Analogue meters/ instruments

From the late 1930s to the mid-1960s, the second generation appeared. These instruments used sensors to convert chemical or physical properties into electrical signals. Electrical circuits were then employed to produce meaningful data (Fig. 8.3). Analysts interacted with these instruments using knobs and switches to obtain data from output devices such as analogue meters and strip chart recorders. 

After suitable amplification, the signal may be read out using a variety of different approaches. Older instruments used an analogue meter as a means of read-out. Considerable advances have been made in read-out systems... 

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). 

Fig. 11.20 (a) Analogue section of an automated PSA Instrument. The operational amplifiers are NS LF 336. The digital panel meter and the differentiating amplifier are optional, (b) Block diagram of the instrument. PI, parallel interface IRQ, interrupt request. Only 4 kb worth of the PROH capacity is used. Bus transceivers are not shown. (Reproduced from [95] with permission of Elsevier). 

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. 

Most meters using an analogue scale incorporate a mirror to eliminate parallax error. The user must look straight at the pointer on the scale when taking readings and the correct position is indicated when the pointer image in the mirror is hidden behind the actual pointer. That is the point at which a reading should be taken from the appropriate scale of the instrument. 

The DC output signal voltage from a lock-in ampli-her is traditionally displayed on an analogue panel meter. On the other hand, digital number-displays become increasingly popular, specifically in digital instruments imder computer control. 

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