Metrohm 767 User Manual

CH-9101 Herisau/Switzerland
E-Mail [e-mail]
Internet www.metrohm.com

767 Calibrated Reference

for mV, pH, S, :S, °C

Instructions for Use

8.767.1023 08.2005 / ars

Teachware
Metrohm AG
Oberdorfstrasse 68
CH-9101 Herisau
[e-mail]

These instructions are protected by copyright. All rights reserved.

Although all the information given in these instructions has been checked with great care, errors
cannot be entirely excluded. Should you notice any mistakes please inform the author at the
address given above.

Contents

Table of contents

1 Overview ............................................................... 1

1.1 Introduction ....................................................................................................1

1.2 Functional description ...................................................................................2

2 General instrument handling............................... 4

2.1 Storage ...........................................................................................................4

2.2 Maintenance ...................................................................................................4

2.3 Calibration ......................................................................................................4

2.4 ‘High-Impedance’, an important basic term’ ................................................4

2.5 Measurement of the insulation resistance ...................................................5

3 Procedure for checking instruments .................. 6

3.1 Basics .............................................................................................................6

3.2 pH Meters and Titrators.................................................................................6

3.2.1 U/mV, pH ..................................................................................................7

3.2.2 Polarization current and voltage source ..................................................8

3.2.3 Temperature (Pt 100/Pt 1000)..................................................................8

3.2.4 Tolerances ................................................................................................9

3.3 Conductivity meters .......................................................................................9

3.3.1 Conductance............................................................................................9

3.3.2 Temperature ...........................................................................................10

3.3.3 Tolerances ..............................................................................................10

3.4 Rancimat 617 and 679 ................................................................................ 10

4 Checking by means of the diagnosis

instructions ........................................................ 12

5 Appendix ............................................................. 13

5.1 Technical specifications ............................................................................. 13

5.1.1 Measuring source...................................................................................13

5.1.2 Temperature coefficient .........................................................................13

5.1.3

5.1.4 Ambient temperature..............................................................................14

5.1.5 Safety specifications ..............................................................................14

5.1.6 Electricity supply.....................................................................................14

5.1.7 Dimensions.............................................................................................14

5.2 Cables for connecting 767 – instrument X ................................................ 14

5.3 Standard equipment ................................................................................... 15

5.4 Warranty and conformity ............................................................................ 16

5.4.1 Warranty..................................................................................................16

5.4.2 Declaration of Conformity ......................................................................17

5.4.3 Quality Management Principles .............................................................18

Longterm stability (2 years)....................................................................13

6 Index ................................................................... 19

767 Calibrated Reference Instructions for Use I

Contents

List of illustrations

Fig. 1: 767 Calibrated Reference...............................................................................................1

Fig. 2 Functional scheme: Position "Voltage source ON" .........................................................1

Fig. 3: Label on cover ................................................................................................................1

Fig. 4: Unloaded potential source .............................................................................................5

Fig. 5: On-load potential source................................................................................................5

Fig. 6: Measurement of the insulation resistance versus earthing ...........................................5

Fig. 7: Cable arrangement for 1st G value (15.3 kS →approx. 66 :S) ..................................11

Fig. 8: Cable arrangement for 2nd G value (14.3 kS → approx. 69 :S) ................................11

II 767 Calibrated Reference Instructions for Use

1 Overview

1 Overview

1.1 Introduction

The measuring source 767.0010 Calibrated Reference for mV, pH,
Ω, µS, °C is a calibrated instrument for the quantities mentioned above.

It is connected instead of the electrodes and can be used for rapidly
and easily checking of the functioning and the basic accuracy of most
Metrohm instruments.

In addition the input resistance of high-impedance measuring amplifiers
(pH Meters, Titrators) and, for separate amplifiers, the insulation of the
reference point from the earth can be checked.

Fig. 1: 767 Calibrated Reference

U R G T U R G T

+U/1G

+U/direct

-U+/direct

(1)

(2)

(3)

(4)

(5)

(6)

solar cell

S1a

S1b

100 Ohm

1000 Ohm

1G

Pt 100

Pt 1000

Fig. 2 Functional scheme: Position "Voltage

source ON"

767 Calibrated Reference Instructions for Use 1

mV pH S :S °C mV pH S :S °C

100 10'000 0 100 10'000 0

1000 1000 0 1000 1000 0

(1200) 0 7 1G

1200 0 7 14'300 70

-341 12.7 0 7 460'000 2

Fig. 3: Label on cover

(exact values are given on the cover)

cover open ¦ cover closed

1 Overview

1.2 Functional description

As mentioned before the input resistance of high-impedance measuring
amplifiers (pH Meters, Titrators) and, for separate amplifiers, the insula­tion 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 avail­able; 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 vir­tually maintenance-free. An internal potential monitor ensures that the
output potential is switched off under inadequate lighting conditions be­fore 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 sock­ets (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 cor­responding 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, tor­sion, etc.) and on the other hand they constantly and unavoidably come
directly into contact with chemicals (spilt solutions, vapors, etc.). Never­theless 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

2 767 Calibrated Reference Instructions for Use

1 Overview

Metrohm electrodes and means that a test can be carried out very eas­ily:

Screw off cable at electrode plug-in head → plug into Calibrated
Reference → measure

If this check produces a variation from the expected result then it is not
immediately clear as to whether the error lies in the instrument to be
checked or in the cable. Therefore specially labeled cables are avail­able in the accessories which are included with the instrument; these
can be temporarily used instead of the original cable (see list of cables
chap. 5.2). These accessory cables are also useful for sensors which
do not have a plug-in head.

Note

It must be mentioned here that under no circumstances can or should
the 767 Calibrated Reference replace the periodic maintenance of the
instrument, but should only be used for determining whether an error
is present or not if functional difficulties occur. In addition, the basic
accuracy and high impedance of the instrument can be checked at
regular intervals.

During maintenance the instrument is subjected to a far more strin­gent check (e.g. linearity of display and A/D converter, etc.). In addi­tion the switches, motors, mechanical components, etc. are also che­cked for corrosion and wear and tear.

767 Calibrated Reference Instructions for Use 3