Null balance

In the dynamic condenser, or the vibrating plate or vibrating condenser method (Fig. 5), also called Kelvin, Zisman, or Kelvin-Zisman probe, the capacity of the condenser created by the investigated surface and the plate (vib. plate) is continuously modulated by periodical vibration (GEN.) of the plate. The ac output is then amplified and fed back to the condenser to obtain null-balance operation (E,V). " " ... 

A change in the ri of the sample stream alters the output of PI and P2, producing a signal at the amplifier output that operates a null-balance system. 

This DSC has two control cycle portions. One portion strives to maintain the null balance between sample and reference, while the other strives to keep the average of the sample and reference temperature at the setpoint. These processes switch back and forth quickly so as to maintain both simultaneously. 

On the other hand, in a genuine DSC instrument, sample and reference are each heated individually. A null balance principle is employed, whereby any change in the heat flow in the sample, (e.g. due to a phase change) is compensated for in the reference. The result is that the temperature of the sample is maintained at that of the reference by changing the heat flow. The signal which is recorded (dH/dt) (the heat flow as a function of time (temperature)), is actually proportional to the difference between the heat input into the two channels as a function of time (temperature). 

The specific conductance (L or K) or conductivity of a solution is always obtained by measuring the resistance (R) of the solution taken in a suitable container of known dimensions called conductivity cell, the cell constant of which has been determined by calibration with a solution of accurately known conductivity e g. a standard KCl solution. The instrument used for electrical conductivity measurement is known as conductivity bridge. A typical system consists of an alternating current (A.C.) Wheatstone bridge, a primary element of conductivity cell and a null balance indicator (as in solubridge ) or an electronic eye as in the conductivity meter. 

Resistance variation can be detected by either null-balance or deflection-balance bridge circuits. In a null-balance bridge the sensor resistance change is balanced (zero output) by a variable resistance in a bridge adjacent arm. The calibrated null adjustment is an indication of the change in sensor resistance. The deflection-balance method, on the other hand, makes use of the amount of bridge unbalance in order to determine the change in sensor resistance. 

It is not obvious why (13.1.31) is called an electrocapillary equation. The name is a historic artifact derived from the early application of this equation to the interpretation of measurements of surface tension at mercury-electrolyte interfaces (1-4, 6-8). The earliest measurements of this sort were carried out by Lippmann, who invented a device called a capillary electrometer for the purpose (9). Its principle involves null balance. The downward pressure created by a mercury column is controlled so that the mercury-solution interface, which is confined to a capillary, does not move. In this balanced condition, the upward force exerted by the surface tension exactly equals the downward mechanical force. Because the method relies on null detection, it is capable of great precision. Elaborated approaches are still used. These instruments yield electrocapillary curves, which are simply plots of surface tension versus potential. 

Several methods for evaluating A and i/j exist (26, 27), but the most precise approaches rely on a null balance like that depicted in Figure 17.1.13. Light that is polarized... 

The effluents of the pump tubes are mixed in the proper reaction sequence, and the final solution enters a tubular flowcell in the colorimeter. A single light source provides twin beams, one passing through the flowcell to a photocell, and the second to an identical reference photocell. A null balance system in the recorder continuously measures the ratio of the sample to reference voltage. 

The differential power supplied is recorded as the ordinate versus the program temperature as the abscissa. The operating principles of the Perkin Elmer system are described [142] and the advantages are indeed very subtle and it is probably true to say that both DSC and DTA instruments can yield valuable information. Figure 17.70 shows a schematic representation of the DSC control loops—one loop controls the average temperature Tp, so that the sample and reference may be increased at a predetermined rate, which is recorded. The second loop ensures that if a temperature difference does occur between the sample and reference, the power imput is adjusted to correct this difference. This is the so-called null-balance principle. 

In the absence of carbon monoxide in the sample cell, the two sensor chambers are heated equally by IR radiation from the two sources. If the sample contains carbon monoxide, however, the right-hand beam is attenuated somewhat and the corresponding sensor chamber becomes cooler with respect to its reference counterpart. As a result, the diaphragm moves to the right and the capacitance of the capacitor changes. This change in capacitance is sensed by the amplifier system. The amplifier output drives a servomotor that moves the beam attenuator into the reference beam until the two compartments are again at the same temperature. The instrument thus operates as a null balance device. 

Zeitgeber zero n Null zero vb auf Null stellen zero adjustment/null balance Nullabgleich zero-order... 

The servomotor itself receives its signal from a conventional servo-amplifier, which in turn receives a chopped error signal from a null type pH electrometer. The electrodes which supply the signal to the electrometer are located in the reaction cell. Since the electrometer is of the null-balance type, a preset end point is required and this presupposes a knowledge of the titration curve. This electrometer system has been described previously [3]. 

The calorimetric methods, DSC and DTA, are schematically presented in Fig. 10.2. DSC relies on the so-called null-balance principle (Fig. 10.3). The temperature of the sample holder is kept the same as that of the reference holder by... 

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