Hydronix Hydro-View HV02, Руководство пользователя

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HYDRO-VIEW USER GUIDE
D0022
REV 1.06
43
Calibration and sampling techniques
When using Hydronix moisture measurement equipment it is customary to calibrate the
equipment to read real moisture percentage, since the sensor reading will be affected by
basic physical (dielectric) properties of the material being measured. Therefore it is
necessary to make independent tests to determine the real moisture content. A common
method of determining moisture content is based on the
desiccation
(drying) of a sample
and a suitable technique for use on site is described here.
With materials such as sand, large variations of moisture content can be encountered in a
single batch. For instance with stockpiles of sand it is quite possible to measure 4%
moisture level at the top of the pile and 16% towards the bottom. When this material is
loaded into the storage bins inevitably material is scooped from both the bottom and the
top of the pile, thus creating localised stratification of moisture in the bin.
As a bin is being emptied, the stagnant material from the sides, which almost certainly will
be at a different moisture level, periodically falls in with the fresh material.
Another feature is the dynamic state of the moisture content in a bin due to the free
properties of the material. This is even noticeable in small laboratory samples in plastic
containers which, if left to stand even for a very short period of time, will vary from top to
bottom.
NOTE: for the reasons given above, collecting a few kilograms of material that is
representative to 0.1% accuracy of several tonnes of material is very difficult to achieve.
Great care should therefore be taken when conducting these tests.
A variety of laboratory techniques exist. The desiccation test is recommended in that it is
an absolute measurement with repeatable results even when carried out by different
operatives and it is not dependent on having to know the specific gravity of the material
being tested.
As will be appreciated no matter how much care is taken in conducting these tests,
sampling and experimental errors will inevitably be present in each result.
Initially the equipment can be calibrated from one actual set of results at the level of
moisture contents encountered on site in conjunction with an estimate of calibration points
at 0% moisture level, thus providing two separate calibration points spaced reasonably
apart. However, improved accuracy will be obtained by taking adding a second true
calibration point at a later date when site conditions allow.
Recommendations for best performance
A
LWAYS
ensure that the sample of material being used for calibration is from a batch with
low moisture variation. This can be checked by using the minimum/maximum or trend
display facilities on the Hydro-View and will help to prevent sampling errors affecting the
results.
A
LWAYS
take care when working with small samples of material not to lose any during the
drying process.
N
EVER
try to calibrate using extremely wet (near saturation) samples of material - the
errors you will make in sampling such material will be very large. A maximum
recommended calibration sample limit for sand would be around 12%.
N
EVER
attempt to calibrate a Hydronix sensor by placing it in a bucket full of material or
packing material on the face of the sensor. The ‘static’ readings obtained in such cases
will not be representative of those obtained in the dynamic situation.