 0°C is 273.15 K. Measurement on this ideal gas scale is best conducted with a constant pressure helium thermometer, although there are small deviations from the absolute scale. A comparison of the four temperature scales discussed above is given in Table 1.1. 

Bimetallic thermometers measure temperature by the change in physical dimension of the sensor. They have often been used [80]. The sensor consists of a composite strip of material, normally in a helical shape, formed by two different metals Differences in the thermal expansion of the two metals cause the curvature of the strip to be a function of temperature. The strip is used as a temperature indicator with a self-contained scale. Bimetallic thermometers have been used at temperatures from -185 to 425°C. The precision and accuracy of such thermometers are described in Ref. 81. 

Ensure all flow and differential pressure indicators etre zeroed and calibrated. All control room pressure indicators need to be checked with a calibrated gage. All dial thermometers and outside pressure gages should be removed and checked for accuracy. The unsatisfactory ones should either be replaced or removed prior to the test and read with calibrated thermocouples, thermometers, or gages during the test. These calibrated devices should adequately fit in the existing fittings. 

Thermostats may be used to indicate temperature but their main application is in the regulation of heating and cooUng and as circuit breakers. They are much less sensitive than liquid-in-glass thermometers and achieving accuracy below 0.1 C is unrealistic. The strip is fixed at one end which is attached to an electrical power supply. The other end is free to move up and down. For heating applications, ffie dial sets the position in which the free end completes an electrical circuit, which will then heat the strip. When the strip temperature rises beyond the set-point (the desired temperature set by the manual dial), the band will move in the opposite direction, thus breaking the contact. At this... 

The shown calorimeter has an accuracy of +0.1% and a temperature range of 170 to 600 K. A sample of 100-300 g is placed in two sets of silver trays, one outside and one inside a cylindrical heater. In the middle of the sample, the tip of the platinum resistance thermometer can be seen. Sample trays, thermometer, and heater are enclosed in a rounded steel shell, which for ease of temperature equilibration is filled with helium of less than one pascal pressure. The shell is covered with a thin silver sheet on the outside, gold-plated to reduce radiation losses. The calorimeter is then hung in the middle of the adiabatic jacket, drawn in heavy black. This adiabatic jacket is heated by electrical heaters and cooled by a cold gas flow, as indicated by the dials of the instmments pictured on the left at the bottom. The whole assembly, calorimeter and adiabatic jacket, is placed in a sufficient vacuum to avoid convection. 

While temperature transmitters are checked and calibrated, sensors may not be checked directly. The operation of some, notably those on the outlets of the electrolyzers, cannot be verified by liquor circulation before startup. A common assumption is that thermocouples (T/Cs) and resistance temperature devices (RTDs) work correctly. If desired, they can be checked by a hot box, if one is available, or by immersion in a flask of hot water or other appropriate fluid. A hot box is a dry-heated temperature-controlled block with drilled holes to accept RTD or T/C sensors. No further calibration of the instrument is necessary after installation, and the DCS indication should be checked against a thermometer of known accuracy. 

The principle of the thermal expansion of solids is used in bimetallic thermometers, which consist of two metal strips of different materials with different coefficients of expansion firmly connected together. Temperature differences bend these bimetallic springs and they are usually wound in spirals or coUs. The useful measuring range is from —73 to - -537°C with an accuracy of +1% of full scale [4). Bimetallic strips are used for temperature compensation in mechanical instrument movements. They are also used in switching relays to indicate whether a limiting value is exceeded. 

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