LTH Electronics MTD75, Operation Manual

Page: 127 / 266

pH Calibration
MTD75 Operation Guide - 125 -
pH Cali
bration
pH Input Channel Calibration
Calibration Procedures
Normal good practices should be observed when calibrating electrode systems against standard
solutions.
Always clean the electrode system before inserting in the standard solution to avoid contamination
and to obtain the best response from the electrode system.
Rinse the electrode system in clean (preferably demineralised) water between standard solutions.
Allow time for the electrode system temperature to stabilise in each standard solution.
Use standard solutions of known quality. If in doubt make up fresh solutions.
Use clean beakers and bottles for standard solutions.
The sensor can be taken offline, which de-energises the relays and holds the current outputs at their
last values. This facility is useful when calibrating the system, however the operator must ensure the
relays are in a safe state when using this feature.
pH Buffers
Please see page 137 for information regarding the range of buffer solutions LTH can supply.
Calibration Principle
The MXD70 Series offers two different but simple methods of calibration for pH electrode systems.
Auto
In Auto calibration the instrument requires that the electrode is first placed in a 4pH buffer and then
secondly in a 9pH buffer. These are nominal values for which the instrument carries the exact LTH
buffer values due to temperature variations (other pH buffer solution values at temperature can also
be entered). From these two points the instrument then calculates the Offset and Slope for the
electrode.
In order for the auto calibration to work correctly, the buffer temperature must either be measured by
the instrument during calibration, or if manual temperature compensation is being used the buffer
temperature must be entered in the Calibration Manual Temperature Input in the calibration menu.
Manual
In Manual calibration mode it is possible to do single or two point calibration, using the Buffer and
Slope menu items. It is important to do the calibration in the correct order.
1. Adjustment of the Buffer (Offset) value at 7.00pH
2. Adjustment of the Slope (Gain) value at any other pH (usually 4 or 9 pH).
The output from an ideal glass/reference electrode pair will normally be 0mV at 7pH, therefore the
slope will have little or no effect at 7pH but increasing influence the further from 7pH the calibration
point is.
If a single point calibration is required the buffer should be adjusted and no adjustment made to the
slope that will remain at the previously entered value. As actual pH buffer values are used no
compensation is made for the buffer solution with temperature in the instrument. It is important
therefore to note the actual buffer value at the temperature of the solution, see page 137 for a table
showing pH variations versus temperature for LTH’s standard solutions.
Temperature has an effect on the output from the pH electrode as well, so it is important that the
buffer temperature is entered into the Calibration Manual Temperature Input if manual temperature
compensation is being employed. Alternatively any automatic temperature compensation element
should be placed in the buffer solution with the pH sensor if Auto TC is being used.