Importance of validation of results
As with the portable instruments, the importance of result
must be stressed. The
calibration certificates that were
Lab 2 and LabMASTER-aw were found to be of no value.
It is recommended that, at least initially, the instruments are
calibration samples every day.
Below follows a summary of the three desktop instruments
tested. Click on instrument image to see measurement results
First a discussion about calibration samples and salt
slurries must be given.
Salt slurries vs. Decagon calibration liquids
||As seen in the results from AquaLab, HygroLab and Lab-
saturated salt slurries (including
Novasina calibration standards)
and Decagon calibration
solution "Dec LiCl 13.41m" with claimed aw = 0.250,
The inconsistency was in the order
A slighter inconsistency was possibly also seen for the
sample "Dec LiCl 8.57m".
|Decagon AquaLab 3TE
||The instrument showed very good reproducibility, also after
The deviations for the Decagon calibrations samples
at aw = 0.75, 0.50 and 0.25
were extremely small.
The deviation at aw = 0.98 raises questions however,
as the results at
aw = 0.996 and 1.00 were extremely good.
A major question with this instrument, however,
seems to be
basis of instrument calibration.
Decagon uses their own calibration samples.
As seen above, the measurement results
at least one of these samples do not match results from saturated salt solutions.
are validated has
been found, despite requests from EminTech to Decagon.
As AquaLab 3TE obviously is calibrated using Decagon calibration samples,
it must be concluded
the factory calibration is not fully documented.
As shown in this link,
a crude correction
of the AquaLab results will give reasonable correlation with
values for the salt slurries and with results from a recently
Once the calibration issues are cleared out, the impressing reproducibility and
makes AquaLab 3TE worth a strong recommendation.
Until then, the
still worth serious consideration for
high quality measurements of water activity.
HygroLab 2 with sample cell AwVC-DIO
||Once HygroLab was calibrated, the instrument gave results
except at higher aw.
Calibration took at least 2 hours.
This instrument was lower specified than AquaLab
but was also considerably less expensive.
The possibility of analysing larger sample volumes was one of the factors
instrument well worth consideration.
||Once LabMASTER-aw was calibrated, measurements
samples gave results according
specifications, except at higher aw.
Calibration took at least 3 hours.
The first analysis series of the salt slurries,
gave reasonable results
with one exception.
Analysis at higher aw still appeared uncertain.
Repeating the measurements after 11 days, gave
higher than previously,
i.e. the instrument required calibration again.
Provided calibration routines are followed, LabMASTER-aw may very well be used
when high quality results
from samples with aw less than 0.98 are required.
The good analysis temperature stability and the
to store various types of
analyses in the instrument are factors that add to the qualities of this instrument.