Let's not forget the old lessons here!
A gypsum block is not like a resistor; it is an electrochemical cell with a
saturated (weak) solution of calcium sulphate, also known as gypsum, forming
the electrolyte.
If you try and measure it with a normal digital voltmeter, which uses DC
excitation, then what you will notice is that the reading drifts
continuously due to polarization of the block (movement of ions towards the
electrodes). Its true - you can then have any reading you want, provided you
wait long enough! But not good measurement practise...
In all workable gypsum block resistance measurements, you need AC excitation
to prevent this ionisation occurring (if you keep changing the direction of
the electric field, average ionic movement is zero).
The second factor is voltage level - too high a voltage, and you "gas" the
gypsum block. This changes the blocks resistance. We use around about 1V
peak to peak
The third factor is frequency - if you change the frequency, you change the
resistance reading. (I don't know why!). We stick with 1kHz.
And finally, if you change the waveform of the AC excitation, you change the
resistance in unpleasant ways. This is because different waveforms contain
(Fourier series) harmonics of lots of higher frequencies than the
fundamental (or basic) frequency. These get distorted within the block, as
you can see if you try and drive a block with a square-wave form something
like a 555 oscillator mentioned by one correspondent.
One final hazard - block interaction.
Data loggers impose quite different conditions on reading gypsum blocks than
do hand-held readers.
With a hand-held reader (such as our GBReader) you are only measuring one
gypsum block at a time.
With a data logger, you are interconnecting multiple gypsum blocks through
wet soil, and so you create extra (and unknowable) resistance paths
"between" blocks. The result is weird readings as blocks at different levels
of moisture content interact. Data looks spiky.
We found this out recently while field-testing testing our new radio-linked
gypsum block measurement systems. (They'd worked perfectly in the lab, but
that's because we let them air-dry to get a full drying curve, and there is
no interblock connection path in air!)
Each (of four) gypsum blocks was isolated by an electronic switch on one
side, before the measurement circuitry. The gypsum blocks were connected
with one side in common. Same problem - interaction.
So you need to galvanically isolate gypsum blocks one from the other if you
want to connect them to a data logger.
There are two accepted ways to do this.
1) If your gypsum blocks are close to the data logger (we keep cable lengths
to under two meters to minimise capacitance effects) then you can use
"analog electronic switches", BUT YOU MUST SWITCH OUT BOTH SIDES OF ANY
GYPSUM BLOCK NOT ACTIVELY BEING MEASURED.
2) You can use "transformer isolation", provided you limit the resistance
range. We have used this system successfully for many years in cabled-gypsum
block networks where individual measurement sites can be as much as two
kilometres apart. (A gypsum block field station within two meters of sets of
four gypsum blocks converts the AC resistance measurement to a current loop
signal for the long-haul transmission over standard irrigation cable back to
a central data logger). Over these sorts of distances, ground potential
differences can be significantly higher than electronic switches can
withstand, so technique 1) above is unsuitable (this problem disappears with
radio-linking).
One final comment - ensure that your calibration of gypsum blocks in a test
lab pressure plate apparatus (kPa versus resistance) is carried out under
exactly the same electrical excitation conditions as you logging gear in the
field will use, otherwise you will be unable to reproduce your calibration
standards under field conditions.
3/04/2001
Thanks to
Andrew Skinner FIICA FIEAust CPEng
Engineering Director
Measurement Engineering Australia
41 Vine Street
MAGILL SA 5072
Ph 08 8332 9044 Fax 08 8332 9577
Andrew.Skinner@mea.com.au
www.mea.com.au
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