Metrohm 767 Calibrated Reference - Manual (Page 6)

1 Overview
2
767 Calibrated Reference Instructions for Use
1.2 Functional description
As mentioned before the input resistance of high-impedance measuring
amplifiers (pH Meters, Titrators) and, for separate amplifiers, the insula-
tion of the reference point from the earth can be checked.
This is done using the potential of a reference diode (approx. 1200 mV)
on the one hand at output socket (5) +U/direct and on the other hand
a high-impedance resistor (1 GΩ) at socket (4) +U/1 GΩ. This potential
is also switched to socket (6) −U÷/direct by a divider. This means that
a lower potential (approx. 341 mV) with inverted polarity is also avail-
able; this can also be converted to a value within the pH scale (approx.
pH 12.7).
The reference diode is fed by a solar cell. This means that neither a
mains supply nor a battery is necessary and makes the instrument vir-
tually maintenance-free. An internal potential monitor ensures that the
output potential is switched off under inadequate lighting conditions be-
fore the tolerance requirements are no longer fulfilled.
The solar cell can be covered and thus switched off. The potential
monitor then switches a second electronic switch so that the internal
resistance of the switched-off source is 14.3 kΩ. This resistance can be
used to check the current and voltage sources built into pH Meters and
Titrators in a very simple manner. The voltage divider at socket (6)
−U÷/direct gives a resistance of about 460 kΩ, which can also be used
for testing purposes.
For checking the temperature measuring amplifier the 0°C resistances
of the temperature sensors Pt 100 and Pt 1000 are built-in, see sock-
ets (1), (2), (3). These are separated from the other circuits in the test
instrument. This means that no unwanted earthing loops can occur
when they are used.
This means that 4 resistance values which can be used for checking
conductivity meters.
For the 767 Calibrated Reference we have done without fine adjustment
and instead have entered the resulting exact values in the table on the
cover. In this way we have gained a considerable degree of accuracy
and stability. In addition we have converted the resistance values into
conductance (µS) and temperature (°C), and the potential into the exact
pH value wherever this makes sense. This means that it is possible to
compare the display of the instrument to be tested directly with the cor-
responding value in this table. Two different tables are provided for the
open and closed covers.
Practice has shown that problems are often caused by the electrode
cable. They are subjected to mechanical stress (tension, pressure, tor-
sion, etc.) and on the other hand they constantly and unavoidably come
directly into contact with chemicals (spilt solutions, vapors, etc.). Never-
theless their insulation value must always remain exactly as good as
that at the measuring amplifier input. Such an exposed element must
therefore be included in a test at all costs. This is why this measuring
source is equipped with sockets which correspond to the plug head of