Re: Excitation for gypsum blocks

[Rick Allen]

NOTE:  To get off this list, send email to majordomo@aqua.ccwr.ac.za
with the body of the message containing the line:
unsubscribe sowacs

Andrew,

I want to be among the first to thank you and congratulate you on a 
very thorough and excellent tutorial and summary on making proper 
measurements with gypsum (and similar) blocks.   Nice job.  I agree 
with your recommendations.

We have experienced the polarization problem with gypsum blocks in 
some of our work with long leads on automated lawn watering 
systems.  This was caused by ground polarity differences between 
the sensor and logging system in the home.  Our "quick fix" was to 
place a hefty capacitor in series with the ground leg of the block.  This 
allowed our exitation AC to get through, but blocked out long term DC 
flow.  This is probably not the best, nor, elegant solution to the 
problem, but it seemed to work.

Thanks again for your contribution.

Rick Allen
Univ. Idaho

> Whoooahhh!!!
> 
> 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.
> 
> And do I know what I'm talking about? Well, as I pointed out with the
> radio-linked gypsum blocks measurements, I'm still making mistakes. But at
> least not the same old ones. Hope this helps...
> 
> Andrew
> 
> 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
>