An Offset Device for Interfacing Pen Recorders to Meters D a d E. Davey, Robln D. Hall, Demls E. Mulcahy, and Gregory R. O'Carnell School of Chemical Technology, S. A. Institute of Technology, P.O. Box 1, lngle Farm, South Australia 5098. Australia
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Althoueh comnuterized data acauisition is referable in cases where a continuous record is required, many scientific instruments. includine s~ectro~hotometers end D H / ~ V meters, are still interfakd-to strip-chart pen recorders, especially in teaching laboratories. Often, however, output signals from the meter will lie offscale,beyond the extent of the recorder's zero control. Although some meters do feature an adjustable offset control, this is seldom sufficient to get the signal within range. Under such circumstances, an external offset device may be required. The unit described here is based on a simple circuit, requiring two 68-kQresistors and a 10-turn 50-kQpotentiometer (Fig. 1).The entire circuit may be housed in a small zippy box, the dry cell holder being attached externally to facilitate the replacement of spent batteries. A flashlight cell
powers the unit, allowing a maximum 1500-mV offset to be accommodated. Connections to the meter and recorder should be made using leads fabricated from shielded dualcore cable. For measurements where a negative offset is required, the input leads from the meter may be reversed. In contrast to other such units, the power drain is negligible. Over the past four years, we have used a number of these devices in interfacing Orion digital pHlmV meters and Turner colorimeters to Omniscribe and MFE chart recorders for both teaching and research purposes (Fig. 2). From our experience, the dry cell can last in excess of a year, regardless of whether the device is used regularly or intermittently. None of the devices we have employed have shown any tendency to generate base-line drift, making them ideal for use in continuous-flowexperiments.
F i p e 1. Circun diagram tor me c h a rscwdsr ~ onsst mvice. The numbers 1. 2. and 3 refer m specitic twmmels on the 50-kn pmentiometer.
Figure 2. An example of me offset device k opration. The peaks ln(a)needm be expanded uslng the 10X m i c a 1 scale expansion facilily on the chsrt recorder. However, with the recwder already at maximum onset (dotted line), the peaks exceed the widlh of the remrder paper (b). The external offset device enables the base line to be reset so that the peaks Ean be brougM within me widlh of the chart paper (c).
784
Journal of Chemical Education
