Delta-T Devices AP4, User Manual

Page: 45 / 64

Hints and Tips
Page 45
CUP TEMPERATURE
The AP4 automatically corrects for deviations of the cup temperature from the
temperature of the calibration run. Above a 5°C shift the AP4 will start to flag a warning
by flashing. The correction is good, i.e. remains within specification, to +10°C and - 5°C.
Beyond this, the correction is likely to become successively less accurate. You should run
one or two calibration plate positions to check their resistances. If these are
unacceptable, you will have to do a full calibration run at the new temperature.
COPING WITH VERY HIGH RESISTANCES
Problems can arise if the resistance is so high that the AP4 cannot complete even a
single cycle. This could be the case for instance if you wanted to record the resistance of
the top surface of a hypostomatous leaf along with the (much lower) under surface
resistances. Terminate the reading by opening the head and breathing on it. Either
discard the reading or add a note as a reminder.
THE LIGHT SENSOR
This is a simple sensor intended for comparative estimates of leaf irradiance. The
spectral and cosine response approximate to that of a PAR quantum sensor, but no
levelling device is provided. After a resistance reading has been accepted and the sensor
head unclipped from the leaf, the light sensor should be oriented in the plane of the leaf.
The irradiance reading will be "frozen" at the moment of storing the data. An irradiance
value will get stored for every reading whether you want it or not - so note which are the
valid readings.
CLEANING THE SENSOR HEAD
Deposits of dirt and plant substances, pollens and resins may accumulate on the rubber
cup seal and even inside the walls of the cup.
Unplug the sensor head lead from the AP4.
Remove the cup seal and cup insert from the sensor head as shown on page 21 of the
AP4 Technical Manual and clean them, by washing with distilled water. Then leave them
to dry out thoroughly for a day or so. Avoid the use of any solvents, which may damage
or be absorbed by the silicone rubber. Spares are included in the consumables kit.
If the RH sensor itself appears dirty, use only a very soft brush to dislodge the dirt.
Scratching the sensor may change its calibration.
Whilst you have the sensor head in pieces you can wipe the RH sensor with distilled
water. Use a cotton bud - or a twist of cotton wool around a matchstick. Be gentle. The
sensor substrate is brittle, like glass. It is glued down on a finger-like extension of the