Voltage readings, digital

If digital voltage readings (V1-V9) are taken at time intervals (At = 0.0001 sec in the example of Figure 44.4), then the area of the true peak (dotted) can be (mathematically) closely approximated to give ion abundance and, similarly, the time (tj to the center of gravity (centroid) of the peak can be determined, thereby giving the m/z value. 

The electronics involved in converting the detector signal to usable voltage or digital reading has a large effect on the operation and response of the detector.. 

The voltage output of the more common types of thermocouple is of the order of 50V/C and the output is either read on a sensitive moving-coil meter or on a digital voltmeter. The reading is converted to temperature using a calibration chart supplied with the thermocouple. Some commercial units are available in which the thermocouple and instrument is supplied as an integral unit with the scale directly calibrated in temperature. If a separate instrument is to be used then it should be noted that the thermocouple resistance is only of the order of 10 and... 

The simple electrical circuit shown in Fig. 16.15(h) is suitable for this procedure. The voltage applied to the titration cell is supplied by two 1.5 V dry cells and is controlled by the potential divider R (a 50-100 ohm variable resistance) it can be measured on the digital voltmeter V. The current flowing is read on the micro-ammeter M. 

The electrical circuit, Figure 17.2 (b), consists of two 1.5 V dry cells that provides a voltage applied to the above titration cell. It is duly controlled and monitored by the potential divider (R) and is conveniently measured with the help of a digital voltmeter (V). Finally, the current flowing through the circuit may be read out on the micro-ammeter (M) installed. 

Another part of the loop program ascertains how much solution is left in the pump. A voltage picked off a ten-turn potentiometer, which is coupled to the piston-drive screw with a timing belt, is fed to the analog-to-digital converter and is compared with the previously stored reading from the thumbwheel switch. 

Figure 8.2 The digital ohmmeter uses a constant-current source to produce a voltage across the unknown resistance proportional to its resistance, and a digital voltmeter to read that voltage.

FIGURE 6.9 Typical wiring of a photomultiplier. Each of the dynodes, beginning in the photocathode, is linked stepwise by resistors RD of the same value. The photocurrent is read as the voltage drop across the load resistor R with high impedance filter (Rf, Cp) and oscilloscope or transient digitizer (detector). 

Sampling rate — The speed at which samples are read and converted into a voltage or a digital expression. The Nyquisfs theorem establishes that the sampling rate must be greater than twice the maximum frequency component in the signal to be acquired to avoid the aliasing of data [i]. 

The reported temperature settings for the graphite furnace were read from the HGA-2100 power supply readout and are approximate. These temperatmres are based upon the applied voltage across the atomizer terminals. All absorbances were obtained from peak heights read from either the strip chart or the digital peak height reader of the Perkin Elmer 503. 

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