17
CALIBRATION
The calibration information is kept in non-volatile EEPROM. If this fails the
instrument displays “Mem Fail” when turned on and will load default calibration
values and future readings may be 25% in error. If this happens a question mark “?” is
displayed on the top right of the display with suspect readings.
Humidity
Like most other RH sensors the 764 should be checked every 6 months or a year. This
can be done by returning the unit to the manufacturer or by using the optional
humidity calibration kit.
The Calibration Kit
This utilises the fact that some salts provide a constant humidity above a saturated
solution. The humidity sensor is placed in a sealed chamber with a saturated solution
of a salt and is then told to calibrate itself, the 764 then measures the current
temperature and calculates what the humidity should be above that salt, allowing it to
work out the calibration constants.
The kit consists of two sample chambers, a support stand, 2 different salt standards
and some pure water. The calibration salts are Magnesium Chloride (MgCl) and
Sodium Chloride (NaCl), these are not particularly toxic (Sodium Chloride is common
table salt) but normal care should be exercised and they should be kept away from
skin, eyes and mouth.
Notes:
It is important that the procedure is done at a constant temperature so that when the
calibration is done the temperature of the saturated salt is the same as the air above it.
The best place is an underground room, if this is not available use somewhere with a
constant temperature, away from sunlight, heaters, open windows etc. A closed
wooden desk draw or cupboard is quite good.
If Magnesium Chloride is left exposed to the air (ie with the lid off) it will absorb
moisture from the air until it overflows and makes a mess.
The calibration procedure is as follows:
1. Read through the whole procedure before doing anything.
2. Put a level teaspoon of Magnesium Chloride in the bottom of one chamber and the
same amount of Sodium Chloride in the bottom of the other. Be very careful not to
contaminate the two salts with each other. Label the 2 chambers.
3. Put a few drops of the pure water onto the salts to make a paste, there should be
plenty of undisolved salt but none of it should be dry. The level of the paste should
be well below the side hole where the sensor is to be inserted, if any paste gets into
the side hole then clean the chamber in running water, dry and start again.
4. Put both the top and side lids on the chambers and leave them on a level surface in
a constant temperature environment for at least 2 hours.
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