Full scale deflection

A number of meter designs have been developed based on this principle. Some are shown in Eigure 17. Certain advantages ate claimed for each, but all share a number of characteristics. Perhaps the most important property is a full-scale deflection on the order of 0.001 mm. The sensors for these meters are extremely sensitive, stable, and capable of being temperature compensated. 

The direction of rotation depends on the direction of the current in the coil, and thus the instrument is only suitable for D.C. It is, however, possible to incorporate a full-wave rectifier arranged as shown in Figure 17.11 in order to allow the instrument to measure A.C. quantities. The quantity measured is the RMS value only if the waveform of the current is truly sinusoidal. In other cases, a considerable error may result. In principle, the scale is linear but, if required, it can be made non-linear by suitably shaping the poles of the permanent magnet. The instrument reading is affected by the performance of the rectifier, which is a non-linear device, and this results in the scale also being non-linear. The error when measuring D.C. quantities can be as low as 0.1 per cent of full-scale deflection and instruments are available for currents between microamperes and up to 600 A. 

Valve voltmeters were widely used in the past, but have been replaced by transistor voltmeters. With instruments of this type it is possible to achieve an input resistance of 50 MQ or more, the current required to operate the instrument being of the order of 10" A. The early instruments had a tendency to zero drift on the lower ranges, but this has been overcome in the modern transistor types. Such instruments are most often used to make potential readings in extremely high-resistance electrolytes. The accuracy of such instruments is of the order of 2% full-scale deflection. It is necessary to ensure that both types are so designed that they do not respond to alternating currents. 

Procedure. Pipette 25.0 mL of the thiosulphate solution into the titration cell e.g. a 150mL Pyrex beaker. Insert two similar platinum wire or foil electrodes into the cell and connect to the apparatus of Fig. 16.17. Apply 0.10 volt across the electrodes. Adjust the range of the micro-ammeter to obtain full-scale deflection for a current of 10-25 milliamperes. Stir the solution with a magnetic stirrer. Add the iodine solution from a 5 mL semimicro burette slowly in the usual manner and read the current (galvanometer deflection) after each addition of the titrant. When the current begins to increase, stop the addition then add the titrant by small increments of 0.05 or 0.10 mL. Plot the titration graph, evaluate the end point, and calculate the concentration of the thiosulphate solution. It will be found that the current is fairly constant until the end point is approached and increases rapidly beyond. 

Repeat procedures (6) and (7) until full-scale deflection and zero settings are obtained. 

All three sources of noise combine to form the type of trace shown at the bottom of figure 3. In general, the sensitivity of the detector should never be set above the level where the combined noise exceeds 2% of the FSD (full scale deflection) of the recorder (if one is used) or appears as more than 2% FSD of the computer simulation of the chromatogram. 

A concentration of 0.15 pg/ml gave a fluorescence of 10% full-scale deflection at maximum instrumental sensitivity. These authors explored analyzing folinic acid in the presence of folic acid and found that excitation at 290 nm effectively shifted the emission band of the compound of interest to 370 nm, thus enabling analysis of a mixture. 

In general, the signal from a gas chromatograph is recorded continuously as a function of time by means of a potentiometric device. Most frequently, a recorder of 1-10 mV full-scale deflection ( 10 inches) and having a response time 1 second or less is quite adequate. 

Inject 5 0 pi of each solution. Adjust the sensitivity of the detector so that the height of the principal peak in the chromatogram obtained with solution (2) is 50-70% of full-scale deflection. In the chromatogram obtained with solution (1) the sum of the area of any peak eluting before the principal peak is not greater than the area of the principal peak in the chromatogram obtained with solution (2) (1.0%). 

Sample flow rates and dilution factors when olfactometer runs correctly and percentage deviation when galvanometer reads full scale deflection... 

During operation, it was difficult to obtain steady null readings on the galvanometer, particularly when the sample flow rates were small. In some positions, small movements of the control valve caused a full scale deflection (FSD) of the galvanometer while in other areas, movement of the valve in the same direction caused the flow to increase and decrease alternately. Fig. 3 shows the variation of flow rate with valve position, within... 

Another feature of major interest is tlie detector sensitivity. This is the total change in a physical parameter required for a full-scale deflection at maximum detector sensitivity and at specified noise level. Detector sensitivity can be considered as a measure of the ability of tlie detector to differentiate between small differences in analyte concentration. Modern detectors have usually high sensitivities, often allowing detection of nanograms of analytes. 

Refill the cell with air-saturated water use this as the 100% (fully saturated) setting and adjust the recorder pen to 95% full-scale deflection. 

This is often termed the full scale deflection (FSD) and is the magnitude of the range of the instrument. In the above example, the input span of the thermocouple is 150°C and the output span is 6 mV. 

Fig. 11. Polarographic record of the course of hydrolysis of pyridoxal-5-phosphate in 0.01 M perchloric acid. 5 X 10 iM pyridoxal-5-phosphate in 0.01 M-HCIO4, p= 1.0 at 79° C after (1) 15 (2) 23 (3) 41 (4) 50.5 (5) 79 (6) 94.5 (7) 110 (8) 125 (9) 152 minutes (10) 5 X 10-4M pyridoxal. Curves were registered using a penrecording polarograph, constructed by Geratebau DAW Berlin, D. D.R. Full scale deflections corresponds to 10 rA...

Time Constant. The time constant t is a measure of the speed of response of a detector. Specifically, it is the time (usually in seconds or milliseconds) a detector takes to respond to 63.2% of a sudden change of signal, as shown in Figure 7.4. The full response (actually 98%) takes four time constants and is referred to as the response time. Unfortunately, some workers define response time as 2.2 time constants (not 4.0), corresponding to 90% of full scale deflection (not 98%) others define a rise time as the time for the signal to rise from 10 to 90%. To further confuse the situation, some use the terms time constant and response time interchangeably. This lack of consistency can be found in the ASTM specifications. 

Sensitivity expressed as the weight of derivative producing a peak with height equivalent to 10% full-scale deflection at an amplification producing a noise level of 5% f.s.d. 

AUFS. An acronym denoting the output from a spectrophotometer in Absorbance Units (per) Full Scale (deflection). 

Maintain the column at 108°. Set the injection port temperature to 225° and the detector to 250°. Use helium as the carrier gas, with a flow rate of 30 mL/min. Set the instrument attenuation setting so that 2.5 xL of the Diluted Standard Preparation containing 200 p,g/mL of each toluenesulfonamide gives a response of 40% to 80% of full-scale deflection. Record the... 

For sample analyses, maintain the temperatures of the column oven, injector port, and detector at 180°, 250°, and 350°, respectively. Adjust the electrometer to provide about half of the full-scale deflection when 0.1 ng of PCP is injected. 

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