14
CALIBRATION
The calibration information is kept in non-volatile ROM. 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 765 should be checked every 6 months or a year. This
can be done by returning the unit to the manufacturer, by using the optional humidity
calibration kit or by comparison to another known good source.
The 765 is fitted with a solid state sensor that should not require recalibration unless it
is damaged by solvents, dirty water or atmospheric pollutants.
If the sensor is contaminated it can usually be reconditioned by the following
procedure:
1. Remove the sensor from the instrument.
2. Bake the sensor at 100-105°C, less than 5% RH for 10 hours
3. Allow the sensor to cool naturally to room temperature.
4. Re-hydrate the sensor at 20-30°C and ~75%RH for 12 hours
5. Replace the sensor onto the 765 and re-check
Alternatively the sensor can be swapped for another, pre-tested one, from your
supplier.
Humidity Test Kit
A test kit is available to test the accuracy of the humidity readings. This utilises the
fact that some salts provide a known humidity above a saturated solution. The
humidity sensor is placed in a sealed chamber with a saturated solution of a salt.
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.
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