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 Copyright
 Introduction
 Servicing a tensiometer
 Testing a tensiometer
 Tensiometer calibration
 Summary
 References


FLAG IFAS PALMM UF



Tensiometer service, testing and calibration
CITATION SEARCH THUMBNAILS PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00047755/00001
 Material Information
Title: Tensiometer service, testing and calibration
Series Title: Florida Cooperative Extension Service bulletin 319
Physical Description: 5 p. : ill. ; 28 cm.
Language: English
Creator: Smajstrla, A. G. (Allen George)
Pitts, Donald J. (Donald James)
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1997
 Subjects
Subjects / Keywords: Agriculture   ( lcsh )
Farm life   ( lcsh )
Farming   ( lcsh )
Tensiometers.   ( lcsh )
University of Florida.   ( lcsh )
Agriculture -- Florida   ( lcsh )
Farm life -- Florida   ( lcsh )
Soil moisture -- Measurement.   ( lcsh )
Spatial Coverage: North America -- United States of America -- Florida
 Notes
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: oclc - 37141810
aleph - 002269505
aleph - ALM2529
System ID: UF00047755:00001

Table of Contents
    Copyright
        Copyright
    Introduction
        Page 1
    Servicing a tensiometer
        Page 2
    Testing a tensiometer
        Page 2
    Tensiometer calibration
        Page 3
        Vacuum test gauge quick calibration
            Page 3
        Vaccum chamber detailed calibration
            Page 4
    Summary
        Page 5
    References
        Page 5
Full Text





HISTORIC NOTE


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida




/I/



3 /1 UNIVERSITY OF

FLORIDA


Bulletin 319


Cooperative Extension Service
Institute of Food and Agricultural Sciences


Tensiometer Service, Testing and Calibration'

A.G. Smajstrla and D.J. Pitts2


Tensiometers are useful instruments for
measuring soil water status in the field. Tensiometers
measure soil water potential or tension, which is a
measure of the amount of energy required for a plant
to overcome capillary and gravitational forces to
extract water from a soil. Thus, tensiometers can be
used to schedule irrigations when the soil water
tension is low -- that is, before plant water stress
occurs. Tensiometers do not measure the osmotic
component of soil water potential, which is due to soil
salinity. However, this would not be expected to be
a limitation to the use of tensiometers in Florida
except in saline soils or where saline irrigation water
is used.

A tensiometer is a water-filled tube with a
vacuum gauge and filling port at the upper end and a
ceramic cup at the lower end (Figure 1). When it is
placed in the soil, the water in the instrument comes
to equilibrium with the water in the soil by flowing
through the ceramic cup. At equilibrium, the water
tension in the instrument is equal to the water tension
in the soil. Then the vacuum gauge measures the soil
water tension.


Vacuum Gauge Tensiometer
Cap


Plriglass
Tube
Vacuum
Gauge



Ceramic
Cup

Figure 1. A vacuum gauge tensiometer.


A tensiometer is used by placing it in the field so
that the ceramic cup is located within the root zone of
the plants to be irrigated. Then the instrument
measures the soil water tension that the plants are
experiencing.

The tensiometer is a fairly simple instrument that
will work well if it is properly installed and in good
condition. The instrument is in good repair if: 1) the
vacuum gauge is accurate, 2) the ceramic cup allows
free water movement between the soil and the
instrument, and 3) there are no air leaks.


1. This document is Bulletin 319, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: April 1997.
2. A.G. Smajstrla, Professor, Agricultural and Biological Engineering Department, Gainesville; DJ. Pitts, Assistant Professor, SW Florida Research
and Education Center, Immokalee, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville
FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean


101
F636b
319


$ ~,F F l.psrp 1""1-I:







F- to (Tensiometer Service, Testing and Calibration
3
3 t New instruments are normally in good repair.
However, with time and usage, mechanical vacuum
SCIENCL E gauges may begin to fail, the ceramic cups may begin
LIBRARY to plug, or air leaks may develop. Therefore,
tensiometers should be periodically tested to insure
that they are working properly, and that soil tensions
are accurately read. As a minimum, tensiometers
should be tested before each crop season for short-
season crops. They should be tested at least every
three or four months for longer-season or perennial
crops. They should also be tested whenever their
readings appear to be unusual, such as if the gauge
remains on zero even though the soil dries as a crop
uses water.

This publication presents procedures for servicing,
testing, and calibration of tensiometers so that the
user can determine that the instrument is working
properly and reading accurately for irrigation
scheduling purposes.

SERVICING A TENSIOMETER

Servicing a tensiometer means preparing it for
field operation or testing. This requires cleaning it if
necessary, then filling it with fluid and expelling any
entrapped air.

First wash the instrument to remove dirt, algae,
bacterial slime and other foreign debris from both the
inside and outside of the ceramic cup and tensiometer
tube. This can be done with plain water and a brush.
Use a small diameter bottle brush to clean inside the
tube and cup. Use a household detergent if necessary
to clean the instrument thoroughly.

If the instrument, especially the ceramic cup, is
slimy this is probably the result of bacterial growth in
the soil and water. Wash the ceramic cup and tube in
a chlorine solution, using about 1/4 cup of household
bleach (5.25% sodium hypochlorite solution) in a
gallon of water. You may want to allow the ceramic
cup to soak in this solution overnight to be sure that
all the bacteria are killed. Then rinse the instrument
with water.

Fill the instrument with clean water or water with
a mild biocide to help prevent organic growths in the
tensiometer fluid. Most tensiometer manufacturers
sell a fluid additive that is both a biocide and coloring
agent that allows the tensiometer fluid to be easily
seen. Deionized water may be used in order to keep
the instruments clean longer in the field. However,
with time they will again become contaminated by


UNIVERSITY OF FLORIDA

1262 05699 3776


Page 2


contact with the soil solution and bacteria in the soil.
It is not mandatory that fluid additives be used,
however additives will reduce the maintenance needed
to keep the instruments clean and working properly.

Allow the ceramic cups to soak in water or
tensiometer fluid for several hours or overnight to be
sure that the ceramic is thoroughly saturated. Then
fill the instrument with tensiometer fluid. A plastic
squeeze bottle is useful for filling the instruments.

Remove excess air from the instrument using a
hand held vacuum test pump available from the
tensiometer manufacturer (Figure 2). The vacuum
test pump has a neoprene suction cup or stopper that
allows it to replace the tensiometer cap. Then as the
pump is operated, air is extracted from the ceramic
cup, tube, and vacuum gauge and pulled to the top of
the tube. Pump the tensiometer several times,
refilling the tube with fluid each time if necessary
until no further air is removed. Then remove the
vacuum pump and refill the tube to the top with
tensiometer fluid. The instrument is now ready to be
tested for leaks or capped for use.

Vacuum Test Pump

'Pump
Plunger


-Pump
Vacuum Body
Gauge \


Suction
Cup
Figure 2. A vacuum test pump with test gauge.


TESTING A TENSIOMETER

To determine whether a tensiometer is working
properly, three tests need-to be conducted: 1) test for
air leaks, 2) test that the mechanical vacuum gauge
works, and 3) test that water can flow through the
ceramic cup.

Begin by testing for large air leaks using the hand-
held vacuum test pump. After the tensiometer has
been cleaned and serviced as previously described, fill
it completely full of fluid, then use the vacuum pump
to make this test. Operate the vacuum pump to







F- to (Tensiometer Service, Testing and Calibration
3
3 t New instruments are normally in good repair.
However, with time and usage, mechanical vacuum
SCIENCL E gauges may begin to fail, the ceramic cups may begin
LIBRARY to plug, or air leaks may develop. Therefore,
tensiometers should be periodically tested to insure
that they are working properly, and that soil tensions
are accurately read. As a minimum, tensiometers
should be tested before each crop season for short-
season crops. They should be tested at least every
three or four months for longer-season or perennial
crops. They should also be tested whenever their
readings appear to be unusual, such as if the gauge
remains on zero even though the soil dries as a crop
uses water.

This publication presents procedures for servicing,
testing, and calibration of tensiometers so that the
user can determine that the instrument is working
properly and reading accurately for irrigation
scheduling purposes.

SERVICING A TENSIOMETER

Servicing a tensiometer means preparing it for
field operation or testing. This requires cleaning it if
necessary, then filling it with fluid and expelling any
entrapped air.

First wash the instrument to remove dirt, algae,
bacterial slime and other foreign debris from both the
inside and outside of the ceramic cup and tensiometer
tube. This can be done with plain water and a brush.
Use a small diameter bottle brush to clean inside the
tube and cup. Use a household detergent if necessary
to clean the instrument thoroughly.

If the instrument, especially the ceramic cup, is
slimy this is probably the result of bacterial growth in
the soil and water. Wash the ceramic cup and tube in
a chlorine solution, using about 1/4 cup of household
bleach (5.25% sodium hypochlorite solution) in a
gallon of water. You may want to allow the ceramic
cup to soak in this solution overnight to be sure that
all the bacteria are killed. Then rinse the instrument
with water.

Fill the instrument with clean water or water with
a mild biocide to help prevent organic growths in the
tensiometer fluid. Most tensiometer manufacturers
sell a fluid additive that is both a biocide and coloring
agent that allows the tensiometer fluid to be easily
seen. Deionized water may be used in order to keep
the instruments clean longer in the field. However,
with time they will again become contaminated by


UNIVERSITY OF FLORIDA

1262 05699 3776


Page 2


contact with the soil solution and bacteria in the soil.
It is not mandatory that fluid additives be used,
however additives will reduce the maintenance needed
to keep the instruments clean and working properly.

Allow the ceramic cups to soak in water or
tensiometer fluid for several hours or overnight to be
sure that the ceramic is thoroughly saturated. Then
fill the instrument with tensiometer fluid. A plastic
squeeze bottle is useful for filling the instruments.

Remove excess air from the instrument using a
hand held vacuum test pump available from the
tensiometer manufacturer (Figure 2). The vacuum
test pump has a neoprene suction cup or stopper that
allows it to replace the tensiometer cap. Then as the
pump is operated, air is extracted from the ceramic
cup, tube, and vacuum gauge and pulled to the top of
the tube. Pump the tensiometer several times,
refilling the tube with fluid each time if necessary
until no further air is removed. Then remove the
vacuum pump and refill the tube to the top with
tensiometer fluid. The instrument is now ready to be
tested for leaks or capped for use.

Vacuum Test Pump

'Pump
Plunger


-Pump
Vacuum Body
Gauge \


Suction
Cup
Figure 2. A vacuum test pump with test gauge.


TESTING A TENSIOMETER

To determine whether a tensiometer is working
properly, three tests need-to be conducted: 1) test for
air leaks, 2) test that the mechanical vacuum gauge
works, and 3) test that water can flow through the
ceramic cup.

Begin by testing for large air leaks using the hand-
held vacuum test pump. After the tensiometer has
been cleaned and serviced as previously described, fill
it completely full of fluid, then use the vacuum pump
to make this test. Operate the vacuum pump to







Tensiometer Service, Testing and Calibration

S create a vacuum in the instrument and look for the
continued flow of air into the instrument. A steady
stream of air bubbles will indicate a large air leak.

When large air leaks occur, they are often around
fittings or gaskets such as where the pressure gauge
is threaded into the plexiglass tube or where an o-ring
seal is used to attach the tip to the tube on some
tensiometer models. If so, these can often be
repaired by using thread sealant or replacing o-rings.
Large air leaks sometimes occur where the ceramic
cup is cemented onto the plexiglass tube. If that is
the case, it can often be repaired with a waterproof
epoxy, however, all components need to be
thoroughly dry before applying epoxy. Several days
may be required to thoroughly dry the ceramic cup
and the epoxy.

If no large air leaks occur, the mechanical vacuum
gauge can be tested by comparing its reading with the
test gauge on the vacuum test pump. Always buy a
test pump with a test gauge installed. Then when the
vacuum pump is used, the tensiometer gauge should
read the same values as the test gauge. This is a
quick and easy test that the tensiometer vacuum
gauge is working and that its readings are
approximately correct. Unfortunately, if the
instrument gauge does not work, or if it is not
accurate, most types must be replaced. Some can be
calibrated, however, most that are commonly used
must be discarded and replaced with a new gauge.

The final instrument test is a test for leaks and
proper operation of the ceramic cup with the
tensiometer sealed and ready for field installation.
After the extraction of air and test of the vacuum
gauge as described above, refill the tensiometer to the
top and seal it with the tensiometer cap. Then place
or hang the instrument in the atmosphere where
water can evaporate from the ceramic cup to simulate
soil drying. Because the instrument is sealed, as
evaporation occurs the vacuum gauge reading should
slowly increase (depending on the rate of drying)
throughout the tensiometer range. This process may
take an hour or more during which time the
tensiometer fluid can be observed for streams of small
air bubbles which indicate small leaks in the
instrument. Such leaks must be found and repaired
because if they are not, the instrument will require
frequent refilling in the field and the gauge reading
will lag the true soil reading.


Page 3


While the above slow-leak test is being conducted,
a test of the ceramic cup flow properties is being
conducted at the same time. If the ceramic cup pores
are plugged, water will not flow through the ceramic
and the instrument will not respond or will respond
only very slowly to drying by the atmosphere. This
will indicate a need for the ceramic to be cleaned
more thoroughly before the instrument will work in
the field. Because of the small pore sizes in the
ceramic, they can readily be plugged by oil, grease, or
other contaminants. Never use oil or grease on or
around these instruments where the ceramics might
become contaminated.

TENSIOMETER CALIBRATION

Because tensiometers commonly use mechanical
vacuum gauges, these instruments can become
inaccurate or fail with time. Calibration is
periodically required when tensiometers are used to
control irrigation scheduling, especially for crops that
are very sensitive to water stress. Two calibrations
can be performed: 1) the vacuum test gauge quick
calibration, and 2) the vacuum chamber detailed
calibration.

Vacuum Test Gauge Quick Calibration

The vacuum test gauge calibration was previously
described in the tensiometer testing section of this
publication. This is a quick test made by comparing
the tensiometer gauge with the vacuum test gauge.
The accuracy of this calibration depends on
maintaining and using an accurate test gauge. If the
tensiometer vacuum test pump used is equipped with
a check valve to sustain the vacuum in the
tensiometer tube, and a small bleed valve to slowly
release the vacuum as desired, then the test
instrument can easily be used to compare the gauges
at several vacuum levels. First, operate the test pump
to create a high level of vacuum in the tensiometer
and read both gauges when they have equilibrated.
Then operate the bleed valve to gradually reduce the
vacuum level, stopping at each new level to read and
record both vacuum gauges.

This procedure allows a quick test of a
tensiometer gauge to be made throughout its range of
operation. It offers the advantage that an
instrument's calibration can quickly be checked in the
field, using only a small, easily-portable vacuum pump
and test gauge.


UNIVERSITY OF Ri.usA L .ALT,',' S







Tensiometer Service, Testing and Calibration

S create a vacuum in the instrument and look for the
continued flow of air into the instrument. A steady
stream of air bubbles will indicate a large air leak.

When large air leaks occur, they are often around
fittings or gaskets such as where the pressure gauge
is threaded into the plexiglass tube or where an o-ring
seal is used to attach the tip to the tube on some
tensiometer models. If so, these can often be
repaired by using thread sealant or replacing o-rings.
Large air leaks sometimes occur where the ceramic
cup is cemented onto the plexiglass tube. If that is
the case, it can often be repaired with a waterproof
epoxy, however, all components need to be
thoroughly dry before applying epoxy. Several days
may be required to thoroughly dry the ceramic cup
and the epoxy.

If no large air leaks occur, the mechanical vacuum
gauge can be tested by comparing its reading with the
test gauge on the vacuum test pump. Always buy a
test pump with a test gauge installed. Then when the
vacuum pump is used, the tensiometer gauge should
read the same values as the test gauge. This is a
quick and easy test that the tensiometer vacuum
gauge is working and that its readings are
approximately correct. Unfortunately, if the
instrument gauge does not work, or if it is not
accurate, most types must be replaced. Some can be
calibrated, however, most that are commonly used
must be discarded and replaced with a new gauge.

The final instrument test is a test for leaks and
proper operation of the ceramic cup with the
tensiometer sealed and ready for field installation.
After the extraction of air and test of the vacuum
gauge as described above, refill the tensiometer to the
top and seal it with the tensiometer cap. Then place
or hang the instrument in the atmosphere where
water can evaporate from the ceramic cup to simulate
soil drying. Because the instrument is sealed, as
evaporation occurs the vacuum gauge reading should
slowly increase (depending on the rate of drying)
throughout the tensiometer range. This process may
take an hour or more during which time the
tensiometer fluid can be observed for streams of small
air bubbles which indicate small leaks in the
instrument. Such leaks must be found and repaired
because if they are not, the instrument will require
frequent refilling in the field and the gauge reading
will lag the true soil reading.


Page 3


While the above slow-leak test is being conducted,
a test of the ceramic cup flow properties is being
conducted at the same time. If the ceramic cup pores
are plugged, water will not flow through the ceramic
and the instrument will not respond or will respond
only very slowly to drying by the atmosphere. This
will indicate a need for the ceramic to be cleaned
more thoroughly before the instrument will work in
the field. Because of the small pore sizes in the
ceramic, they can readily be plugged by oil, grease, or
other contaminants. Never use oil or grease on or
around these instruments where the ceramics might
become contaminated.

TENSIOMETER CALIBRATION

Because tensiometers commonly use mechanical
vacuum gauges, these instruments can become
inaccurate or fail with time. Calibration is
periodically required when tensiometers are used to
control irrigation scheduling, especially for crops that
are very sensitive to water stress. Two calibrations
can be performed: 1) the vacuum test gauge quick
calibration, and 2) the vacuum chamber detailed
calibration.

Vacuum Test Gauge Quick Calibration

The vacuum test gauge calibration was previously
described in the tensiometer testing section of this
publication. This is a quick test made by comparing
the tensiometer gauge with the vacuum test gauge.
The accuracy of this calibration depends on
maintaining and using an accurate test gauge. If the
tensiometer vacuum test pump used is equipped with
a check valve to sustain the vacuum in the
tensiometer tube, and a small bleed valve to slowly
release the vacuum as desired, then the test
instrument can easily be used to compare the gauges
at several vacuum levels. First, operate the test pump
to create a high level of vacuum in the tensiometer
and read both gauges when they have equilibrated.
Then operate the bleed valve to gradually reduce the
vacuum level, stopping at each new level to read and
record both vacuum gauges.

This procedure allows a quick test of a
tensiometer gauge to be made throughout its range of
operation. It offers the advantage that an
instrument's calibration can quickly be checked in the
field, using only a small, easily-portable vacuum pump
and test gauge.


UNIVERSITY OF Ri.usA L .ALT,',' S







Tensiometer Service, Testing and Calibration

Vacuum Chamber Detailed Calibration

Figure 3 shows a vacuum chamber that can be
constructed to allow detailed tensiometer calibrations
and extended leak tests to be performed. The calibra-
tion chamber is constructed from pressure-rated PVC
pipe and PVC end caps. Holes are drilled along the
length of the pipe for tensiometer ports, and other
small ports are drilled for the installation of a vacuum
regulator, vacuum gauge, and drain valve. Number 11
neoprene rubber stoppers with 3/4 or 7/8-inch
diameter holes in them are used to seal tensiometers
into the chamber, and solid stoppers are used to plug


Page 4


pump and setting the vacuum regulator, then the
tensiometer gauges are allowed to come to
equilibrium with the vacuum level set, and both the
chamber vacuum gauge and the tensiometer gauges
are read. This process is repeated at several vacuum
levels to verify that each instrument is accurate or to
create a calibration curve for each instrument tested.

The advantage of this method is that the entire
instrument is inserted into the chamber during the
test procedure. Thus, leak tests, ceramic cup flow
properties, and tensiometer vacuum gauges are all
tested at the same time. The vacuum level can be


Figure 3. Top, side and end views of a tensiometers calibration chamber designed to calibrate six instruments at the same time.


tensiometer access ports when they are not in use. A
wooden stand is used as a base for the chamber.

A vacuum pump is used to create the vacuum
required in the chamber, and a vacuum regulator is
used to precisely set the vacuum level during
calibration. Water is placed in the chamber to a level
approximately midway up the ceramic cup.

Tensiometers are calibrated by servicing them and
installing them in the chamber as shown in Figure 3.
A vacuum level is created by operating the vacuum


maintained for a long time (such as overnight) to test
for slow leaks.

Both manually-read tensiometers and automatic
switching tensiometers or pressure transducers can be
calibrated using the chamber described here. Since
all of these instruments measure soil water tension,
all of the calibrations performed are independent of
soil type, thus no further field calibration is required.
When manual or pressure-transducer tensiometers are
used, calibration curves can be determined by
measuring the chamber vacuum level and the gauge


TENSIOMETER CALIBRATION CHAMBER


TOP VIEW


TENSION R PORTS TENSIOMETER
TO VACUUM @
PUMP @







Tensiometer Service, Testing and Calibration

or transducer outputs at the same time. A mercury
manometer or high precision test gauge can be used
to accurately monitor the chamber vacuum level if
that degree of accuracy is required.

Automatic switching tensiometers use a magnetic
pickup switch to indicate when a preset vacuum level
has been reached. At that point, an irrigation can be
automatically scheduled. To calibrate these switching
tensiometers, the desired vacuum level is set in the
chamber and the tensiometer vacuum gauges are
allowed to equilibrate with it. Then the magnetic
pickup switch is slowly rotated until switch closure
occurs. This setting is marked on the vacuum gauge.
This procedure insures that the switch setting will
occur at the same vacuum level in the field. Again,
no field calibration of these instruments is required.
All calibration can be done in the convenience and
comfort of the shop or laboratory.

SUMMARY

Tensiometers are useful instruments for irrigation
scheduling under field conditions, however, they
require servicing,-testing and calibration to insure that
they are working properly. A tensiometer is a fairly
simple instrument that will work well if it is properly
installed and in good repair. The instrument is in
good repair if: 1) the vacuum gauge is accurate, 2) the
ceramic cup allows free water movement between the
soil and the instrument, and 3) there are no air leaks.
This publication presents both field and laboratory
procedures for servicing, testing, and calibration of
tensiometers so that the user can determine that the
instrument is working properly and reading accurately
for irrigation scheduling purposes.


Page 5


Because tensiometers commonly use mechanical
vacuum gauges, these instruments can become
inaccurate or fail with time. Calibration is
periodically required if these instruments will be used
to control irrigation scheduling, especially for crops
that are very sensitive to water stress. Two
calibrations can be performed: 1) the vacuum test
gauge calibration, and 2) the vacuum chamber
calibration. The vacuum test gauge procedure allows
a quick test of a tensiometer gauge to be made
throughout its range of operation. It offers the
advantage that an instrument's calibration can quickly
be checked in the field, using only a small, easily-
portable vacuum pump and test gauge.

With the vacuum chamber method, the entire
instrument is inserted into the chamber during the
test procedure. Thus, leak tests, ceramic cup flow
properties, and tensiometer vacuum gauges are all
tested at the same time. The vacuum level can be
maintained for a long time (such as overnight) to test
for slow leaks.

All of the calibrations performed are independent
of soil type, thus no field calibration is required. All
calibrations can be done in the convenience and
comfort of the shop or laboratory.

REFERENCES

Pitts, D.J., T.A. Obreza, D. Parker and A.G.
Smajstrla. 1996. A vacuum chamber for testing
tensiometers. Technical paper presented at the
Florida Section ASAE annual meeting, May,
1996. Cocoa Beach, FL.

Smajstrla, A.G., D.S. Harrison and F.X. Duran. 1984.
Tensiometers for soil moisture measurement and
irrigation scheduling. Ext. Circ. 487. Fla. Coop.
Ext. Svc., Univ. Of Fla., Gainesville.







Tensiometer Service, Testing and Calibration

or transducer outputs at the same time. A mercury
manometer or high precision test gauge can be used
to accurately monitor the chamber vacuum level if
that degree of accuracy is required.

Automatic switching tensiometers use a magnetic
pickup switch to indicate when a preset vacuum level
has been reached. At that point, an irrigation can be
automatically scheduled. To calibrate these switching
tensiometers, the desired vacuum level is set in the
chamber and the tensiometer vacuum gauges are
allowed to equilibrate with it. Then the magnetic
pickup switch is slowly rotated until switch closure
occurs. This setting is marked on the vacuum gauge.
This procedure insures that the switch setting will
occur at the same vacuum level in the field. Again,
no field calibration of these instruments is required.
All calibration can be done in the convenience and
comfort of the shop or laboratory.

SUMMARY

Tensiometers are useful instruments for irrigation
scheduling under field conditions, however, they
require servicing,-testing and calibration to insure that
they are working properly. A tensiometer is a fairly
simple instrument that will work well if it is properly
installed and in good repair. The instrument is in
good repair if: 1) the vacuum gauge is accurate, 2) the
ceramic cup allows free water movement between the
soil and the instrument, and 3) there are no air leaks.
This publication presents both field and laboratory
procedures for servicing, testing, and calibration of
tensiometers so that the user can determine that the
instrument is working properly and reading accurately
for irrigation scheduling purposes.


Page 5


Because tensiometers commonly use mechanical
vacuum gauges, these instruments can become
inaccurate or fail with time. Calibration is
periodically required if these instruments will be used
to control irrigation scheduling, especially for crops
that are very sensitive to water stress. Two
calibrations can be performed: 1) the vacuum test
gauge calibration, and 2) the vacuum chamber
calibration. The vacuum test gauge procedure allows
a quick test of a tensiometer gauge to be made
throughout its range of operation. It offers the
advantage that an instrument's calibration can quickly
be checked in the field, using only a small, easily-
portable vacuum pump and test gauge.

With the vacuum chamber method, the entire
instrument is inserted into the chamber during the
test procedure. Thus, leak tests, ceramic cup flow
properties, and tensiometer vacuum gauges are all
tested at the same time. The vacuum level can be
maintained for a long time (such as overnight) to test
for slow leaks.

All of the calibrations performed are independent
of soil type, thus no field calibration is required. All
calibrations can be done in the convenience and
comfort of the shop or laboratory.

REFERENCES

Pitts, D.J., T.A. Obreza, D. Parker and A.G.
Smajstrla. 1996. A vacuum chamber for testing
tensiometers. Technical paper presented at the
Florida Section ASAE annual meeting, May,
1996. Cocoa Beach, FL.

Smajstrla, A.G., D.S. Harrison and F.X. Duran. 1984.
Tensiometers for soil moisture measurement and
irrigation scheduling. Ext. Circ. 487. Fla. Coop.
Ext. Svc., Univ. Of Fla., Gainesville.