Filled-System 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 

TABLE 8-9 Recommended Temperature for RTDs and Thermocouples Measurement Ranges 

Fluid-filled bulbs deliver enough power to drive controller mechanisms and even directly actuate control valves. These devices are characterized by large thermal capacity, which sometimes leads to slow response, particularly when they are enclosed in a thermal well for process measurements. Filled-system thermometers are used extensively in industrial processes for a number of reasons. The simplicity of these devices allows rugged construction, minimizing the possibility of failure with a low level of maintenance, and inexpensive overall design of control equipment. In case of system failure, the entire unit must be replaced or repaired. 

As they are normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also the maximum temperature is somewhat limited. 

Pyrometers Planck s distribution law gives the radiated energy flux qb(X,T)dX in the wavelength range X to X + dX from a black surface  

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Filled-system thermometer Rotameter Bourdon tube Lever Magnetic resonance analysis (MRA)... 

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

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. 

Resistance thermometers Filled-system thermometers Bimetal thermometers low temperatures... 

Filled-system thermometer Turbine —bellows —magnetically Infrared (IR) spectroscopy... 

Filled-system thermometers may be separated into two types those in which the Bourdon responds to volume changes and those that respond to pressure changes. The systems that respond to volume changes are completely filled with mercury or other liquid, and the system that responds to pressure changes is either filled with a gas or partially filled with a volatile liquid. 

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. 

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