Metrohm 767 Calibrated Reference User Manual download pdf (Page 8)

2 General instrument handling

4 767 Calibrated Reference Instructions for Use

2 General instrument handling

2.1 Storage

It is best to store the Calibrated Reference in its own case (with closed

cover) together with its accessory cables. In this way it is protected

against dirt, mechanical stress and moisture.

2.2 Maintenance

The instrument needs no real maintenance (it also contains no batter-

ies). Finger prints or other dirt on the solar cell should be removed with

a cloth which has been slightly moistened with window-cleaning liquid

or alcohol. The colored tables on the cover should not be exposed to

cleaning agents.

2.3 Calibration

The calibration certificate is printed on the cover and contains data re-

quired for retraceability purposes. The separately printed calibration

certificate also contains the dates of the last and next calibrations. We

recommend to carry out a service every 5 years.

It is best to send the instrument back to Metrohm for a new calibration.

Please include all cables belonging to the set so that they can also be

checked. It is expedient to transport the instrument in its own case,

which should be packed in suitable transport packing material.

2.4 ‘High-Impedance’, an important basic term

pH electrodes are potential sources with a very high internal resistance.

If possible, no current should flow from the source in order not to falsify

the measuring potential. This means that the whole measuring circuit

consisting of electrode, cable, plug, socket, switching element up to the

measuring amplifier itself must be extremely well insulated. Only high

quality insulation material such as Teflon, polyethylene, glass, sili-

conized ceramics, etc. come into question. The intention is to achieve

an insulation resistance of up to 10

Ohm. This is quite an extreme re-

quirement. This value can be regarded as being infinite in the following

observations. From the Figure 4, p. 5 it can be seen that the potential E

in the amplifier is always effective, even when R

changes very noticea-

bly with the temperature (which is normal with electrodes).

Minute contamination caused by atmospheric deposits or spilt liquids

can influence the insulation values.

What happens in such a case?

An on-load potential source is formed and there is therefore a potential

drop at R

(see Fig. 5, p. 5). The measuring potential effective at the

amplifier will be falsified by this amount.

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