Delta-T Devices AP4, User Manual

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Hints and Tips
Page 42
Hints and Tips
IN TRANSIT
Avoid shocks and keep the lid shut. The padded carrying case will give the AP4 some
measure of protection when travelling to and from the field.
It is not bomb-proof however and will not necessarily save the AP4 if you drop it!
Keep the zip fasteners done up. They hold the AP4's lid shut and help to reduce the
ingress of dirt and dust.
Avoid high temperatures. Leaving the AP4 in a closed vehicle in the sun can easily expose
it to >50°C. Likewise avoid the opposite extreme, where falling temperatures could lead
to dew or condensation forming on the instrument. Bringing a cold porometer into a
warm greenhouse may also cause condensation. Give it time to equilibrate.
THE FIRST RUN OF THE DAY
Make up the calibration plate well ahead of time. Leave it in its plastic wallet, stored in
the head pocket in the AP4 case. A freshly made up plate needs about 1 hour to
stabilise. If you do not do this the measured resistance (after calibrating) may be in error
by 15%.
At 20°C and 50 % RH we found that the calibration plate will stay fresh, with its
measured resistance within ± 5 %, for several days in its wallet, and up to 18 hours out of
the wallet. It will be shorter at higher temperatures and lower RH's.
Make up the plate the day before. This may also be a convenient time to make sure the
correct cup shape is installed in the sensor head.
When starting up the AP4, first use it to measure the ambient RH. Select the cycling level
nearest to this RH, then leave the instrument to cycle on one of the calibration plate
positions for maybe 10 or 15 minutes. This gives the cup time to acclimatise to the
current RH. During storage it may have been quite different, resulting in different
equilibrium concentrations of moisture in the cup walls.
The calibration run can now be done with the expectation that a stable reading will be
obtained within a few cycles of changing to each new plate position. It's a matter of
personal preference whether you work through the calibration plate from position 1 to
6, or in the reverse order - it doesn't affect the final result. The "6 to 1" lobby say the
high resistance end, position 1, takes longest to equilibrate, so do it last. The "1 to 6"