�From:�	William R. Pogue (IRRIGATION-L)
�To:�		IRRIGATION-L@LISTSERV.GMD.DE
�Date:�	Tuesday, July 16, 1996  1:51 am
�Subject:�	Jochen Eberhard---Problems with Tensiometers

Jochen is having problems in getting his tensios to respond to anirrigation
event when reading has reached 64 kPa.  I offer the followinginformation
which may serve as a checklist to identify what may be happening.
First: Check the following items:
         1. Instrument gauge and cup properly deaired at the onset
         2. Instrument cup not plugged by deposits from hard water
             containing dissolved solids which have precipitatedout of
             solution.  This can be checked by venting theinstrument
             to the atmosphere and allowing it to drain.  With anIRROMETER
             (BRAND NAME)  the instrument should drain at least2/3 of
             the reservoir portion in 12 hours for the tip to beconsidered
             adequately porous.
         3. Instrument gauge operating properly.  Test with eithera hand
             vacuum pump to see if gauge can reach a 80 kPa level
             or a vacuum source in a laboratory.
Second: Field Site/Installation:
          1. Cup in the active root system with snug fit betweencup and
soil.
              A loose fit will cause the instrument to respondvery slowly.
          2. Cup not sitting on top of a rock or against a largeroot, which
              affects cup to soil  contact.
          3. Cup in area representatively wetted by the irrigationevent
              ( emission point source with drip); guideline isthat
instrument(s)
              are set 30-45 cm from emitter.  If too close, youcan get false
              readings on the wet side; if too distant, they canbe false on
the
              dry side.
          4. Soil type can influence how much air is present whenavailable
              soil water has depleted to a given suction value. In very
light soils
              you can have lots of air present at readings in the15-25 kPa
              range and thus air beginning to enter theinstrument.  In
heavier
              soils this may not occur until the 50+ kPa level. The entry of

              air into the water column presents a problem.
          5. At readings of 64kPa there is probably some airentry, which can
              be viewed as a void at the top of the water column. Any air
              present in the column will desentize the responsetime of the
              instrument to any irrigation event.  Thus, anyinstrument
reading
              64 kPa, with air entrapped at the top of the watercolumn will
              be very, very slow to respond to wetting in the areaof the
cup.
           6.  Is there the possibility that air has also enteredthe Bourdon
                tube of the gauge ?   With some instrument designsthis
               can happen.  If it does, gauge may  need  deairingagain.  If
               the instrument design presents this as an ongoingproblem,
               routine deairing of the gauge may be required atevery
reading.
               The Irrometer brand is not of this design.  Checkthis factor.
           7. If air entrappment is ruled out, try pulling theinstrument and
               putting it in a container of water ( instrumentreading 64
kPa-
               or some such higher reading).  If reading quicklydrops (
which
               is what should happen with a good operatinginstrument free
               of entrapped air)  then you know that the wettingfront from
the
               emitter is not reaching the area where theinstrument cup is
               located.
           8. There could be a soil interface, or soil "lens",between the
emitter
               and the instrument cup which is impeding or slowingdown the
               water movement in the soil.  This is common withlayered soils
               or where there are soil type changes, hard pans,plow pans,
etc.
               Investigate this possibility depending on theresult from Item
7
               above.
This will at least get you started in trying to find out thesource of the
problem.  If there are other issues you can let me know directly.
Regards, Bill Pogue, Irrometer Company