+ durable industrial design
+ battery voltage indicator
- significant instability
- results outside specification
- inadequate manual
- extra charge for calibration
The design submitted a feeling
The manual for the instrument Testo 650 covered a lot of features for
the instrument, like e.g. attaching an optional printer. No information
about the use of the probe 0628.0024 was however included.
Both instrument and sensor were delivered with calibration certificates. For further validations, instruc-
tions of how to calibrate the instrument was only supplied after buying a calibration kit (about € 220).
The measurements were easily accomplished. The results were, however, disappointing.
As seen in the diagram below, the results varied significantly for NaCl (0.755 aw). For each
measurement series the read result increased by more than 0.01 aw. Additional measurements
and other salt samples confirmed this behaviour. The results for NaCl samples varied between
0.771 and 0.791. Resting the sensor for 6 days gave 0.772 for a NaCl sample. It appears that
other phenomena than sensor saturation were responsible for the result variations.
At aw values higher than 0.97, the results were unreliable, in spite of the stated measurement
interval 0 - 1.000 aw. A cut-off in the instrument showed the result 0.999, irrespective of sample
aw = 0.975, 0.996 or 1.000. The cut-off gave a maximal error in the three samples of 0.024,
0.003 and -0.001, respectively. The apparent low deviations at aw higher than 0.95 in the
diagram shown below are hence not true observations but results of this cut-off.
|Deviation versus aw for Testo 650 with measure-
ment cell 0628.0024 in three measurement series.
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