Measuring bridge calibration

The procedure for each solution is the same as that described above for the standardization measurement on 0.02 M KCl. For several of the HAc solutions and for the most dilute strong electrolytes, the resistance may be so high that it is not possible to balance the bridge withXnear 500. One can either use two decade resistance boxes in series for arm 2 or merely take slide-wire readings at largerX values. In either case be sure that your bridge calibration procedure will provide any necessary correction factors. 

There is ample literatime by the American Society of Mechanical Engineers (ASME), the American Society for Testing and Materials, the NBS, and others that deals with calibration methods, specifications for construction and usage of measuring instruments and temperature comparators, and processing of calibration data. It is advisable in each case to have the major components of the system (primary and secondary standards), potentiometers, and Mueller bridges calibrated periodically by the NBS or other qualified laboratory. 

The sensor usually consists of a coil of wire made from the material that is wound on a former and the whole sealed to prevent oxidization, although a film of the metal deposited on a ceramic substrate can also be used. The resistor is connected in a Wheatstone bridge network (Figure 17.17), using fixed resistors in the other three arms. The instrument connected across the bridge is calibrated directly in terms of temperature. The range is limited by the linearity of the device and the upper temperature, which can be measured, must be well below the melting point of the material. 

A glass electrode, a thin-walled glass bulb containing an electrolyte, is much easier to use than a hydrogen electrode and has a potential that varies linearly with the pH of the solution outside the glass bulb (Fig. 12.11). Often there is a calomel electrode built into the probe that makes contact with the test solution through a miniature salt bridge. A pH meter therefore usually has only one probe, which forms a complete electrochemical cell once it is dipped into a solution. The meter is calibrated with a buffer of known pH, and the measured cell emf is then automatically converted into the pH of the solution, which is displayed. 

The method determines the partial pressure of methane in the gas phase above the solution (Henry s law). Methane catalytically oxidizes on a heated platinum filament, that is part of a Wheatstone bridge. The heat generated increases the electrical resistance of the filament which is measured and compared against calibrated standards. 

This change in resistance is then measured by a precision resistance measuring device that is calibrated to give the proper temperature reading. This device is normally a bridge circuit, which will be covered in detail later in this text. 

SEC is measured with a conductivity meter, which normally consists of an AC bridge and a conductivity cell or electrodes. The conductance is measured between two electrodes. Two solutions of known conductivity should be used, one to calibrate the metre and the other to check the slope. It is important to correct all data for water temperature, either by calculation or by automatically using the metre s auto-temperature correction mode, since SEC is highly dependent on temperature. SEC increases by about 2% per degree centigrade rise in temperature due principally to an increase in water viscosity. 

The sample cup is then precompressed by means of a spanner wrench. The resistance change of the strain elements imbalances the bridge current, providing a deflection of the potentiometer, galvanometer, or oscilloscope beam. The same electrical equipment was used to calibrate the gage. In this way, the initial precompression and pressure vs. time during impact and explosion are accurately measured. 

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