Delta-T Devices AP4, User Manual

Page: 22 / 64

CALIBRATION
Page 22
INSTALL CALIBRATION
After you select FIT CURVE the AP4 accepts the last reading at the last plate position, and
with the readings from all the other positions, it calculates the new calibration constants
for the cup. The new calibration is then used to recalculate the measured values in the
left-hand results column.
Compare the measured and true values in the left and right columns. Look for glaring
discrepancies which might indicate a possible experimental error on one of the plate
positions.
To help you decide whether you like the quality of the new calibration data, the
porometer calculates a statistical estimate of the average error over all the readings.
It is defined as follows:
𝜎 = [
∑ {(𝑟
𝑚
− 𝑟
𝑡
) 𝑟
𝑡
⁄ }
2
𝑚=𝑁
𝑚=1
(𝑁 − 3)
]
½
× 100 %
where r
t
= true or theoretical plate resistance
r
m
= measured plate resistance
N = number of positions used.
On the porometer screen this is called the curve error. A fuller description is the
standard deviation of the residual errors.
This figure is useful when calibrating to check that the calibration ran OK. If the error is
less than 10% then go ahead and INSTALL it. If between 10 to 15% then redo the worst
reading in the results column. The porometer is accurate to ± 20% once it is accurately
calibrated. The spread on your readings should be better than this.
If the error is greater than 10 % the cursor will (suggestively) surround the REDO option.
Otherwise it will suggest you accept the INSTALL option. When you do this, it saves the
new calibration settings.
Things to do if the curve fitting error is unacceptably large :
•
Redo calibration on the worst plate position.
•
Ensure calibration plate is properly seated against the cup and protect it from
the wind.
•
Check the cup seal is OK.
•
Check the calibration plate is clean, moist and has had about 1 hour to stabilise
since it was last moistened.
•
Check for thermistor damage. (Is the δT erratic or greater than 0.5°C on the
shift-OFF screen when the cup is in thermal equilibrium?)
See also Hints and Tips on page 42 and How Results are Calculated on page 32 of the
AP4 Technical Manual.