ISO 23505:2025

International
Standard
ISO 23505
First edition
Petroleum and liquid petroleum
2025-08
products — Calibration of spherical
tanks — External electro-optical
distance-ranging method
Pétrole et produits pétroliers liquides — Jaugeage des réservoirs
sphériques — Méthode par mesurage électro-optique externe de
la distance
Reference number
© ISO 2025
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Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Precautions . 2
6 Equipment . 2
7 General considerations . 3
8 Determination of the number and location of stations . 3
9 Determination of the number and position of tangential target points . 4
10 Calibration procedure . 5
10.1 Instrument set-up .5
10.2 Establishment of the reference target point .5
10.3 Measurement procedure .5
10.4 Measurement of the height of the spherical tank on the vertical centre line .8
11 Other measurements. 9
12 Calculation and development of capacity tables . 9
12.1 General requirements .9
12.2 Calculation of the inner radius of the spherical tank .10
12.2.1 Spherical radius at tangential target point .10
12.2.2 Spherical radius obtained at the station .10
12.2.3 Outer radius of spherical tank.10
12.2.4 Average wall thickness of spherical tank .10
12.2.5 Inner radius of spherical tank .11
12.2.6 Inner radius of spherical tank at reference temperature .11
12.3 Calculation of the total internal height of the spherical tank on the vertical centre line .11
12.4 Inspection of calibration data . 12
12.5 Calculation of the total volume of the spherical tank at reference temperature and
working pressure . 12
12.6 Calculation of partial volume of spherical tank at reference temperature and working
pressure . 13
Annex A (informative) Calibration uncertainties . 14
Annex B (informative) Effect of thermal changes on spherical tank shells .23
Annex C (normative) Certificate of calibration .24
Bibliography .25

iii
Foreword
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iv
International Standard ISO 23505:2025(en)
Petroleum and liquid petroleum products — Calibration
of spherical tanks — External electro-optical distance-
ranging method
1 Scope
This document specifies a method for the calibration of spherical tanks with a radius greater than 1 m by
means of external measurements using an electro-optical distance-ranging instrument (EODR). It also
specifies the subsequent calculation and the compilation of tank capacity tables.
This document applies to spherical tanks built above ground, that are non-insulated, single-shell and with
a radial variation up to 1 %. The method specified in this document can also apply to spherical tanks built
above the deck when the carrying ship is in the dock.
This document does not apply to the calibration of spheroidal tanks.
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 4512, Petroleum and liquid petroleum products — Equipment for measurement of liquid levels in storage
tanks — Manual methods
ISO 7507-1:2003, Petroleum and liquid petroleum products — Calibration of vertical cylindrical tanks — Part 1:
Strapping method
ISO 7507-4:2010, Petroleum and liquid petroleum products — Calibration of vertical cylindrical tanks — Part 4:
Internal electro-optical distance-ranging method
ISO 7507-5, Petroleum and liquid petroleum products — Calibration of vertical cylindrical tanks — Part 5:
External electro-optical distance-ranging method
IEC 60825-1, Safety of laser products— Part 1: Equipment classification and requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 7507-1, ISO 7507-5 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
station
position from which measurements of angles and slope distance are made

3.2
tangential target point
spherical tangential position of the electro-optical distance-ranging instrument aimed along the spherical
circumference
3.3
ranging target point
tank wall ranging point from the electro-optical distance-ranging instrument to the spherical tank centre
direction
3.4
reference target point
fixed point clearly marked on the pillar or wall of the spherical tank and visible to the electro-optical
distance-ranging instrument
4 Principle
A minimum of three stations, each with a sufficiently large viewing sphere, shall be equally spaced around
the spherical tank and superimposed to cover a circle of the spherical tank. The EODR instrument is operated
on the station to measure the spherical tank. The direction of the centre of the sphere is determined by the
intersection of the vertical bisectors of the horizontal and vertical strings, and ranging in this direction
provides the distance from the instrument to the tank wall. The horizontal and vertical angles of at least
eight evenly spaced tangent points on the sphere are then measured.
Given that the centre of the instrument, the centre of the sphere and the point of tangency form a right-
angled triangle, and assuming that the two radii making up the triangle are equal, the radius of the spherical
tank at the point of tangency can be calculated from:
— the horizontal angle, vertical angle and slope distance in the direction of the centre of the sphere; and
— the horizontal and vertical angles in the direction of the tangent.
The average of the radii of the tangent points obtained at all stations is calculated as the average radius of
the spherical tank, and the tank capacity table is then developed accordingly.
5 Precautions
The general and safety precautions contained in ISO 7507-1:2003, Clause 4, shall apply to this document. In
addition, the laser emitted by the distance ranging unit shall conform to IEC 60825-1 for a Class 1 or Class 2 laser.
The EODR instrument and other electrical equipment shall have a level of electrical protection appropriate
for the area classification of the location of the tank to be calibrated (see IEC 60079-10-1).
6 Equipment
6.1 Electro-optical distance-ranging instrument, with angular and distance-measuring parts that
shall conform to ISO 7507-4:2010, 5.1, where the accuracy of the angle measurement part shall be equal to or
−5
greater than 1,572 × 10 rad (1 mgon, or 3 '), and the accuracy of the distance measurement part (without
prism) shall be equal to or greater than 2 mm.
6.2 Tripod and its leg brace.
6.3 Dip-tape and dip-weight that shall conform to ISO 4512.
6.4 Single-corner cubic prisms, (for use as target prisms) mounted on a prism holder.
NOTE The prism holder is expected to be mounted on a ranging pole or a tribrach mounted on a tripod.

6.5 Ranging pole.
6.6 Ancillary equipment, which shall include:
a) a paint thickness meter; and
b) a plate thickness meter.
7 General considerations
7.1 The EODR instrument should be maintained so that its performance can be traced to national or
international measurement standards, or both. The accuracy of the distance-measuring unit as well as
the angle-measuring unit shall be verified using the procedures recommended by the manufacturer or as
specified in ISO 7507-4:2010, Annex A.
NOTE The distance-measuring unit is expected to hold its calibration. It is not normally necessary to verify it in
the field.
7.2 Calibration shall be carried out after the tank pressure test and before the tank is insulated.
7.3 The influence of changes in tank pressure on calibration should be within an acceptable range.
7.4 Calibration shall be carried out without interruption.
8 Determination of the number and location of stations
8.1 The stations are normally placed on the ground. The number and location of the stations should
ensure that the spheres observed by the stations are evenly interwoven and cover the circumference of the
spherical tank, and that the spheres observed are of similar size and as large as possible (see Figure 1).
8.2 The minimum number of stations shall be not less than 3. If the circumference of the spherical tank is
large and the space around it is limited, the number of stations may be increased accordingly.
8.3 The stations shall be equally spaced around the spherical tank, and the shortest vertical distance
from the stations to the wall of the tank shall be the longer one between 5 m and 2 times the radius of
the spherical tank, but shall not exceed 40 m. Adjust the location of the station properly to ensure that the
ranging direction is not interrupted, and the deviation distance between the ranging target point and the
weld and other obstacles on the spherical tank is more than 300 mm.

Key
1 spherical tank
2 pillar
3 tangential direction
4 vertical centre line direction
5 station
Figure 1 — Station positioning
9 Determination of the number and position of tangential target points
9.1 At each station, after the orientation of the centre of the sphere and the measurement of distance and
angle have been completed, the tangential direction of the surface of the sphere should be aimed at and the
horizontal angle and vertical angle of the tangential target point measured.
9.2 The tangential target points should be a minimum of 8 points, evenly distributed around the
circumference of the apparent sphere (see Figure 2). These points should in principle be at 0°, 45°, 90°, 135°,
180°, 225°, 270° and 315° of the apparent circumference.
9.3 The sighting lines to the tangential target points shall not be interrupted or fall on welds and other
obstacles on the sphere and the tangential target points shall deviate from them by more than 300 mm. If
the tangential target point is adjusted for this purpose, the same adjustment shall be made for the tangential
target point symmetrical to the centre of the apparent spherical circle.

9.4 If the radius of the spherical tank exceeds 8 m, the tangential target point can be increased to 16 points,
which is equivalent to increasing one point in the middle of each adjacent two points of the original 8 points.
Key
1 surface of spherical tank in EODR instrument field of view
T1, T2, T3 etc. tangential target points
NOTE T1 and T5, T2 and T6, T3 and T7, and T4 and T8 can only be adjusted in the same direction and in the
same amount.
Figure 2 — Tangential target point positioning on the visible sphere
10 Calibration procedure
10.1 Instrument set-up
10.1.1 The instrument and its supporting tripod are stably set up in the station position (see Clause 8). The
three legs of the tripod should be inserted into the ground or fixed with a leg brace.
10.1.2 The instrument shall be free from external vibration.
10.1.3 The sighting lines from the instrument to the tank shell wall shall not be obstructed.
10.1.4 At least the minimum settling time recommended by the manufacturer should be allowed before the
instrument is used.
10.1.5 The instrument shall be set horizontal, thus ensuring that the vertical axis (standing axis) is vertical.
10.2 Establishment of the reference target point
A fixed point is clearly marked on the pillar or wall of the spherical tank as the reference target point. This
point should be visible to the instrument and as close as possible to the height of the instrument.
10.3 Measurement procedure
10.3.1 All measurements should be carried out without interruption and as quickly as possible.

10.3.2 Measure and record the slope distance, horizontal angle and vertical angle to the reference target points.
10.3.3 To determine the direction of the centre of the sphere, proceed as follows (see Figure 3).
a) Aim tangentially at the left sphere as close as possible to the transverse diameter, measure and record
the horizontal angle (A ). Keeping the vertical angle constant, aim tangentially at the right sphere,
h,l
measure and record the horizontal angle (A ). Calculate the average of the two horizontal angles as the
h,r
horizontal angle (A ) towards the centre of the sphere.
h,c
b) Aim tangentially at the upper sphere as close as possible to the vertical diameter, measure and record
the vertical angle (A ). Keeping the horizontal angle constant, aim tangentially at the lower sphere,
v,u
measure and record the vertical angle (A ). Calculate the average of the two vertical angles as the
v,l
vertical angle (A ) towards the centre of the sphere.
v,c
Key
1 surface of sphere
2 right tangency of horizontal string
3 lower tangency of vertical string
4 vertical bisector of horizontal string
5 direction of sphere centre
6 vertical bisector of vertical string
7 left tangency of horizontal string
8 upper tangency of vertical string
Figure 3 — Positioning in the direction of the centre of the visible sphere
10.3.4 Align towards the centre of the sphere (set the horizontal and vertical angles of the instrument to
the horizontal and vertical angles towards the centre of the sphere determined in 10.3.3) and measure the
distance, as illustrated in Figure 4. Record the measured horizontal angle, vertical angle and slope distance
as the horizontal angle (A ), vertical angle (A ) and distance (D) in the spherical centre direction, where D
h,c v,c
is the distance to the tank wall (ranging target point).
10.3.5 Aim at tangential target points on the circumference of the sphere (see Clause 9) and measure and
record horizontal (A ) and vertical (A ) angles until all these points on the sphere have been measured, as
h,t v,t
illustrated in Figure 4.
10.3.6 When the measurement of all tangential target points in this station is completed, the measurement
of the reference target point shall be repeated.
Key
1 spherical tank
2 ranging target points (C)
3 vertical angle zero direction
4 station and the EODR instrument installed on it
5 horizontal angle zero direction
6 tangential target point (T)
7 centre of sphere (O)
A horizontal angle
h
A horizontal angle towards the centre of the sphere
h,c
A horizontal angle of the tangential target point
h,t
A vertical angle
v
A vertical angle towards the centre of the sphere
v,c
A vertical angle of the tangential target point
v,t
θ angle between tangential target point direction and spherical centre direction
D distance from the EODR instrument to the tank wall in the direction of the centre of the sphere
r radius of spherical tank at tangential target point
Figure 4 — Principle diagram of radius measurement of spherical tank at tangential target point

10.3.7 If the repeated slope distances to the reference target points do not agree with the measurements
taken in 10.3.2, within 2 mm, repeat the procedure given in 10.3.1 to 10.3.6. Record the difference of the two
measurements as uncorrected instrument drift, e(R ). Add the corresponding standard uncertainty, u(R ),
dr dr
calculated using Formula (1), to the resulting uncertainty of the tank radius (see Annex A).
eR()
dr
uR()= (1)
dr
10.3.8 If the horizontal or vertical angles to the reference target points do not agree with the measurements
−4
taken in 10.3.2, to within 1,571 × 10 rad (0,01 gon), repeat the procedure given in 10.3.1 to 10.3.6. Record
the difference of the two measurements as uncorrected instrument drift, e(A ) (horizontal angles)
h,dr
and e(A ) (vertical angles). Add the corresponding standard uncertainty, u(A ), calculated by using
v,dr h,dr
Formula (2) for horizontal angles, or Formula (3) for vertical angles, to the resulting uncertainty of the
angular measurements (see Annex A).
eA()
hd, r
uA = (2)
()
hd, r
eA
()
vd, r
uA = (3)
()
vd, r
10.3.9 Measurements of spherical tanks at other stations (see Clause 8) shall be made in accordance with
10.1 to 10.3.
10.4 Measurement of the height of the spherical tank on the vertical centre line
10.4.1 Where possible, the height, H, of the spherical tank on the vertical centre line should be measured
directly. When there are maintenance holes or other accessories on the centre line, the vertical height, H ,
d
can be measured at a convenient distance, D , from the centre line, and then converted to the height on the
d
vertical centre line.
10.4.2 The height of a spherical tank at or off the centreline can be measured directly with a dip-tape and
dip-weight or indirectly with an EODR instrument at a convenient measuring station as follows:
a) adjust the horizontal angle of the instrument to the direction of the centre of the sphere and keep it
unchanged, that is, aligned with the vertical centre line;
b) align the ranging pole with the line of sight of the instrument, and stand its base at the measuring
position on the top of the spherical tank, and then measure and record the horizontal angle, vertical
angle and slope distance, or, if possible, horizontal distance and elevation of the target prism on the
ranging pole;
NOTE Instruments usually automatically convert and display horizontal distances and elevations.
c) align the ranging pole with the line of sight of the instrument and place its base upward against the
bottom of the spherical tank below the measuring point on the top of the tank. Move the ranging pole
and measure the horizontal distance to the target prism on it until it is consistent with the measured
horizontal distance at the top. Measure and record the horizontal angle, vertical angle and slope
distance, or, if possible, the elevation of the target prism on the ranging pole;
d) if it is necessary to alter the height of the ranging-pole for any reason (i.e. obstruction of the line of
sight), record the alteration in height.
Any alteration in the height will require a compensating correction to be applied during the calculation of
the height of the spherical tank.

10.4.3 When the measuring point of the height of the spherical tank deviates from the vertical centre line,
the deviation distance should be measured and recorded.
10.4.4 When the height of the spherical tank is measured externally, the thickness of the upper and
lower tank walls and the height of the accessories involved in the external height should also be measured
separately.
10.4.5 All data involved in the measurement of the height of the spherical tank shall be measured and
recorded to the nearest millimetre.
11 Other measurements
11.1 The following data shall be determined and processed in accordance with ISO 7507-1:
a) tank shell and atmospheric temperature;
b) the height of each course;
c) the plate thickness of each course;
d) deadwood.
11.2 Determine and record the working pressure of the spherical tank.
11.3 Measure and record the length of the ranging pole, to the nearest 1 mm.
12 Calculation and development of capacity tables
12.1 General requirements
12.1.1 All calculations shall be made in accordance with accepted mathematical principles.
NOTE Calibration uncertainties can be computed as described in Annex A and a statement of these uncertainties
can be added to the computed calibration table.
12.1.2 The calculation methods given below give minimum requirements for precision, but it is permitted
to use alternative procedures which produce a final tank capacity table of similar or greater precision. The
values involved in the calculation are rounded as follows:
−7
a) angle rounded to 1 × 10 rad (0,01 mgon);
b) radius rounded to 0,000 1 m;
c) board thickness rounded to 0,000 5 m;
d) temperature rounded to 1 °C;
e) height rounded to 0,001 m;
f) volume rounded to 0,001 m .
12.1.3 The reference temperature and the working pressure for which the tank capacity table has been
calculated shall be recorded at the head of the table. To enable any necessary corrections to be made, the
temperature of the tank shell should be calculated as described in Annex B.
12.1.4 The expansion of a spherical tank caused by liquid head is small and can be ignored in the preparation
of tank capacity tables. The expansion due to internal gas pressure may be significant and shall generally be

considered. The pressure-related volum
...