SIST ISO 9110-1:2021

INTERNATIONAL ISO
STANDARD 9110-1
Second edition
2020-05
Hydraulic fluid power — Measurement
techniques —
Part 1:
General measurement principles
Transmissions hydrauliques — Techniques de mesurage —
Partie 1: Principes généraux de mesurage
Reference number
ISO 9110-1:2020(E)
©
ISO 2020

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ISO 9110-1:2020(E)

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ISO 9110-1:2020(E)

Contents Page
Foreword .iv
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Uncertainty of limit specifications . 2
4.1 General aspects . 2
4.2 Classes of measurement accuracy . 2
5 General measurement considerations and requirements . 2
5.1 Calibration . 2
6 Complete calibration procedure . 5
6.1 Selection of reference standard . 5
6.2 Procedure . 5
7 Instrument calibration uncertainty models . 6
7.1 General . 6
7.2 General procedure . 6
7.3 First order uncertainty model . 7
7.4 First order calibration uncertainty evaluation . 7
7.5 Second order uncertainty model . 7
7.6 Second order calibration uncertainty evaluation . 8
7.7 Third order uncertainty model . 8
7.8 Third order calibration uncertainty evaluation . 9
8 Readability uncertainty evaluation .10
8.1 General .10
8.2 Analog readout devices.10
8.3 Digital readout devices .12
8.4 Readout device records .12
9 Assurance control techniques .13
10 Total measurement uncertainty .13
10.1 Determination of measurement system uncertainty .13
Annex A (informative) Measurement system acceptance designated information sheet .14
Annex B (informative) Uncertainty propagation .15
Annex C (informative) Best practices tutorial .16
Bibliography .20
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ISO 9110-1:2020(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. Each 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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee
SC 8, Product testing.
This second edition cancels and replaces the first edition (ISO 9110-1:1990), which has been technically
revised.
The main changes compared to the previous edition are:
— new normative and informative references have been added;
— new definitions have been added;
— classes of accuracy to measurement have been renamed;
— assessment of uncertainties has been revised and expanded and general measurement considerations
and requirements have been renamed;
— guidance on gravity correction has been added;
— readability uncertainty evaluation has been added;
— determination of uncertainty limits and classification of uncertainties has been combined and
uncertainty limit specifications have been renamed;
— frequency of calibration has been revised and assurance control techniques have been renamed;
— total measurement uncertainty clause has been added;
— original Annex A has been deleted;
— new Annex A - Measurement System Acceptance Designated Information Sheet, has been added;
— new Annex B - Uncertainty Propagation, has been added;
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ISO 9110-1:2020(E)

— new Annex C - Best Practices Tutorial, has been added.
A list of all parts in the ISO 9110 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 9110-1:2020(E)

Introduction
Universal measurement standards are required if meaningful comparisons are to be made and valid
conclusions deduced. A fundamental aspect of fluid power technology is the need to quantify the
performance characteristics of hydraulic components and systems to provide a basis for action or
decision-making. The method of measurement used is capable of reliably determining such performance
characteristics.
This document provides guidance for identifying uncertainty sources and magnitudes in the calibration
of instruments and their use in measurement situations encountered in hydraulic fluid power testing.
Methods are described for assessing the uncertainty in measurements and derived results.
It is widely recognized that no measurement, irrespective of the amount of care exercised, can ever
be absolutely accurate and free of error. Different circumstances each have unique uncertainty
requirements. The value of a measurement is dictated by the use that will be made of it, as well as the
particular circumstance. Therefore, the maximum value of a reported measure can only be realized if
it can be applied under many different circumstances, requiring that the uncertainty associated with a
measure be assessed and reported.
This document is intended to be used in conjunction with others that address the measurement of
specific physical parameters: flow, pressure, torque, speed and temperature.
This document (ISO 9110-1) relates to general principles for the measurement of static or steady-state
conditions. ISO 9110-2 deals with the measurement of average steady-state static pressure in a closed
conduit.
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INTERNATIONAL STANDARD ISO 9110-1:2020(E)
Hydraulic fluid power — Measurement techniques —
Part 1:
General measurement principles
1 Scope
This document establishes general principles for the measurement of performance parameters under
static or steady-state conditions.
This document provides guidance on the sources and magnitudes of uncertainty to be expected in
the calibration of and measurements using hydraulic fluid power components. It describes practical
requirements for assessing the capability of the measuring system, and hence the level of uncertainty
of the measurement system, or for assisting in developing a system which will meet a prescribed level
of uncertainty.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 5598, Fluid power systems and components — Vocabulary
ISO 7870-1, Control charts — Part 1: General guidelines
ISO 7870-2, Control charts — Part 2: Shewhart control charts
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5598 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
data reduction errors
errors that stem from any processing of test data to the final result, as from digital computer resolution,
numerical rounding of results, and uncertainty in model curve fitting and interpolation
3.2
indicated value
magnitude of the measure and the parameter subject to measurement
3.3
parallax
phenomenon responsible for reading errors when the observer’s eye is not perpendicular to the meter
face, and is not directly in line with a pointer whose tip is not in the same plane as the instrument scale
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ISO 9110-1:2020(E)

3.4
readability
ability of a human observer to discern a numeric value to the quantity displayed on the readout device
3.5
uncertainty model
chart, graph or equation that relates the indicated value (3.2) to the value of the measure and the
parameter being measured
4 Uncertainty of limit specifications
4.1 General aspects
4.1.1 Each performance test standard that incorporates this document as a normative reference shall
have its own uncertainty defined for each of the three classes of measurement accuracy described herein,
and instrumentation selection criteria stated.
4.1.2 The maximum uncertainty which may be allowed in a fluid power test measurement can only be
established by considering the component or system under test, the expected use of the test results, and
the economics of the test program.
4.1.3 Each test procedure complying with this document shall include a table of permissible
uncertainty that provides the limits for each of the three classes of measurement accuracy relevant to
this test procedure: A, B, and C (see 4.2.1, 4.2.2, and 4.2.3). The limits should be based upon the maximum
uncertainty allowable for each measurement.
4.2 Classes of measurement accuracy
4.2.1 Class A is the most restrictive and is intended for those measurement situations that are
scientific in nature and directed at investigating phenomena. Equipment capabilities and technical
expertise required to perform class A measurements would generally be used only in the most stringent
applications.
4.2.2 Class B is intended to encompass performance measurements required for selection and
application of components and for quality audits. The requirements for class B measurements should be
within the capabilities of most fluid power testing laboratories.
4.2.3 Class C would apply to diagnostic situations where the objective is to determine if hardware is
functioning properly or has failed, and to monitor the operational status of equipment. Users with limited
expertise in fluid power measurements using standard commercial instrumentation would possess the
required capabilities.
5 General measurement considerations and requirements
5.1 Calibration
The uncertainty inherent in a measurement system may be associated with individual elements of that
system or the system as a whole. In general, calibrating and evaluating the uncertainty of the system as
a whole results in smaller errors and reduced uncertainty.
All reference standards and measuring instruments shall be calibrated utilizing traceable standards
of known uncertainty and environmental influences. The reference standard shall be traceable to a
nationally or internationally certified calibration agency or have been derived from accepted values of
natural physical constants or have been derived by the ratio type of calibration technique. Reference
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ISO 9110-1:2020(E)

standards or physical constants are those recognized by the International Committee for Weights and
Measure (CIPM), the International Bureau of Weights and Measures (BIPM), or the National Standard
Institute of the respective country. The reference standard used for calibration shall be recorded.
It is recommended that measurement and calibration laboratories establish a measurement assurance
program. Analyzing calibration data using control chart methods may be used to characterize the short-
and long-term behaviour of instruments. This time dependent behaviour may be used to establish and
validate calibration intervals.
The reference standard uncertainty included in the total measurement system uncertainty summation
in Clause 10 is obtained either from the manufacturer or certifying agency that provided certification
traceable to the reference standards laboratory.
5.1.1 The calibration interval of reference standards is determined by:
a) consideration of usage and environmental factors;
b) manufacturer's recommendations;
c) governing contract, government regulation, or specific industry specifications/customer
requirements;
d) inherent stability of the standard.
5.1.2 The complete calibration interval of measuring instruments shall be determined by using the
results of intermediate calibrations as per Clause 9. Calibration intervals may also be based on a time
interval considering the following factors:
— equipment stability and drift using historical trend analysis or control charts;
— industry and government-related organizations' recommendations;
— quality standards, customer/contract requirements, and industry regulations;
— experience with instrument usage and frequency;
— environmental operating conditions in the application;
— criticality and complexity of the calibration process;
— risks associated with using un-calibrated instruments;
— risk for damage.
For Class A measurements, intermediate calibration should be conducted immediately prior to
instrument use. If this is not practical in the test situation, e.g. calibration carried out by an external
agency, an intermediate calibration at the end of testing is recommended.
For Class B and C measurements, intermediate calibrations are normally based on a time interval.
NOTE All test results acquired in the preceding calibration interval are suspect if at the next calibration the
results fall outside the required allowable measurement uncertainty or control chart limits.
The risk of acquiring suspect data can be assessed considering the following factors:
a) instrument manufacturer’s recommendations and specifications;
b) instrument past operating experience and calibration control chart history;
c) calibration data history of similar existing instruments.
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5.1.3 New instruments and those without a prior calibration history shall be calibrated at no less than
ten calibration points and five repeated trials at each point. Calibration can be conducted internally, or by
the instrument manufacturer or an outside calibration agency.
See OIML D10, NCSL International RP-1, ANSI Z540.3, and ISO 10012.
5.1.4 Calibration increments for instruments with linear characteristics shall be spaced in a linear
manner. For non-linear instruments, such as turbine flow meters, logarithmically spaced increments are
recommended to provide better definition in the non-linear range. The calibration increments selected
shall include the end points encountered in the measurement situation.
For instruments with prior calibration history, an intermediate calibration performed at 25 %, 50 %,
and 100 % of full scale with three repeated trials is sufficient.
5.1.5 Eliminate systematic standard uncertainty observed during calibration by instrument adjustment
or by correcting all data obtained. If systematic standard uncertainty correction is not implemented,
include the maximum value of the systematic standard uncertainty in the computation of the total
measurement system uncertainty in Clause 10. For example, if the calibration of an instrument reveals a
3 % deviation at mid-range and 1 % at the end points, and the data obtained using the instrument is to be
used without correction, the 3 % deviation shall be used in the uncertainty computation.
5.1.6 Correct standard uncertainties which are the result of a physical relationship with another
independent variable by using a known mathematical function. This class of uncertainties is normally
due to environmental factors. If the standard uncertainty is neglected and no correction is made for its
effect, the maximum value of the uncertainty shall be included in the computation of total measurement
system uncertainty in Clause 10. The effect of temperature on a transducer strain gage bridge is an
example of such an effect.
Gravity varies depending upon the location on earth. Therefore, the need for gravity correction arises
because gravity at the location of a reference standard or instrument varies from the internationally
accepted standard value.
The value for local gravity may be calculated using Formula (1), the International Gravity Formula (IGF)
and the current World Geodetic System model WGS84, which accounts for the rotation of the earth,
height above sea level, and the spheroidal shape of the globe.
2
2
 
10+ ,[0019385138639 sin()θ ] R
 
g =9,7803267714 (1)
 
 
l
2
Re+
 
10− ,[006694379990139 sin()θ ]
 
where
2
g local gravity value (m/s );
l
θ is the geographic latitude;
e is the elevation above sea level (m);
R is the nominal radius of the earth (6 378 137,0 m).
See References [6], [7], [8] and [10].
Gravity correction is accomplished using a ratiometric method in Formulae (2a) and (2b). For example,
in torque or pressure measure calibration, which relies upon reference dead weight, the following
relationship for correction applies:
m⋅g
l
m = (2a)
C
 g
s
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pg⋅
l
p = (2b)
C
 g
s
where
m and p are the corrected values for mass and pressure;
C C
m is the mass under standard conditions (kg);
p is the pressure (MPa);
2
g is the local gravity value (m/s );
I
2
g is the international standard gravity value (9,808665 m/s ).
s
Gravity correction applies to fluid elevation head instruments such as manometers. Gravity correction
is accomplished using the relationship in Formula (3):
hg⋅
tl
h = (3)
C,t
g
s
where
h is the corrected value for the height of the indicating fluid (cm, or m);
C,t
h is the height of the indicating fluid (cm, or m);
t
2
g is the local gravity value (m/s );
l
2
g is the international default value for gravity (9,806 65 m/s ).
s
5.1.7 If a testing agency is not equipped to perform either an intermediate or a complete calibration,
the instrument manufacturer or other agency may be contracted to perform these services. The testing
agency and its independent contractor are not exempted from any of the requirements set forth herein.
6 Complete calibration procedure
6.1 Selection of reference standard
Select a reference standard which:
a) is free of physical damage, or the damage was previously noted in the calibration records and is not
considered to affect its function;
b) is certified and traceable as per the requirement of 5.1;
c) has its total uncertainty evaluated and documented.
6.2 Procedure
6.2.1 Mount the reference standard in an attitude indicated in its calibration record or as recommended
by its manufacturer.
6.2.2 Select the measuring instrument to be calibrated.
6.2.3 Mount the measuring instrument in an attitude recommended by the manufacturer or in an
attitude expected in the measurement situation.
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6.2.4 Make zero value checks with the measuring instrument physically uncoupled from any possible
loading effects.
6.2.5 Couple the measuring instrument to the reference standard and begin calibration data collection.
6.2.6 For instruments which are subject to hysteresis effects (e.g. material characteristics or static
friction), conduct the calibration for both increasing and decreasing reference values. Evaluate the results
of the first calibration trial to assess hysteresis effects.
6.2.7 Correct for systematic standard uncertainty. Take advantage of any correction charts or
uncertainty models which resulted from calibration of the reference standard.
6.2.8 Correct the reference values for any other systematic standard uncertainties when the
relationships with other physical variables are known and the physical variables themselves are known
(measured) at the time of instrument calibration.
In circumstances where reading correction is undesirable or the reference instrument is subject to
uncontrolled variations, include the maximum expected value of the systematic standard uncertainty
in computing the total measurement system uncertainty as per Clause 10.
6.2.9 Record the reference value, after correction as per 6.2.7 and 6.2.8, and the corresponding
instrument indicated value for each calibration increment.
6.2.10 Develop an uncertainty model in accordance with Clause 7.
6.2.11 Sign and date the calibration sheets. Record all pertinent information concerning the reference
standard used for calibration, any physical damage observed to the instrument calibrated or unusual
characteristics, environmental conditions, and mounting attitude of the reference standard and
instrument. Place these records in a permanent file or in an instrument calibration database.
6.2.12 A label affixed to the instrument is recommended. The information may also be entered in an
instrument calibration database. Attach the label to the instrument's readout device in a manner which
will discourage its inadvertent removal and not interfere with readability. The label should contain the
following information:
a) date of last complete calibration;
b) instrument identification information;
c) identification of the person or agency responsible for calibration of the instrument.
7 Instrument calibration uncertainty models
7.1 General
This clause sets forth the procedures for deriving uncertainty models of a measuring instrument and,
when significant, for evaluating the effects of environmental factors. Based on the uncertainty model
selected, the measuring instrument calibration uncertainty can be determined.
7.2 General procedure
7.2.1 Select a suitable uncertainty model from either First order (7.3), Second order (7.5), or Third
order (7.7).
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NOTE The amount of calibration uncertainty in most instruments depends upon the model selected. Higher
order models yield smaller uncertainties.
7.2.2 Enter the calibration uncertainty on the instrument's calibration record or database.
7.3 First order uncertainty model
A first order uncertainty model makes direct use of the indicated value of the instrument readout
device without any corrections to the measured value. The model includes the measuring instrument,
interconnect cabling and the readout device as a measuring system.
7.4 First order calibration uncertainty evaluation
7.4.1 Use the calibration data as recorded in Clause 6.
7.4.2 Calculate the difference between the indicated value and the reference value of the fiv
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