Null detectors

Figure Bl.27.8. Schematic view of Picker s flow microcalorimeter. A, reference liquid B, liquid under study P, constant flow circulating pump and 2, Zener diodes acting as heaters T and T2, thennistors acting as temperature sensing devices F, feedback control N, null detector R, recorder Q, themiostat. In the above A is the reference liquid and C2is the reference cell. When B circulates in cell C this cell is the working cell. (Reproduced by pemiission from Picker P, Leduc P-A, Philip P R and Desnoyers J E 1971 J. Chem. Thermo. B41.)...

H = A Null detector to record the intensity of the element under study and the internal-standard (Lithium),... 

Apparatus. All electrical resistances were measured with an electrolytic conductivity bridge (Leeds and Northrup model 4666) which was constructed according to specifications set forth by Jones (28) and described by Dike (29). The audio-frequency source was a General Radio Co. type 1311-A audio oscillator used with the frequency regulated at 1000 Hz and the output at about 5 V. The detector circuit consisted of a high-gain low-noise tuned amplifier and null detector (General Radio Co. type 1232-A) and an oscilloscope (Heathkit model O-ll) ... 

The Wheatstone bridge shown in Figure 8.5 provides the most direct and best known circuit for comparison of unknown resistances against standard resistances. Resistances RA, RB, and R are standard resistance values that are used in the measurement of the unknown resistance R Resistance R is made variable and is adjusted until the null detector indicates that the bridge is balanced. 

When the circuit is at balance, there is no current through the null detector and no potential difference between terminals x and y. At balance, four significant conditions exist ... 

At balance, the points at d and b on the ac bridge must be equal in magnitude as well as in phase. The simplest method for determining when this condition exists is to use an oscilloscope connected as shown in the figure as the null detector. If precautions are taken to ensure that the excitation and the null signal are electronically isolated, the balance condition is easily, albeit slowly, obtained [16]. 

Calorimeter temperature was measured with a Leeds and Northrup G-l Mueller bridge used in conjunction with a d.c. Null Detector (No. 9834) or with a moving coil galvanometer (No. 2284-D) and lamp and scale. 

The sampling loops were replaced by two stainless steel U-tubes of 1.5- and 20-cc. capacity. The expansion bomb is a 1.7-liter stainless steel cylinder. The trap between the helium supply and the Beckman valve is 1/4-inch stainless steel tubing. A null detector is used to measure pressures in the inlet system. Samples are obtained in 10-ml. stainless steel cylinders fitted with a Vg-inch stainless steel Hoke valve with a V-stem and Teflon packing. When the sample is liquid, it is entirely vaporized into the 1.7-liter expansion bomb, and a gaseous sample is taken for infrared, near infrared, and gas chromatographic analysis. 

Conductivity Measurements. Cell resistance measurements were made with a General Radio type 1650-A impedance bridge. It is equipped with an internal, 1000-cycle signal source and tuned null detector. For more sensitive balance at high resistances, a Hewlett Packard 400L vacuum tube voltmeter is used as an external null detector. 

Two methods of using the oscilloscope as a null detector are described below. 

The accuracy of a voltage measurement depends most of all on the design and the quality of the potentiometer unit used. For a precision potentiometer, the relative accuracy of measurement (ability to measure very small changes in voltage) wUl depend a great deal on the null detector sensitivity, while the absolute accuracy will depend more on the accuracy of the standard-cell voltage value and the linearity of the slide-wire. The most accurate potentiometers replace the slide-wire by discrete resistors and switches. 

The general bridge balance condition, where no ac voltage is observed by the null detector, is... 

Emf measurements were made with a Leeds and Northrup K-5 potentiometer equipped with a Leeds and Northrup DC null detector (Model 9829). The temperature of the bath was regulated to within 0.02 K. Details of the experimental procedure, including preparation of the electrodes (19), cell design, preparation of solutions, purification of the hydrogen gas, and other experimental aspects, have been reported elsewhere (13,14). 

The single-pan balance is a commonly used balance in the clinical laboratory. It is most often electronically operated and self-balancing. Such a balance may be coupled directly to a computer or recording device. In the electronic singlepan balance, a load on the pan causes the beam to tilt downward. A null detector senses the position of the beam and indicates when the beam has deviated from the equilibrium point. 

Solution resistances were measured at 1000 Hz with an AC Wheatstone bridge an HP 200 AB oscillator provided the input and a tuned amplifier with a null detector was used to find the bridge balance (with simultaneous resistance and capacitance balancing). A homemade cell with Pt electrodes was used, similar to the one described in Ref. 27a. The cell was calibrated with 0.10N NaCl solutions. 

Ewing (57) has reviewed electronic laboratory balances. The various types of null detectors, such as optical, inductive, and capacitance types, as well as the electronic readouts are discussed. 

The temperature distribution within the cryostat was measured with six 20-gauge copper-constantan thermocouples located throughout the cryostat, these thermocouples being referenced to the resistance thermometer. Thermocouple emf s were measured with an L N Type K-3 potentiometer in conjunction with a Model 9834 electronic dc null detector. Inasmuch as these differential thermocouples generate small emf s, their accuracy is limited by the potentiometer, this accuracy being 0.02°C in this investigation. During operation the temperature distribution in the bath liquid was within 0.1°C of the resistance thermometer. 

The resistance ratio for Chromel-P thermocouple wire was determined with the circuit shown in Fig. 1. A fixed current was supplied to the specimen with a stabilized power supply (Trygon Model H20-1.5) having remote sensing. Two laboratory-type rheostats were used to give additional fine current adjustments the specimen current was maintained at a selected value of 0.10000 amp by adjusting the potential across the standard 1-ohm resistor, Rs, to 0.10000 v. The potential across the standard resistor and the specimen coil was determined with a potentiometer (L N K-3), a null detector (L N Model 9834-1), and an Eppley standard cell. 

Measurement of conductance, which is a measurement of the reciprocal of resistance, is done with a Wheatstone bridge such as that shown in Figure 3. In its traditional form, the bridge consists of four resistances, the unknown cell resistance Rq and three precision variable resistances, Ri, Rx, and R3, which are manipulated to show zero voltage drop across BD as observed by the null detector ND. Calculation of the unknown resistance is then derived from Ohm s law, where V is the voltage and / the electrical current across a resistance R ... 

The resistances B, D, and E can be measured, and from these measuranents, the resistance and hence the conductance of the cell can be calculated. A small superimposed AC voltage ( 20 mV peak to peak) at 1000 Hz is best as a signal, because then faradaic polarization at the electrodes is minimized. The null detector may be a sensitive oscilloscope or a tuned amplifier and meter. Stirring is often used to minimize polarization. 

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