Test summary

water activity
measuring instruments




Importance of validation of results
As with the portable instruments, the importance of result
validation during measurements must be stressed. The
calibration certificates that were enclosed with both Hygro-
Lab 2 and LabMASTER-aw were found to be of no value.

It is recommended that, at least initially, the instruments are
validated using calibration samples every day.

Below follows a summary of the three desktop instruments
tested. Click on instrument image to see measurement results
diagram. First a discussion about calibration samples and salt
slurries must be given.
Navigation: Water activity
  What is water activity
  Test Introduction
Test details
  Salt solutions used
  Calibration samples used
  Desktop instruments
  Portable instruments
  Desktop instruments
  Decagon AquaLab 3TE
  Novasina LabMASTER-aw
  Rotronic HygroLab 2
  Portable instruments
  Decagon PawKit
  Rotronic HygroPalm
  Testo 650
    Desktop instruments
  Portable instruments

Salt slurries vs. Decagon calibration liquids

  As seen in the results from AquaLab, HygroLab and Lab-
(SatSalt2), saturated salt slurries (including
Novasina calibration standards) and Decagon calibration
"Dec LiCl 13.41m" with claimed aw = 0.250, gave
inconsistent results. The inconsistency was in the order
0.01 aw.

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
11 days. 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 the basis of instrument calibration.
Decagon uses their own calibration samples. As seen above, the measurement results
from at least one of these samples do not match results from saturated salt solutions.

Documentation showing how the calibration samples are validated has
not been found, despite requests from EminTech to Decagon.
As AquaLab 3TE obviously is calibrated using Decagon calibration samples,
it must be concluded that the factory calibration is not fully documented.

As shown in this link, a crude correction of the AquaLab results will give reasonable correlation with
literature values for the salt slurries and with results from a recently calibrated LabMASTER-aw.

Once the calibration issues are cleared out, the impressing reproducibility and easy handling of
the instrument makes AquaLab 3TE worth a strong recommendation. Until then, the instrument
is 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
within specifications, except at higher aw.

Calibration took at least 2 hours.

This instrument was lower specified than AquaLab and
LabMASTER-aw, but was also considerably less expensive.
The possibility of analysing larger sample volumes was one of the factors that makes this
instrument well worth consideration.
Novasina LabMASTER-aw
  Once LabMASTER-aw was calibrated, measurements
using the calibration samples gave results according
to specifications, except at higher aw.

Calibration took at least 3 hours.

The first analysis series of the salt slurries, SatSalt2,
gave reasonable results with one exception.
Analysis at higher aw still appeared uncertain.
Repeating the measurements after 11 days, gave results that were
about 0.03 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 possibility
to store various types of analyses in the instrument are factors that add to the qualities of this instrument.
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