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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 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