Temperature compensation circuit

P-type silicon piezoresistive elements in high concentration of 1020 cm-3 are used to make a bridge configuration. The resistance of those elements increases with temperature (positive temperature characteristic). On the other hand, the piezoresistive coefficient 7r44 determines the sensibility decrease with temperature (negative temperature characteristic). The temperature-compensation circuit is made by making use of these characteristics. Fig. 7.3.8 shows the principle of the temperature-compensation circuit and the concept of temperature compensation. 

All the tests recommended by the manufacturer should be performed before the cruise. In particular, the temperature compensation circuit requires attention. 

The instrument works with a temperature compensating circuit that corrects for changes in the cell temperature relative to the temperature at which the RSIO was standardized. This tolerance is given in the manual, and is of the order of 3K. Keeping the laboratory temperature stable within these limits to the standardization temperature is essential for the accuracy of the results. If the laboratory temperature is beyond the limits, the drift of the RSIO needs to be checked with SSW, and a new standardization made. 

The reference oscillator (sometimes referred to as the time base oscillator) is used to provide a known frequency or time interval. Some form of ciystal oscillator is generally used to provide a reference frequency of 1 or 10 MHz. Although the stability of such oscillators is inherently good, it can be improved by temperature compensation circuits or, when used in the top-quahfy instruments, by being installed in a constant-temperature oven. 

Ni alloys of 30—32 wt % are used as temperature-compensator alloys and are characterized by a steep decrease ia magnetic permeabiUty with temperature. These alloys are ideally suited ia electrical circuits as shunts which maintain constant magnetic strength ia devices such as electric meters, voltage regulators, and speedometers. 

The Nernst equation shows that the glass electrode potential for a given pH value will be dependent upon the temperature of the solution. A pH meter, therefore, includes a biasing control so that the scale of the meter can be adjusted to correspond to the temperature of the solution under test. This may take the form of a manual control, calibrated in 0 C, and which is set to the temperature of the solution as determined with an ordinary mercury thermometer. In some instruments, arrangements are made for automatic temperature compensation by inserting a temperature probe (a resistance thermometer) into the solution, and the output from this is fed into the pH meter circuit. 

A typical thermocouple installation for an industrial application is shown in Fig. 6.23. Instead of placing the reference junction in a temperature controlled environment (which is often inconvenient), an automatic reference junction compensation circuit is fitted. This provides a second source of emf Sj,° in series with the thermocouple emf E. The meter thus measures 0 = E 0 where E%-0... 

Flow-through conductivity sensors suitable for insertion in pipelines (see Fig. 6.47a) are now available for use at temperatures up to 480 K. and pressures up to 1700 kN/m2(64). As conductivity is temperature sensitive, a thermistor is usually included in the detector circuit as part of a temperature compensator. Screw-in cells (Fig. 6.476) will withstand higher pressures. More recently, electrodeless methods of measuring conductivity have become available. In this case the solution is placed between two energised toroids. The output voltage of the instrument (from the output toroid circuit) is proportional to the conductivity of the solution provided that the input voltage remains constant. This type of conductivity meter can be used under much more severe conditions, e.g. with highly corrosive or dirty systems 43 . 

Magnetic shunts for temperature compensation in magnetic circuits... 

Various external attachments can be fitted to moulded case circuit breakers, e.g. pad-locking tabs, shunt trip coils, hazardous area enclosure with an on and off operating handle, withdrawable rack mountings, undervoltage tripping units, auxiliary switches of the normally open and normally closed types, interlocking devices, ambient temperature compensation for the protection curves. Some... 

Commercially available instruments, one of which is shown in Figure 10, have useful ranges extending from 50 pS cm to 2 S cm with relative accuracy of a few tenths of a percent of full-scale, after-temperature compensation. A temperature sensor is incorporated in the toroid probe, and a compensation circuit corrects readings to the standard reference temperature of 25 °C. 

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