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The 99% opinion is that it's a typo (> piezometer). I would agree, however there's an uncanny collection of plezometers around the subject of drill wells, underground waterways etc. Of course it could be an epidemy, started by one single typo some time ago and propagated by copycats...
I checked with a colleague who is both a language expert and speleologist (a rare occurence, but then Slovenia with its Kras IS where speleology has started in the first place) and according to his research and ample experience, it is a typo. 4 KudoZ points were awarded for this answer
Like the one GaryD quoted below:>www.sci.kumamoto-u.ac.jp/~hydrolab/KagabuISMAR2007.pdf "Plezo meter N. SS. -i. Fig. 7 Geologie columnar section at the site of piezometers U and N." They repeat the word, and lo and behold, now it is "piezometers".
Convincing title: Advances in plezo-nanopositioning technology the follow-up text reveals: ...point where the piezo effect totally disappears. ieeexplore.ieee.org/iel5/8678/27509/01225537.pdf? arnumber=1225537 Loads more: The sensor can be constituted of a piezo-element, electrically connected to a... The centre of the plezo electric bar is attached to the centre of the base...>www.freshpatents.com/Sensor-arrangement-dt20060921ptan20060... Plezo-Electrlc. Strain Gage. PA89. SG71. Piezoelectric signal conditioners.... gees.usc.edu/soilab/Old%20Documentation%20Files/Validyne/SignalConditioning.pdfhe Piezoelectric Loudspeaker .... piezo-electric pressure-sensitive elements. The plezo-electric elements measure... arjournals.annualreviews.org/doi/pdf/10.1146/annurev.fl.03.010171.001051
In some text you may find piezo 20 times and plezo once. If you google for plezo, you will get it!
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Answers
3 mins confidence: peer agreement (net): +4
piezometer
Explanation: Plezometer is not in any of my dictionaries.
-------------------------------------------------- Note added at 16 hrs (2009-05-02 15:44:28 GMT) --------------------------------------------------
On a philosophical note, there are a plenty of things that you can prove do not exist.
For example, a sky scraper in the middle of Central Park. Just go and look, there is not one there.
Some words exist in certain languages and not in others (I mean direct translations).
And although I keep checking, I am pretty sure there is not ten million euros in my bank account!
Mark Nathan Local time: 03:06 Native speaker of: English PRO pts in category: 4
Grading comment
I checked with a colleague who is both a language expert and speleologist (a rare occurence, but then Slovenia with its Kras IS where speleology has started in the first place) and according to his research and ample experience, it is a typo.
Notes to answerer
Asker: "Plezometer is not in any of my dictionaries" - nor in any of mine. But that's no proof, just an "on the balance of probabilities" hypothesis. Which is why I posted the question in the first place. Note that a non-existence (of anything) is not proveable (sg).
The translation workplace 
peer agreement (net): +4