Digital meters/instruments

Several types of readout devices are found in modern instruments. Some of those devices include the D Arsonval meter, digital meters, recorders, cathode-ray lubcs, LCD panels, and computer displays. 

Figure 9- 1.1b is a schematic of a typical double-beam-intime instrument. The beam from the hollow-cathode source is split by a mirrored chopper, one half passing through Ihe tlame and the other half around it. The two beams are then recombined by a half-silvered mirror and passed into a Czerny-Turner grating monochromator a photomultiplier tube serves as Ihe transducer. The output from the latter is the input to a lock-in amplifier that is synchronized with the chopper drive. The ratio between the reference and sample signal is then amplified and fed to the readout, which may be a digital meter or a computer. 

Since most electrochemical measurements relating to corrosion of metals are satisfied with a sensitivity of 1 jiV or 1 xA, modern instrumentation usually employs electronic operational amplifiers where the noise limits control the range of measurements. The function of the operational amplifier is to amplify the potential (Vg) applied at the input so that it can be displayed on a low impedance analogue or digital meter (V ) as shown in Fig. 1.1. The output potential of the operational amplifier is proportional to the source potential and is required to have sufficient input impedance to avoid polarisation of the potential source. 

Modern digital test instruments are reasonably robust, but to maintain them in good working order they must be treated with care. An approved test instrument costs equally as much as a good-quality camera it should, therefore, receive the same care and consideration. Let us look at the requirements of four often used test meters. 

Load n-hexane into the instrument, obtain a vapor pressure reading and then adjust the zero potentiometer for the transducer to obtain a calibration value of 5.00 -0.02 on the digital meter display. 

Load R-pentane into the instrument, obtain a pressure reading on the digital meter, and then adjust the transducer span potentiometer to achieve a value of 15.40 -0.05 on the digital meter. 

At the completion of the test, record the uncorrected dry vapor pressure reading fix>m the digital meter to the nearest 0.1 kPa (0.01 pd). If the instrument does not automaticaUy calculate the DVPE, record the uncorrected vapor pressure reading and calculate the DVPE using Eq 1 (see 13.1). 

For the practical implementation of the above mentioned facts, the portable electronic digital coercive force meter with self-contained power supply, attached converter and closed type magnetic core. Instrument error is 5 %. ... 

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 pH of a solution can be measured by an instrument called a pH meter. A pH meter translates the H+ ion concentration of a solution into an electrical signal that is converted into either a digital display or a deflection on a meter that reads pH directly (Figure 13.4). Later, in Chapter 18, we will consider the principle on which the pH meter works. 

Data acquisition is not treated in this book. The most common technique is to convert a physicochemical signal into a voltage by means of a sensor, and feed the electrical signal into a digital volt meter or a chart recorder. With today s instrumentation this is no longer the problem it used to be. 

Belton Road West, Loughborough Leicestershire LEll OTR Digital Panel Meter - Model 2751-K Dlgltron Instrumentation Mead Lane, Hertford Hertforshire SG13 7AW... 

In this series of instruments for analytical electrochemistry, Philips also supplies the microprocessor-controlled PW 9527 digital conductivity meter with 16 push-buttons and on the rear an analogue output for connection to a recorder and a 25-way connector providing a two-way RS 232 serial connection (see Philips leaflet 9498 362 9326). 

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