Design Of An Instrument For

A research program was designed to answer the following questions:

1. Can a simple instrument for clinical use be built on the indications arising from the preliminary results highlighted above?

2. Are measurements of ESR repeatable intra- and inter-rater?

To answer these questions a measuring device for clinical use was built with the following specifications:

• A probe with a 1 mm diameter steel tip, loaded with a spring to ensure a contact force of about 20 g.

• A hand-held reference electrode made from a 30 mm diameter steel cylinder.

• A range of measurement with 20% accuracy from 10 kO to 100 MO.

• A computer displayed a resistance vs. time diagram with a logarithmic scale, with the possibility of displaying numerical value of resistance every second and the average of the last 10 s.

The block diagram is shown in Figure 6.2.

In such a circuit the measurements of low values of ESR are taken at practically constant current, if the reference resistance (RR) is much higher than the tested resistance. The high values are measured practically at constant voltage, since RR in this case is much smaller than the measured resistance. The former situation is obtained when RR is a few MO and skin resistance value is a few tens of kO and the latter when ESR is several tens of MO. So there are controlled conditions (constant current or voltage respectively) in the high and low ranges where

030 mm reference electrode

Fig. 6.2 Block diagram of the device for measuring resistance at steady contact pressure. RR is a reference resistance in series with ESR.

030 mm reference electrode

Precision DC

amplifier

A/D converter

Fig. 6.2 Block diagram of the device for measuring resistance at steady contact pressure. RR is a reference resistance in series with ESR.

ESR tends to cluster, and more uncertain conditions (both current and voltage change) in the middle range.

The probe design is shown in Figure 6.3 and its physical aspect in Figure 6.4.

10 mm gap corresponding to 20 grams spring-loading c

* 015 mm insulated tube

Steel spring

^ r

030 mm - steel cylinder

Measuring device

This hardware is connected with a laptop through a National Instrument acquisition A/D converter, the USB8000, which provides also the 2.5 V supply. The laptop must be used disconnected from the mains and operated only by its internal 12 V battery for safety reasons.

Fig. 6.3 Measuring system, showing the hand-held electrode and the probe design.

Fig. 6.4 Simple implementation of the hand-held probe, with an external spring to keep constant pressure on skin.

A switch button on the box of the device allows the operator to trigger a series of 10 resistance readings, at 1 s intervals. At the end of the 10 readings the software calculates the average and shows it on the computer monitor. An averaged value of resistance over a time range seems more reliable than a single reading, especially when an unsteady hand can cause continuous small slips of the probe tip on the skin.

In this research we did not investigate the possible optimization of the time interval, but adopted 10 s as a compromise between an excessively long measurement time in a clinical setting and the desire of extending measurements until an acceptable stabilization of the resistance value.

For the same reason it was decided to wait 5 s before triggering the series of 10 measurements, for avoiding transient effects of ESR at its first generally rapid descent when the probe is applied to the skin. The value so obtained may not be a true stabilized value of ESR, but at least it is measured under controlled conditions and gives an overall idea of the speed at which the resistance is dropping.

A typical diagram obtained with this device is shown in Figure 6.5. In this figure it is possible to appreciate the instant of probe tip application and withdrawal, at about 185 s and 200 s. Initially the resistance does not seem to drop in any significant way, due to some hesitation of the operator's hand. Then good contact is established, at about 188 s, and resistance drops suddenly. Two changes in inclinations are visible within the first 4 s. For the remaining time of contact there is not a true stabilization, but the resistance continues to fall at a slow pace. The 10 measurements are indicated, together with the final average, concluding the test at about 199 s. The average is what will be called the ESR of the specific skin spot.

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