SIST EN ISO 11276:2014

Soil quality - Determination of pore water pressure - Tensiometer method (ISO 11276:1995)

Describes methods for the determination of pore water pressure (point measurements) in unsaturated and saturated soil using tensiometers. Applicable for in situ measurements in the field and, e. g. soil cores, used in experimental examinations.

Bodenbeschaffenheit - Bestimmung des Porenwasserdrucks - Tensiometerverfahren (ISO 11276:1995)

Diese Internationale Norm beschreibt Tensiometerverfahren zur Bestimmung des Porenwasserdrucks sowohl in ungesättigtem als auch in gesättigtem Boden. Die Verfahren sind auf In-situ-Messungen des Porenwasserdrucks im Freiland sowie für die Bestimmung des Porenwasserdrucks z. B. in Pflanzenbehältern oder Bodenkernen für experimentelle Untersuchungen anwendbar.
Bei normalem atmosphärischen Druck (d. h. etwa 100 kPa) ist die Anwendung dieser Verfahren auf einen Bereich von Drücken bis etwa  85 kPa begrenzt. Der Bereich verringert sich bei geringerem atmosphärischen Druck. Tensiometer sind nicht einsetzbar, wenn in der Messtiefe Temperaturen unter 0 °C auftreten. Ihre Genauigkeit wird von Temperaturschwankungen des Bodens und der Luft beeinflusst. Die Einstellzeit von Tensiometern reicht von einigen Sekunden bis zu mehreren Tagen. Zuverlässige Messungen unter Freiland-bedingungen erfordern eine regelmäßige Wartung der Tensiometer.
Tensiometer liefern Punktmessungen des Porenwasserdrucks. Um den Porenwasserdruck in unterschiedlichen Tiefen messen zu können, sind mehrere Tensiometer erforderlich. Im Freiland werden parallele Gerätesätze erforderlich, wenn die räumliche Variabilität des Bodens zu berücksichtigen ist.

Qualité du sol - Détermination de la pression d'eau dans les pores - Méthode du tensiomètre (ISO 11276:1995)

La présente Norme internationale prescrit les méthodes pour la détermination, à l'aide de tensiomètres, de la pression d'eau dans les pores, dans des sols non saturés ou saturés. Ces méthodes sont applicables aux mesurages in situ de la pression d'eau dans les pores, ainsi qu'à la surveillance de la pression d'eau dans les pores dans, par exemple, des pots de plantes ou des carottes de sol utilisées dans des procédures expérimentales. Aux pressions atmosphériques normales, c'est-à-dire aux alentours de 100 kPa, l'emploi de ces méthodes est limité à une gamme de pressions d'eau dans les pores allant jusqu'à environ -- 85 kPa. Cette gamme se trouve encore réduite à des pressions atmosphériques plus faibles. Les tensiomètres ne fonctionnent pas s'il règne à la profondeur des mesurages des températures inférieures à zéro. La précision des mesures est influencée par les fluctuations de température du sol et de l'air. Le temps de réponse d'un tensiomètre s'échelonne entre quelques secondes et plusieurs jours. Il est nécessaire de vérifier régulièrement le bon fonctionnement des tensiomètres pour obtenir des mesurages fiables. Vu le caractère ponctuel des mesurages de la pression d'eau dans les pores effectués par un tensiomètre, il est nécessaire d'en prévoir plusieurs pour mesurer la pression d'eau dans   180les pores à plusieurs profondeurs. Sur le terrain, il faudra des ensembles d'instruments identiques si l'on veut tenir compte de la variabilité spatiale du sol.

Kakovost tal - Določanje tlaka vode v porah - Tenziometrijska metoda (ISO 11276:1995)

Standard EN-ISO 11276 določa metode za določanje tlaka vode v porah v nenasičenih in nasičenih tleh z uporabo tenziometrov. Te metode se uporabljajo za meritve tlaka vode v porah na kraju samem na terenu ter za spremljanje tlaka vode v porah v na primer posodah za rastline ali jedrih tal, uporabljenih v preskusnih postopkih. Pri normalnem atmosferskem tlaku, to je približno 100 kPA, je uporaba teh metod omejena na vrednosti tlaka do –85 kPa. Vrednost se zniža pri nižjih atmosferskih tlakih. Tenziometri ne bodo delovali, če bodo temperature na globini merjenja pod ničlo. Njihova natančnost je odvisna od temperaturnih nihanj tal in zraka. Odzivni čas tenziometra je lahko od nekaj sekund do nekaj dni. Za zanesljive meritve na terenu morajo biti tenziometri pogosto servisirani.

General Information

• Status: Published
• Public Enquiry End Date: 31-Oct-2013
• Publication Date: 08-Apr-2014
• ICS: 13.080.20 - Physical properties of soils
• Technical Committee: KAT - Soil quality
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 31-Mar-2014
• Due Date: 05-Jun-2014
• Completion Date: 09-Apr-2014
• Ref Project: EN ISO 11276:2014 - Soil quality - Determination of pore water pressure - Tensiometer method (ISO 11276:1995)

Relations

• Revised: SIST ISO 11276:1998 - Soil quality -- Determination of pore water pressure -- Tensiometer method
• Effective Date: 01-May-2014

Overview

EN ISO 11276:2014 (identical to ISO 11276:1995) is a CEN/ISO standard for soil quality that specifies the tensiometer method for determination of pore water pressure in both unsaturated and saturated soils. It covers point measurements using tensiometers for in situ field work and laboratory/experimental setups such as soil cores and plant containers. The standard describes instrument components, installation, calibration, limitations and maintenance to obtain reliable pore water pressure data.

Key topics and technical requirements

• Scope & limits
  • Applicable to pore water pressures down to about –85 kPa at normal atmospheric pressure; reduced range at lower atmospheric pressures.
  • Tensiometers do not function if sub-zero temperatures occur at measurement depth.
• Principle
  • A porous cup filled with de-aired water equilibrates with soil water; the internal water pressure (corrected for height differences) equals local pore water pressure (matric + pneumatic).
• Tensiometer construction
  • Porous cup (typically unglazed ceramic or alternative materials described in annexes) must be air-entry rated and durable in soil.
  • Connecting/body tubes of low permeability with leakproof joints; minimize number of joints.
  • Pressure sensors: mercury manometers, Bourdon gauges, and electrical pressure transducers are described; sensor accuracy determines measurement accuracy.
• Installation & measurement
  • Install vertically or horizontally with porous cup centered at desired depth while minimizing soil disturbance and ensuring good cup-soil contact.
  • Use replicate tensiometers to capture spatial variability.
• Calibration, servicing & maintenance
  • Use de-aired water (boiled or vacuum-degassed) and avoid entrapped air when filling.
  • Regular servicing is required to remove air accumulation (air-trap limit: >100 mm3 requires refilling).
  • Verify accuracy of Bourdon gauges and transducers before installation and at least annually; test assemblies for leaks under pressure/vacuum.
• Units & conversions
  • Uses SI units (Pascal) and provides conversions between soil water potential, pressure and head equivalents.

Applications and users

• Practical uses:
  • Monitoring soil moisture tension for agronomy, irrigation scheduling and greenhouse management.
  • Geotechnical and environmental investigations (slope stability, unsaturated soil mechanics).
  • Laboratory testing of soil cores, lysimeters and experimental soil columns.
• Typical users:
  • Soil scientists, agronomists, geotechnical engineers, environmental consultants, laboratory technicians and researchers working with soil testing, pore water monitoring and instrumentation.

Related standards

• Adopted by CEN as EN ISO 11276:2014 from ISO 11276:1995 (prepared by ISO/TC 190, Soil quality). For broader context, consult other ISO/CEN soil quality standards and CEN/TC 345 guidance on soil characterization.

Keywords: soil quality, pore water pressure, tensiometer method, EN ISO 11276:2014, in situ measurements, unsaturated soil, soil cores, soil testing.

Standards Content (Sample)

SLOVENSKI STANDARD
01-maj-2014
.DNRYRVWWDO'RORþDQMHWODNDYRGHYSRUDK7HQ]LRPHWULMVNDPHWRGD,62

Soil quality - Determination of pore water pressure - Tensiometer method (ISO
11276:1995)
Bodenbeschaffenheit - Bestimmung des Porenwasserdrucks - Tensiometerverfahren
(ISO 11276:1995)
Qualité du sol - Détermination de la pression d'eau dans les pores - Méthode du
tensiomètre (ISO 11276:1995)
Ta slovenski standard je istoveten z: EN ISO 11276:2014
ICS:
13.080.20 Fizikalne lastnosti tal Physical properties of soils
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 11276
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2014
ICS 13.080.20
English Version
Soil quality - Determination of pore water pressure - Tensiometer
method (ISO 11276:1995)
Qualité du sol - Détermination de la pression d'eau dans les Bodenbeschaffenheit - Bestimmung des
pores - Méthode du tensiomètre (ISO 11276:1995) Porenwasserdrucks - Tensiometerverfahren (ISO
11276:1995)
This European Standard was approved by CEN on 13 March 2014.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same
status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2014 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 11276:2014 E
worldwide for CEN national Members.

Contents Page
Foreword .3
Foreword
The text of ISO 11276:1995 has been prepared by Technical Committee ISO/TC 190 “Soil quality” of the
International Organization for Standardization (ISO) and has been taken over as EN ISO 11276:2014 by
Technical Committee CEN/TC 345 “Characterization of soils” the secretariat of which is held by NEN.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by September 2014, and conflicting national standards shall be
withdrawn at the latest by September 2014.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech
Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.
Endorsement notice
The text of ISO 11276:1995 has been approved by CEN as EN ISO 11276:2014 without any modification.
INTERNATIONAL
ISO
STANDARD
First edition
1995-09-01
Seil quality - Determination of pore water
pressure - Tensiometer method
Quake du sol - Determination de Ia Pression d’eau dans les pores -
M6 thode du tensiomk tre
Reference number
ISO 11276: 1995(E)
ISO 11276:1995(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide
federation of national Standards bodies (ISO member bodies). The work
of preparing International Standards is normally carried out through ISO
technical committees. Esch member body interested in a subject for
which a technical committee has been established has the right to be
represented on that committee. International organizations, governmental
and non-governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting
a vote.
International Standard ISO 11276 was prepared by Technical Committee
lSO/TC 190, Seil quality, Subcommittee SC 5, Physical methods.
Annex A forms an integral part of this International Standard. Annexes B,
C, D, E and F are for information only.
0 ISO 1995
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronie or mechanical, including photocopying and
microfilm, without Permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-l 211 Geneve 20 l Switzerland
Printed in Switzerland
ii
INTERNATIONAL STANDARD 0 ISO ISO 11276:1995(E)
Soil quality - Determination of pore water
pressure - Tensiometer method
NOTES
1 Scope
2 Pore water pressure is also referred to as tensiometer
This International Standard specifies methods for the
pressure.
determination of pore water pressure in both unsatu-
rated and saturated soil using tensiometers. The 3 The pore water pressure represents the sum of the
pressures due to interfacial forces acting between the
methods are applicable for in situ pore water pressure
water, air and solid phases of the soil (matric pressure), the
measurements in the field, as well as for monitoring
part of the mass of overlying material not carried by the soil
pore water pressure in, for example, plant Containers
Skeleton and therefore carried by the soil water (overburden
or soil cores used in experimental procedures.
pressure; this pressure is often considered as part of the
matric pressure) and the local air pressure within the soil
At normal atmospheric pressures, i.e. about 100 kPa,
(pneumatic pressure). Under most circumstances, the
the application of these methods is limited to a range
overburden and pneumatic pressures are Zero.
of pressures down to about - 85 kPa. The range is
lower atmospheric pressures.
reduced at
Tensiometers will not function if sub-zero tempera-
2.2 matric pressure: The amount of work that must
tures occur at the measurement depth. Their accuracy
be done in Order to transport reversibly and
is influenced by seil and air temperature fluctuations.
isothermally an infinitesimal quantity of water, ident-
Tensiometer response time ranges from a few sec-
ical in composition to the soil water, from a pool at the
onds to several days. To obtain reliable measure-
elevation and the external gas pressure of the Point
ments under field conditions, tensiometers require
under consideration, to the soil water at the Point
frequent servicing.
under consideration, divided by the volume of water
transported.
A tensiometer provides Point measurements of pore
water pressure. To measure pore water pressure at
several tensiometers will be 2.3 pneumatic pressure: The amount of work that
different depths,
must be done in Order to transport reversibly and
necessary. In the field, replicate sets of instruments
isothermally an infinitesimal quantity of water, ident-
will be required if the spatial variability of the soil is to
be allowed for. ical in composition to the soil water, from a pool at
atmospheric pressure and at the elevation of the Point
under consideration, to a similar pool at an extemal
gas pressure of the Point under consideration, divided
by the volume of water transported.
2 Definitions
NOTE 4 Soil water pressure tan be considered as a
For the purposes of this International Standard, the
pressure equivalent of soil water potential. The same ap-
plies to the soil water head, the head equivalent of soil
following definitions apply.
water potential.
NOTE 1 Additional definitions are given in E.2, for infor-
mation only. The relationship between these is
‘P-p, = p - h-g-p,
2.1 pore water pressure: The sum of matric and
pneumatic pressures. where
0 ISO
ISO 11276:1995(El)
Y is the soil water potential, in joules per kilogram
4 Apparatus
on a mass basis;
4.1 Tensiometer, usually consisting of a porous
is the pressure equivalent of soil water poten-
P
CUP, a connecting tube and/or a body tube, a pressure
tial, in joules per cubic metre on a volume basis
(1 J/m3 = 1 N/m2 = 1 Pa);
Sensor and a mechanism for expelling any air which
accumulates within the tensiometer. The details of
h is the head equivalent of soil water potential, in
the design depend primarily on whether the instru-
joules per newton on a forte basis
ment is intended for field or indoor use and the type
(1 J/N=l m);
of pressure Sensor employed; examples are shown in
is the density of water, in kilograms per cubic
figure 1. Annex B provides information on materials
Pw
metre;
for the construction of tensiometers and on their
construction.
is the acceleration due to gravity, in metres per
S
second squared.
4.1.1 Porous CUP, made of a porous material of air-
In this International Standard pressure equivalents and soil
entry value (i.e. the pressure required to forte air
water Potentials are used. The corresponding unit of
through the water-saturated CUP) larger in magnitude
measurement is the Pascal (Pa). Table 1 provides conver-
than the lowest pore water pressure to be measured
sions between soil water potential and its pressure and
and the known hydraulic conductivity. The material
head equivalents.
shall be rigid and not subject to degradation in soil.
Usually unglazed ceramic is used; alternatives are de-
scribed in annex B.
3 Principle
4.1.2 Connecting and body tubes, made from ap-
A tensiometer comprises a porous cup that is per-
propriate materials of low permeability to water and
meable to water connected to a pressure-measuring
gas and connected by leakproof joints. Rigid or semi-
device. The pores of the wall of the cup are small
rigid tubing shall be used to connect the tensiometer
enough to prevent air passing through when it is wet.
to the pressure Sensor (see annex B). The function
The porous cup is filled with water. When the cup is
of the connecting tube may, in part or totally, be
placed in the soil, water within the tensiometer flows
served by the body tube.
through the porous wall to the soil, or soil water flows
into the tensiometer, until the pressure of the water The body tube usually fills the hole remaining above
on both sides of the porous wall is equal. When or behind the tensiometer cup after inserting it into
equilibrium has been reached, the measured pressure the soil. lt is a rigid tube with the same outside di-
of the water inside the tensiometer, after correction ameter as the porous CUP. In many designs, it is filled
with water, but in others it forms a casing for smaller
for the differente in height between the pressure
tubes connected to the porous cup and/or cables at-
Sensor and the porous cup equals the pore water
tached to a pressure transducer located behind the
pressure of the soil water at the Position of the
CUP.
porous CUP.
Table 1 - Conversions between soil water potential and its pressure and head equivalents
Pressure equivalent Head equivalent
Potential
Parameter to be converted
Pa m
Jh
1 0,102 0 x 1o-3
Pressure equivalent (Pa) 1 o-3
Head equivalent (m) 9 807 1
9,807
Potential (J/kg) IO3 0,102 0
NOTES
potential, multiply by the factor
1 To convert from the potential or its equivalent in the first vertical to another equivalent or
given, for example:
a potential of 1 J/kg has a pressure equivalent of IO3 Pa and a head equivalent of 0,102 0 m.
2 Acceleration due to gravity = 9,807 m/s2
Density of water = 1 000 kg/m3
0 ISO
ISO 11276:1995(E)
4.1.3 Pressure Sensors. Several forms are used in If a tensiometer assembly of new design or of untried
tensiometers, the most common being mercury materials is to be used, it shall be tested for leaks
manometers, Bourdon gauges and electrical pressure under pressure and/or under vacuum before instal-
transducers. The use of other types of manometer is lation. This procedure is recommended for all instal-
lations.
permissible. The accuracy of the pressure Sensor de-
termines how accurately the pressure of the water
within the tensiometer tan be measured.
5 Procedure
Annex A details the construction and use of mercury
5.1 Installation of tensiometers
manometers for use with tensiometers. The other
pressure Sensors are described in annex C.
Tensiometers may be installed vertically or horizon-
tally, whichever is most suitable for the required pur-
The accuracy of Bourdon gauge and pressure
pose. Install each tensiometer so that the centre of
transducer tensiometers shall be verified before in-
the porous cup is at the depth at which measurement
stallation and at least annually thereafter.
is required. Ensure minimal disturbance to the soil
NOTE 5 The accuracy of instruments used in the field
that will surround the tensiometer, both at the soil
may be tested with a mercury manometer reference. The
surface and at depth. Maximize the contact between
complete tensiometer assembly tan be tested in the field
the porous cup and the soil but minimize the smearing
by inserting a “T” piece into the connecting tube. When
of the soil around the CUP.
required, another connecting tube is attached to it for con-
nection to a mercury manometer. Should greater accuracy
NOTE 6 Usually, a hole of the same diameter as the
be required for laboratory purposes, specialized testing
tensiometer is carefully bored and the tensiometer is in-
equipment will be necessary.
serted into it. Details of alternative procedures for preparing
holes in which tensiometers tan be inserted in the field are
4.2 Tensiometer construction given in annex D. Methods similar to those described in
annex D, but scaled down, should usually be Chosen when
installing tensiometers in plant Containers, soil cores,
Details of materials for constructing tensiometers and
lysimeters, etc.
of their construction are given in annex B. Since the
interior of a tensiometer installed in unsaturated soil
Care shall be taken to protect the tensiometer System
is under a partial vacuum, it is essential that all poss-
from temperature fluctuations. Fluctuations induce
ible leakage Points are made as secure as possible.
thermal expansion and contraction of Parts of the
The number of joints in the System shall be kept to
System and the water within, which influence the
the minimum possible. Adhesive joints shall be made
pressure measurement. In the field, all exposed Parts
so that the void space between components is filled
of the tensiometer shall, as far as practicable, be
completely. Joints relying on a tight fit of two ma-
shielded from solar radiation. (This reduces thermal
terials, for example Stoppers, shall be correctly sized,
disturbance to the tensiometer reading and also pro-
with as large an area of contact as possible.
longs the life of the components.) Precautions shall
also be taken to prevent the percolation of rain or irri-
The System is used in a damp environment. Hence
all materials shall be Chosen to resist moisture. This gation water down the side of the tensiometer to the
applies particularly to adhesives, some kinds of which CUP. All equipment and the area around the
may soften or swell (leading to failure of cemented tensiometer shall be protected from darnage by
Parts) in damp conditions. rodents and other animals.
ISO 11276:1995(E)
Removable airtight cap
\
Connecting tube
\I 1
1 - Vacuum gauge
H- Rubber bung
Groundsurface
Air trap
Body tube
\
Inlett valve
De-aired water
Transducer cable
(with vent)
Outlet valve A+
Transducer (ventilated /
- Porous cup -
to atmosphere)
b) Bourdon gauge type tensiometer
a) Simple tensiometer with an air trap and
connecting tube for use with a manometer
typepressuresensor
Pressure-resistant
Connecting s crew
screw connection
for fixture
Water-f illed body tube
cell holder
Pressure 1 Sealed upper body tubing
L-
transducer
2 Water-filled lower body tubing
d) Tensiometer with pressure transducer and
c) Small tensiometer with pressure transducer for use in soil cores
System for air removal for use in the field
- The main elements of tensiometers incorporated into a variety of designs intended for field
Figure 1
and laboratory use
0 ISO
ISO 11276:1995(E)
5.2 Preparation of tensiometers for use 5.4 Servicing
and maintenance of
tensiometers
5.2.1 Preparation of de-aired water
The major recurrent Problem
when operating
tensiometers is air accumulation within them. Those
Remove dissolved air from all the water used in the
having the pressure Sensor placed behind the porous
tensiometers, either by boiling it or with a vacuum
cup are less susceptible to this Problem, but it is es-
System. Store the de-aired water so that no air tan
sential to ensure that any air accumulation is mini-
come into contact with it. Pour the de-aired water
mized by occasional purging.
carefully and smoothly to minimize contact with air.
With other types of equipment, small air bubbles in
the air trap will not affect the accuracy of the
5.2.2 Filling the System with water
tensiometer, but will lengthen its response time. The
tensiometer shall be refilled with de-aired water
lt is essential, when filling the assembled tensiometer
whenever an air bubble of volume greater than
System with the de-aired water, to avoid air being
100 mm3 (0.1 cm3) has collected in the air trap. The
trapped inside it. Under field conditions, flush mercury
procedure IS the same as that described in 5.2.2.
manometer tensiometer Systems as described in an-
nex A.
Persistent low readings of tensiometers (i.e. very
negative readings) may be due to poor contact with
NOTE 7 Under experimental laboratory conditions, it is
the soil or leaks in the System. In the latter case, large
preferable not to flush the System, as doing so tan influ-
amounts of air will collect in the tensiometer. If either
ence the water balance of a soil core.
Problem is suspected, the tensiometer shall be re-
moved and repaired.
lt is possible to remove air from field Systems
equipped with Bourdon gauges or electrical pressure
Examination, and servicing if necessary, shall usually
transducers, using a vacuum pump. This Causes the
be performed at least once a week.
air in the System to expand and bubble out. Water
replaces the air when the vacuum is released. Some-
times, several such evacuation and repressurization
6 Expression of results
cycles will be necessary to remove all the air.
6.1 Method of calculation
5.3 Reading tensiometers
The pressure Sensor reading gives the sum of the
pressure in the porous cup of the tensiometer and
lt is important to wait until the tensiometer System
that of the water column between the pressure sen-
reaches hydraulic equilibrium before making readings.
sor and porous cup (see figure2). The pore water
NOTES
pressure of the soil water at the Position of the
porous cup is calculated using the following formula:
8 In a wet, coarse soil, reliable readings may be obtained
within 1 h or less of setting up or servicing, whereas in drier Dm =
Px + &l/=g-a
‘tJ
soil, several days may be needed. lt is recommended that
an interval of at least 4 h and preferably 16 h (overnight) is
where
allowed before reading field tensiometers, after setting up
or servicing.
is the pore water pressure, in Pascals, at
PP
the Position of measurement, i.e. at the
9 The frequency with which readings are made will de-
porous Cup;
pend on the purpose for which they are collected. In the
upper 0,5 m or more, readings will Change quickly in re-
is the pressure, in Pascals, of the water in
Px
sponse to rainfall (hourly time-scale) and slightly less quickly
the pressure Sensor in equilibrium with the
in response to evaporation (daily time-scale). Changes will
porous CUP, relative to atmospheric press-
be slower at lower depths. However, if intervals of longer
ure;
than a week elapse between readings, it will often be
necessary to Service manometer and Bourdon gauge type
a is the vertical distance, in metres, be-
tensiometers before reliable readings tan be obtained. To
tween the pressure Sensor and the porous
minimize the effect of diurnal temperature fluctuations and
pore water pressure oscillations due to extraction of water
CUP;
by plants, it is preferable that tensiometers be read at the
is the density of water, in kilograms per
same hour each day that they are monitored, if the reading
Pw
frequency is daily or less. cubic metre (approximately 1 000 kg/m3);
ISO 11276:1995(E) 0 ISO
is the acceleration due to gravity, in metres
(i.e. less negative) than the soil pore water pressure
g
second squared (approximately will be recorded. Alternatively, or in addition, the soil
Per
9,81 m/s*). pore water pressure may be changing quite rapidly in
time as a consequence of, for example, a wetting
front moving through the soil, in which case equilib-
6.2 Precision
rium between the soil and tensiometer cannot be ob-
tained.
lt is not possible to state the precision of a
tensiometer measurement of pore water pressure.
Several factors may individually, or in combination,
7 Test report
affect the precision, i.e. the degree to which the
pressure in the tensiometer differs from the true pore
The test repor-t shall contain the following information:
water pressure at the Position of the porous CUP. The
accuracy of the measurement of the water pressure
a) a reference to this International Standard;
within a tensiometer is determined by the accuracy
of the pressure Sensor System employed.
b) an accurate site description of the measuring lo-
cation and of the soil Profile;
All tensiometer Systems take time to equilibriate with
the external conditions. This response time depends
c) an accurate description of the tensiometers and
on
pressure Sensors used;
- the type of pressure Sensor, which determines the
d) the depth of the tensiometers and an accurate
volume of water displaced for a given Change in
description of the installation procedure;
soil water potential;
the pore water pressure measured in kilopascals,
e)
- the capacity of the tensiometer System;
as a function of depth and time;
- the water conductivity of the porous material of
f) any remarks that are important to the interpret-
the CUP;
ation of the results, such as whether the
tensiometers were recently purged of air, and
- the surface area of the porous CUP.
observations with respect to the hydrological and
meteorological conditions before and during the
In addition, in a given soil the response time is influ-
measurements;
enced by the contact with the soil and the hydraulic
conductivity of the soil, which is a function of the soil
g) any special details which may have been noted
water content.
during the measurements;
If insufficient time has been allowed for the
tensiometer and pressure Sensor System to come h) details of any relevant operations not specified in
into equilibrium with the soil, after either initially set- this International Standard, or regarded as op-
ting up or servicing the equipment, a pressure higher
tional.
ISO 11276:1995(E)
Vacuumgauge
Reference level
Groundsurface
Cl
Figure 2 - Components of the pressure measured by a Sensor attached to a tensiometer

ISO 11276:1995(E)
Annex A
(normative)
Construction and use of mercury manometers
mately 0,5 mm and 2,0 mm, shall be of low per-
A. 1 Introduction
meability to gas and water and be sufficiently
transparent that the water/mercury interface tan be
WARNING - Mercury is hazardous to People,
seen easily. The internal surface of the manometer
animals and the environment and accordingly
tube shall be smooth, to discourage collection of dir-t
great care is required when using mercury
inside the tube.
manometers. All users should be aware of the
nature of the hazard, and be familiar with the
NOTE 11 Polyamide 12, Polyamide 66 (both are types of
precautions necessary to prevent Spills and with
nylon) and glass are suitable materials for capillary tubes.
procedures for cleaning up any mercury spillage.
The two types of nylon are also suitable for connecting
tubes (see B.4).
The mercury manometer is suitable for many appli-
cations. Under constant temperature conditions, as in
The manometer tube is mounted on a scale, gradu-
a laboratory, the tensiometer water pressure tan be
ated in small units, often millimetres. There shall be
measured as accurately as the mercuty Ievel tan be
no gap between the two, to minimize parallax errors
measured against a graduated scale, i.e. to an accu-
when reading the mercury level.
racy of 0,l kPa. Under field conditions, the accuracy
of a mercury manometer is about 0,4 % plus the The bott
...