Metrohm 767 Calibrated Reference - Manual (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 14 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
i
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
i
(see Fig. 5, p. 5). The measuring potential effective at the
amplifier will be falsified by this amount.