ISO 1928:2020

Coal and coke - Determination of gross calorific value

This document specifies a method for the determination of the gross calorific value of a solid mineral fuel at constant volume and at the reference temperature of 25 °C in a combustion vessel calorimeter calibrated by combustion of certified benzoic acid. The result obtained is the gross calorific value of the analysis sample at constant volume with all the water of the combustion products as liquid water. In practice, fuel is burned at constant (atmospheric) pressure and the water is not condensed but is removed as vapour with the flue gases. Under these conditions, the operative heat of combustion is the net calorific value of the fuel at constant pressure. The net calorific value at constant volume can also be used; formulae are given for calculating both values. General principles and procedures for the calibrations and the fuel tests are specified in the main text, whereas those pertaining to the use of a particular type of calorimetric instrument are described in Annexes A to C. Annex D contains checklists for performing calibration and fuel tests using specified types of calorimeters. Annex E gives examples illustrating some of the calculations. Annex F provides guidance around safe use, maintenance and testing of the calorimeter combustion vessel. NOTE Descriptors: solid fuels, coal, coke, tests, determination, calorific value, rules of calculation, calorimetry.

Charbon et coke — Détermination du pouvoir calorifique supérieur

General Information

• Status: Published
• Publication Date: 04-Oct-2020
• ICS: 75.160.10 - Solid fuels
• Technical Committee: ISO/TC 27/SC 5 - Methods of analysis
• Drafting Committee: ISO/TC 27/SC 5/WG 12 - Miscellaneous methods
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 13-Aug-2025
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 1928:2025 - Coal and coke - Determination of gross calorific value
• Effective Date: 27-May-2023
• Revises: ISO 1928:2009 - Solid mineral fuels - Determination of gross calorific value by the bomb calorimetric method and calculation of net calorific value
• Effective Date: 26-May-2018

Overview

ISO 1928:2020 - "Coal and coke - Determination of gross calorific value" specifies a laboratory method for measuring the gross calorific value (higher heating value) of solid mineral fuels using a combustion-vessel calorimeter. The procedure determines the gross calorific value at constant volume and at the reference temperature of 25 °C, with the calorimeter calibrated by combustion of certified benzoic acid. The standard also provides formulae to calculate net calorific values (operative heat of combustion) and guidance for converting results to different bases.

Key technical topics and requirements

• Scope and principle: Measurement of gross calorific value of coal and coke in a calibrated combustion vessel at constant volume (25 °C reference).
• Calibration: Use of certified benzoic acid as the calibrant; determination of the calorimeter’s effective heat capacity and valid working range.
• Apparatus and reagents: Requirements for calorimeters, thermometers and reagents used in the determination.
• Sample preparation: Procedures for preparing representative analysis samples of solid fuels.
• Measurement procedure: Instructions for assembling the calorimeter, performing combustions, recording temperature rises, and analysing combustion products.
• Calculations: Formulae for gross calorific value at constant volume and for converting to net calorific value at constant pressure or volume.
• Precision and reporting: Repeatability and reproducibility considerations and mandatory test report content.
• Instrument-specific guidance: Annexes A–C describe adiabatic, isoperibol/static-jacket, and automated calorimeters; Annex D offers checklists; Annex E gives worked calculation examples; Annex F provides safety, maintenance and testing guidance for combustion vessels.

Practical applications

• Quality control and specification of coal and coke used in power generation, metallurgy and industrial heating.
• Contract verification of energy content for fuel procurement and trade.
• Fuel characterization for process optimization in power plants, coking plants, and industrial boilers.
• Laboratory accreditation and regulatory compliance where standardized calorific-value measurement is required.
• Research and development on solid fuel performance, combustion efficiency, and emissions modelling.

Who should use ISO 1928:2020

• Analytical laboratories performing calorific-value testing.
• Coal and coke producers and traders verifying product energy content.
• Power utilities, steel and cement plants assessing fuel quality.
• QA/QC engineers, procurement specialists, and regulatory agencies requiring standardized test methods.

Related standards

ISO 1928:2020 references other ISO standards for thermometers, sampling and moisture determination (e.g., ISO 651, ISO 13909‑4, ISO 17247). Users should consult these related documents for complementary requirements in sampling, moisture analysis and thermometer specifications.

Keywords: ISO 1928:2020, gross calorific value, coal, coke, calorimetry, combustion vessel calorimeter, benzoic acid calibration, net calorific value, solid mineral fuels, calorimeter calibration.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 1928
Fourth edition
2020-10
Coal and coke — Determination of
gross calorific value
Charbon et coke — Détermination du pouvoir calorifique supérieur
Reference number
©
ISO 2020
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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CH-1214 Vernier, Geneva
Phone: [phone]
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Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 2
3.1 Terms and definitions . 2
3.2 Symbols . 3
4 Principle . 6
4.1 Gross calorific value . 6
4.2 Net calorific value . 7
5 Reagents . 7
6 Apparatus . 9
7 Preparation of test sample .12
8 Calorimetric procedure .13
8.1 General .13
8.2 Preparing the combustion vessel for measurement .15
8.2.1 General procedure .15
8.2.2 Using a combustion aid .15
8.3 Assembling the calorimeter .15
8.4 Combustion reaction and temperature measurements .16
8.5 Analysis of products of combustion .16
8.6 Corrected temperature rise .17
8.6.1 Observed temperature rise, t − t .17
f i
8.6.2 Isoperibol and static-jacket calorimeters .17
8.6.3 Adiabatic calorimeters .18
8.6.4 Thermometer corrections .19
8.7 Reference temperature .19
9 Calibration .19
9.1 Principle .19
9.2 Calibrant .19
9.2.1 Certification conditions.19
9.2.2 Calibration conditions .19
9.3 Valid working range of the effective heat capacity .20
9.4 Ancillary contributions .20
9.5 Calibration procedure .21
9.6 Calculation of effective heat capacity for the individual test .21
9.6.1 Constant mass-of-calorimeter-water basis .21
9.6.2 Constant total-calorimeter-mass basis .22
9.7 Precision of the mean value of the effective heat capacity .23
9.7.1 Constant value of ε .23
9.7.2 ε as a function of the observed temperature rise .23
9.8 Redetermination of the effective heat capacity .23
10 Gross calorific value .24
10.1 General .24
10.2 Coal combustions .24
10.3 Coke combustions .24
10.4 Calculation of gross calorific value .25
10.4.1 General.25
10.4.2 Constant mass-of-calorimeter-water basis .25
10.4.3 Constant total-calorimeter-mass basis .26
10.4.4 ε as a function of the observed temperature rise .26
10.5 Expression of results .27
10.6 Calculation to other bases .27
11 Precision .28
11.1 Repeatability limit .28
11.2 Reproducibility limit .28
12 Calculation of net calorific value.28
12.1 General .28
12.2 Calculations .28
12.2.1 Calculation of net calorific value at constant pressure .28
12.2.2 Calculation of net calorific value at constant volume .30
13 Test report .31
Annex A (informative) Adiabatic calorimeters .32
Annex B (informative) Isoperibol and static-jacket calorimeters .36
Annex C (informative) Automated calorimeters .42
Annex D (informative) Checklists for the design of combustion tests and their procedures .45
Annex E (informative) Examples to illustrate some of the calculations used in this document .50
Annex F (informative) Safe use, maintenance and testing of calorimeter combustion vessels .56
Bibliography .62
iv © ISO 2020 – All rights reserved

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. 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. 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 on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO's adherence to the WTO principles in the Technical
Barriers to Trade (TBT) see the following URL: Foreword — Supplementary information
This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 5,
Methods of analysis.
This fourth edition cancels and replaces the third edition (ISO 1928:2009), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— change the document title within the scope of TC 27,
— editorially update symbols within formulae,
— update references,
— expand on some derivations,
— remove ambiguity around crucible masses, and
— specify the analysis sample.
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.
INTERNATIONAL STANDARD ISO 1928:2020(E)
Coal and coke — Determination of gross calorific value
WARNING — Strict adherence to all of the provisions specified in this document should ensure
against explosive rupture of the combustion vessel, or a blow-out, provided that the combustion
vessel is of proper design and construction and in good mechanical condition.
1 Scope
This document specifies a method for the determination of the gross calorific value of a solid mineral
fuel at constant volume and at the reference temperature of 25 °C in a combustion vessel calorimeter
calibrated by combustion of certified benzoic acid.
The result obtained is the gross calorific value of the analysis sample at constant volume with all the
water of the combustion products as liquid water. In practice, fuel is burned at constant (atmospheric)
pressure and the water is not condensed but is removed as vapour with the flue gases. Under these
conditions, the operative heat of combustion is the net calorific value of the fuel at constant pressure. The
net calorific value at constant volume can also be used; formulae are given for calculating both values.
General principles and procedures for the calibrations and the fuel tests are specified in the main text,
whereas those pertaining to the use of a particular type of calorimetric instrument are described in
Annexes A to C. Annex D contains checklists for performing calibration and fuel tests using specified
types of calorimeters. Annex E gives examples illustrating some of the calculations. Annex F provides
guidance around safe use, maintenance and testing of the calorimeter combustion vessel.
NOTE Descriptors: solid fuels, coal, coke, tests, determination, calorific value, rules of calculation,
calorimetry.
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 651, Solid-stem calorimeter thermometers
ISO 652, Enclosed-scale calorimeter thermometers
ISO 687, Solid mineral fuels — Coke — Determination of moisture in the general analysis test sample
ISO 1770, Solid-stem general purpose thermometers
ISO 1771, Enclosed-scale general purpose thermometers
ISO 5068-2, Brown coals and lignites — Determination of moisture content — Part 2: Indirect gravimetric
method for moisture in the analysis sample
ISO 11722, Solid mineral fuels — Hard coal — Determination of moisture in the general analysis test
sample by drying in nitrogen
ISO 13909-4, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples
ISO 17247, Coal and coke — Ultimate analysis
ISO 18283, Hard coal and coke — Manual sampling
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions 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/
3.1.1
gross calorific value at constant volume
absolute value of the specific energy of combustion for unit mass of a solid fuel burned in oxygen in a
calorimetric combustion vessel under the conditions specified
Note 1 to entry: The products of combustion are assumed to consist of gaseous oxygen, nitrogen, carbon dioxide
and sulfur dioxide, of liquid water (in equilibrium with its vapour) saturated with carbon dioxide under the
conditions of the combustion vessel reaction, and of solid ash, all at the reference temperature.
Note 2 to entry: Gross calorific value is expressed in units of joules/gram.
3.1.2
gross calorific value at constant pressure
absolute value of the specific energy of combustion, for unit mass of a solid fuel burned in oxygen at
constant pressure, instead of constant volume in a calorimetric combustion vessel
Note 1 to entry: The hydrogen in the fuel, reacting with gaseous oxygen to give liquid water, causes a decrease in
the volume of the system. When the fuel carbon reacts with gaseous oxygen, an equal volume of gaseous carbon
dioxide is formed and, hence, no change in volume occurs in combustion of the carbon. The oxygen and nitrogen
in the fuel both give rise to an increase in volume.
3.1.3
net calorific value at constant volume
absolute value of the specific energy of combustion, for unit mass of a solid fuel burned in oxygen under
conditions of constant volume and such that all the water of the reaction products remains as water
vapour (in a hypothetical state at 0,1 MPa), the other products being as for the gross calorific value, all
at the reference temperature (3.1.8)
3.1.4
net calorific value at constant pressure
absolute value of the specific heat (enthalpy) of combustion, for unit mass of the fuel burned in oxygen
at constant pressure under such conditions that all the water of the reaction products remains as
water vapour (at 0,1 MPa), the other products being as for the gross calorific value, all at the reference
temperature (3.1.8)
3.1.5
adiabatic calorimeter
calorimeter that has a rapidly changing jacket temperature
Note 1 to entry: The inner calorimeter chamber and the jacket exchange no energy because the water temperature
in both is identical during the test. The water in the external jacket is heated or cooled to match the temperature
change in the calorimeter proper.
3.1.6
isoperibol calorimeter
calorimeter that has a jacket of uniform and constant temperature
Note 1 to entry: These calorimeters have the inner chamber surrounded by a water jacket in which the
temperature is maintained at ambient temperature. The outer jacket acts like a thermostat and the thermal
conductivity of the interspace between the two chambers is kept as small as possible.
2 © ISO 2020 – All rights reserved

3.1.7
aneroid calorimeter
calorimeter system without fluid, where the calorimeter can, stirrer and water are replaced by a metal
block and the combustion vessel itself constitutes the calorimeter
Note 1 to entry: Characteristically, these calorimeters have a small heat capacity, leading to large changes in
temperature. Therefore, smaller masses of sample are used. A calorimeter of this kind requires more frequent
calibrations.
3.1.8
reference temperature
international reference temperature for thermochemistry, 25 °C
Note 1 to entry: See 8.7.
Note 2 to entry: The temperature dependence of the calorific value of coal or coke is small, about 1 J/(g·K).
3.1.9
effective heat capacity of the calorimeter
amount of energy required to cause unit change in temperature of the calorimeter
3.1.10
corrected temperature rise
change in calorimeter temperature caused solely by the processes taking place within the
combustion vessel
Note 1 to entry: The change in temperature can be expressed in terms of other units: resistance of a platinum or
thermistor thermometer, frequency of a quartz crystal resonator, etc., provided that a functional relationship is
established between this quantity and a change in temperature. The effective heat capacity of the calorimeter
can be expressed in units of energy per such an arbitrary unit. Criteria for the required linearity and closeness in
conditions between calibrations and fuel tests are given in 9.3.
3.2 Symbols
c specific heat capacity of water at constant pressure
p,aq
c specific heat capacity of the sample
p,s
c dt heat capacity times the temperature change
p
c specific heat capacity of the crucible
p,cr
ΔC is the difference in heat capacity (m × c ) of the crucible used in the calibrations and
cr p,cr
that used in combustion of the fuel
dq heat flow into the calorimeter
dT
(dt/dτ) the initial drift rate
i
G specific rate constant, which is evaluated from the time-temperature measurements of the
rating periods, the fore- and the after-period
g drift rate (dt/dτ) in the rating periods
g final drift rate (drift rate in the after-period)
f
g initial drift rate (drift rate in the fore-period)
i
k is the Newton’s law cooling constant
l length of ignition wire (fuse)
wire
L is the latent heat of vaporization of water at 25 °C and constant pressure (43 988 J/mol)
L is the latent heat of vaporization at 25 °C and constant pressure of the water present in the
s
analysis sample and formed from the hydrogen in it
M moisture in the analysis sample
m mass of combustion vessel water
aq
M total moisture mass fraction of the fuel for which the calculation is required
T
m mass of benzoic acid
ba
m mass of crucible
cr
m mass of sample
s
m mass of wire (fuse)
fuse
m mass of fuel sample burned
m mass of combustion aid
p initial pressure of oxygen
O
P power of stirring
st
Q contribution from combustion of the fuse
fuse
Q contribution from oxidation of the ignition wire
ign
Q contribution from formation of nitric acid (from liquid water and gaseous nitrogen and
N
oxygen)
Q correction for taking the sulfur from the aqueous sulfuric acid in the combustion vessel to
S
gaseous sulfur dioxide
q gross calorific value at constant pressure of the dry (moisture-free) fuel
p,gr,d
q net calorific value at constant pressure for air-dried fuel with moisture mass fraction
p,net,M
q net calorific value at constant pressure of the dry (moisture-free) fuel
p,net,d
q net calorific value at constant pressure of the fuel with moisture mass fraction M
T
p,net,M
T
q certified gross calorific value at constant volume for benzoic acid
V,ba
q gross calorific value at constant volume of the fuel as analysed
V,gr
q gross calorific value at constant volume of the dry (moisture-free) fuel
V,gr,d
q gross calorific value at constant volume of the fuel with moisture mass fraction M
V,gr,m T
q net calorific value at constant volume
V,net
q net calorific value at constant volume of the dry (moisture-free) fuel
V,net,d
q net calorific value at constant volume for air-dried fuel with moisture mass fraction
V,net,M
4 © ISO 2020 – All rights reserved

q net calorific value at constant volume of the fuel with moisture mass fraction M
T
V,net,M
T
q gross calorific value at constant volume of a combustion aid
V,2
R the universal gas constant, equal to 8,315 J/mol K
T the reference temperature for calorific value, i.e. 298,15 K (25 °C)
Δn contraction in volume of the gaseous phase for the combustion reaction, expressed in
g
terms of moles per gram of sample, on an air-dried basis
t calorimeter temperature
t the correction of 1 applied after the ignition of the sample
Δm heat capacity from the mass of the crucible
cr
Δt heat-leak correction, which is the contribution from the heat exchange
ex
t final temperature of the main period (equal to the reference temperature)
f
τ time, a minutes after the end of the main period
a
t temperature, a minutes after the end of the main period
f+τ
a
t − t observed temperature rise
f i
Δt observed temperature rise
t initial temperature of the main period (at the time of firing the charge)
i
t thermostat (jacket) temperature
j
t − t thermal head
j
t successive temperature readings, taken at 1 min intervals during the main period
k
t the integrated mean temperature
m
t is equal to t and is the temperature at the beginning of the main period
0 i
t is the temperature reading, taken during the main period, at the nth one-minute interval,
n
t (= t ) being the reading taken at the end
n f
t mean temperature in the after-period
mf
t mean temperature in the fore-period
mi
t temperature at the time τ ,
x x
t is the temperature that the calorimeter eventually attains if left running for an extended
∞
period of time, which is the asymptotic temperature of an isoperibol calorimeter
(at “infinite” time)
t reference temperature
ref
V is the volume of the barium hydroxide solution used
V is the volume, of the hydrochloric acid solution used
V volume of combustion vessel water, may be substituted, as appropriate, for m
aq, aq
V Combustion vessel volume
cv
W The work done against the atmosphere when the water is expanded at constant pressure to
vapour at 25 °C
W Δn multiplied by RT to interpret the volume change in terms of the associated work done
2 g
by the atmosphere to maintain constant pressure
w hydrogen mass fraction of the sample less the hydrogen contained in the moisture mass
H
fraction
w hydrogen, mass fraction of the moisture-free fuel (includes the hydrogen from the water of
H,d
hydration of the mineral matter as well as hydrogen in the coal substance)
w nitrogen, mass fraction of the moisture-free fuel
N,d
w oxygen, mass fraction of the moisture-free fuel
O,d
w the volatile-matter mass fraction of the sample with moisture mass fraction, M
V T
w the ash mass fraction of the sample with moisture mass fraction, M
A T
ε effective heat capacity of the calorimeter
ˆ
ε best estimate (corresponds to “mean” value) of ε from linear regression of ε as a function of
the observed temperature rise (t − t )
f i
ε effective heat capacity of calorimeter on a “total-calorimeter-mass” basis
*
ε mean effective heat capacity of the calorimeter based on n determinations of ε
n
ε effective heat capacity of hypothetical calorimeter with no crucible in the combustion vessel
O
ε mean effective heat capacity of the calorimeter based on n determinations of ε
O,n O
θ corrected temperature rise
τ time
Δτ length of the main period
τ time at the end of the main period
f
τ time at the beginning of the main period
i
τ Dickinson extrapolation time
x
4 Principle
4.1 Gross calorific value
A weighed portion of the general analysis sample of the solid fuel is burned in high-pressure oxygen in a
combustion vessel calorimeter under specified conditions. The effective heat capacity of the calorimeter
is determined in calibration tests by combustion of certified benzoic acid under similar conditions,
accounted for in the certificate. The corrected temperature rise is established from observations of
temperature before, during, and after the combustion reaction takes place. The duration and frequency
of the temperature observations depend on the type of calorimeter used. Water is added to the
combustion vessel initially to give a saturated vapour phase prior to combustion, thereby allowing all
the water formed from the hydrogen and moisture in the sample to be regarded as liquid water.
6 © ISO 2020 – All rights reserved

The gross calorific value is calculated from the corrected temperature rise and the effective heat
capacity of the calorimeter, with allowances made for contributions from ignition energy, combustion of
the fuse(s) and for thermal effects from side reactions such as the formation of nitric acid. Furthermore,
a correction is applied to account for the difference in energy between the aqueous sulfuric acid formed
in the combustion vessel reaction and gaseous sulfur dioxide, i.e. the required reaction product of
sulfur in the fuel.
4.2 Net calorific value
The net calorific value at constant volume and the net calorific value at constant pressure of the fuel are
obtained by calculation from the gross calorific value at constant volume determined on the analysis
sample. The calculation of the net calorific value at constant volume requires information about the
moisture and hydrogen mass fractions of the analysis sample. In principle, the calculation of the net
calorific value at constant pressure also requires information about the oxygen and nitrogen mass
fractions of the sample.
5 Reagents
5.1 Oxygen, at a pressure high enough to fill the combustion vessel to 3 MPa, pure, with an assay of at
least 99,5 % volume fraction, and free from combustible matter.
NOTE Oxygen made by the electrolytic process can contain up to 4 % volume fraction of hydrogen.
5.2 Fuse.
5.2.1 Ignition wire, of nickel-chromium 0,16 mm to 0,20 mm in diameter, platinum 0,05 mm to
0,10 mm in diameter, or another suitable conducting wire with well characterized thermal behaviour
during combustion.
5.2.2 Cotton fuse, of white cellulose cotton, or equivalent, if required; see 8.2.1, fourth paragraph.
5.3 Crucible lining material, for use in aiding total combustion of coke, anthracite, high ash coal and
other less reactive fuels.
5.3.1 Paste, of fused aluminosilicate cement passing a 63 µm test sieve and suitable for use up to a
temperature of 1 400 °C, mixed with water.
5.3.2 Aluminium oxide, fused, of analytical reagent quality, passing a 180 µm test sieve and retained
on a 106 µm test sieve.
5.3.3 Silica fibre disk, an ash-free, silica-fibre.
5.4 Standard volumetric solutions and indicators, only for use when analysis of final combustion
vessel solutions is required.
5.4.1 Barium hydroxide solution, c[Ba(OH) ] = 0,05 mol/l, prepared by dissolving 18 g of barium
hydroxide, Ba(OH) ·8H O, in about 1 l of hot water in a large flask.
2 2
Stopper the flask and allow the solution to stand for two days or until all the barium carbonate has
completely settled out. Decant or siphon off the clear solution through a fine-grained (slow flowrate)
filter paper into a storage bottle fitted with a soda-lime guard tube to prevent ingress of carbon dioxide.
Standardize the solution against 0,1 mol/l hydrochloric acid solution (5.4.4) using phenolphthalein
solution (5.4.6) as an indicator.
5.4.2 Sodium carbonate solution, c(Na CO ) = 0,05 mol/l, prepared by dissolving 5,3 g of anhydrous
2 3
sodium carbonate, Na CO , dried for 30 min at 260 °C to 270 °C, but not exceeding 270 °C, in water.
2 3
Transfer the resulting solution quantitatively to a 1 l volumetric flask and make up to volume with water.
5.4.3 Sodium hydroxide solution, c(NaOH) = 0,1 mol/l, prepared from a standard concentrated
volumetric solution as directed by the manufacturer.
Alternatively, prepare from anhydrous sodium hydroxide by dissolving 4,0 g of sodium hydroxide, NaOH,
in water; transfer the resulting solution to a 1 l volumetric flask and make up to volume with water.
Standardize the resulting solution against 0,1 mol/l hydrochloric acid solution (5.4.4) using
phenolphthalein solution (5.4.6) as an indicator.
5.4.4 Hydrochloric acid solution, c(HCl) = 0,1 mol/l, prepared from a standard concentrated
volumetric solution, as directed by the manufacturer.
Alternatively, prepare by diluting 9 ml of hydrochloric acid (ρ = 1,18 g/ml) to 1 l with water. Standardize
the resulting solution against anhydrous sodium carbonate or against sodium carbonate solution
(5.4.2) using a screened indicator solution (5.4.5).
5.4.5 Methyl orange indicator, screened, 1 g/l solution.
Dissolve 0,25 g of methyl orange and 0,15 g of xylene cyanole FF in 50 ml of 95 % volume fraction
ethanol and dilute to 250 ml with water.
5.4.6 Phenolphthalein, 10 g/l solution.
Dissolve 2,5 g of phenolphthalein in 250 ml of 95 % volume fraction ethanol or 2,5 g of the water-soluble
salt of phenolphthalein in 250 ml of water.
5.4.7 Water, deionised, distilled or water of equivalent purity, with a specific conductivity not higher
than 0,2 mS/m at 25 °C.
5.5 Benzoic acid, of calorimetric-standard quality, certified by a recognized standardizing authority
(or with unambiguously traceable certification).
Benzoic acid is the sole substance recommended for calibration of an oxygen-combustion vessel
calorimeter. For the purpose of checking the overall reliability of the calorimetric measurements,
test substances, e.g. n-dodecane, are used. Test substances are used mainly to prove that certain
characteristics of a sample, e.g. burning rate or chemical composition, do not introduce bias in the
results. A test substance should have a certified purity and a well-established energy of combustion.
The benzoic acid is burned in the form of pellets. The benzoic acid is normally used without drying or
any treatment other than pelletizing; consult the sample certificate. The benzoic acid does not absorb
moisture from the atmosphere at a relative humidity below 90 %, but it is recommended that the
benzoic acid be stored in a moisture-free environment (desiccator) until use.
The benzoic acid shall be used as close to certification conditions as is feasible; significant departures
from these conditions shall be accounted for in accordance with the directions in the certificate. The
energy of combustion of the benzoic acid, as defined by the certificate for the conditions utilized, shall
be adopted in calculating the effective heat capacity of the calorimeter; see 9.2.
8 © ISO 2020 – All rights reserved

6 Apparatus
6.1 General
The calorimeter (see Figure 1), consists of the assembled combustion vessel, the calorimeter can
(with or without a lid), the calorimeter stirrer, water, temperature sensor and leads with connectors
inside the calorimeter can required for ignition of the sample or as part of temperature measurement
or control circuits. During measurements, the calorimeter is enclosed in a thermostat. The manner in
which the thermostat temperature is controlled defines the working principle of the instrument and,
hence, the strategy for evaluating the corrected temperature rise.
In aneroid systems (systems without a fluid), the calorimeter can, stirrer and water are replaced by a
metal block. The combustion vessel itself constitutes the calorimeter in some aneroid systems.
In combustion calorimetric instruments with a high degree of automation, especially in the evaluation
of the results, the calorimeter is, in a few cases, not as well defined as the traditional, classical-type
calorimeter. Using such an automated calorimeter is, however, within the scope of this document as
long as the basic requirements are met with respect to calibration conditions, comparability between
calibration and fuel tests, ratio of sample mass to combustion vessel volume, oxygen pressure,
combustion vessel liquid, reference temperature of the measurements and accuracy of the results. A
printout of some specified parameters from the individual measurements is essential. Details are given
in Annex C.
Equipment, adequate for determinations of calorific value in accordance with this document, is
specified below.
6.2 Calorimeter with thermostat.
6.2.1 Combustion vessel, capable of withstanding safely the pressures developed during combustion;
see Figure 1.
The design shall permit complete recovery of all liquid products. The material of construction shall
resist corrosion by the acids produced in the combustion of coal and coke. A suitable internal volume of
the combustion vessel is from 250 ml to 350 ml.
WARNING — Combustion vessel parts shall be inspected regularly for wear and corrosion;
particular attention shall be paid to the condition of the threads of the main closure.
Manufacturers’ instructions and any local regulations regarding the safe handling and use of
the combustion vessel shall be observed. When more than one combustion vessel of the same
design is used, it is imperative to use each combustion vessel as a complete unit. Colour coding is
recommended. Swapping of parts can lead to a serious accident.
Key
1 thermostat lid 4 calorimeter can
2 ignition leads 5 thermostat
3 thermometer 6 stirrer
Figure 1 — Classical-type combustion-vessel calorimeter with thermostat
6.2.2 Calorimeter can, made of metal, highly polished on the outside and capable of holding an
amount of water sufficient to completely cover the flat upper surface of the combustion vessel while the
water is being stirred.
A lid generally helps reduce evaporation of calorimeter water but, unless it is in good thermal contact
with the can, it lags behind in temperature during combustion, giving rise to undefined heat exchange
with the thermostat and a prolonged main period.
6.2.3 Stirrer, working at constant speed.
The stirrer shaft should have a low-heat-conduction and/or a low-mass section below the cover of the
surrounding thermostat to minimize transmission of heat to or from the system. This is of particular
importance when the stirrer shaft is in direct contact with the stirrer motor. When a lid is used for the
calorimeter can, this section of the shaft should be above the lid.
The rate of stirring for a stirred-water-type calorimeter should be large enough to make sure that hot
spots do not develop during the rapid part of the change in temperature of the calorimeter. A rate of
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stirring such that the length of the main period can be limited to 10 min or less is usually adequate; see
Annexes A and B.
6.2.4 Thermostat (water jacket), completely surrounding the calorimeter, with an air gap of
approximately 10 mm separating calorimeter and thermostat.
The mass of water of a thermostat intended for isothermal operation shall be sufficiently large to
outbalance thermal disturbances from the outside. The temperature should be controlled to within
±0,1 K or better throughout the test. A passive constant temperature (“static”) thermostat shall have
a heat capacity large enough to restrict the change in temperature of its water. Criteria for satisfactory
behaviour of this type of water jacket are given in Annex B.
NOTE 1 For an insulated metal static jacket, satisfactory properties are usually ensured by making a wide
annular jacket with a capacity for water of at least 12,5 l.
NOTE 2 Calorimeters surrounded by insulating material, creating a thermal barrier, are regarded as static-
jacket calorimeters.
When the thermostat (water jacket) is required to follow closely the temperature of the calorimeter,
it should be of low mass and preferably have immersion heaters. Energy shall be supplied at a rate
sufficient to maintain the temperature of the water in the thermostat to within 0,1 K of that of the
calorimeter water after the charge has been fired. When in a steady state at 25 °C, the calculated mean
drift in temperature of the calorimeter shall not exceed 0,000 5 K/min; see A.3.2.
6.2.5 Temperature-measuring instrument, capable of indicating temperature with a resolution of at
least 0,001 K so that temperature intervals of 2 K to 3 K can be determined with a resolution of 0,002 K
or better.
The absolute temperature shall be known to the nearest 0,1 K at the reference temperature of the
calorimetric measurements. The temperature-measuring device should be linear, or linearized, in its
response to changes in temperature over the interval it is used.
As alternatives to the traditional mercury-in-glass thermometers, suitable temperature sensors are
platinum-resistance thermometers, thermistors, quartz crystal resonators, etc., which, together
with a suitable resistance bridge, null detector, frequency counter, or other electronic equipment,
provide the required resolution. The short-term repeatability of this type of device shall be 0,001 K
or better. Long-term drift shall not exceed the equivalent of 0,05 K for a period of six months. Sensors
with linear response (in terms of temperature) are less likely to drift, causing bias in the calorimetric
measurements, than are non-linear sensors.
For adiabatic systems, a suitable arrangement is as follows: Mercury-in-glass thermometers in
accordance with ISO 651, ISO 652, ISO 1770 or ISO 1771 shall satisfy the measurement requirements. A
viewer with magnification about 5x is needed for reading the temperature with the resolution required.
Also, a mechanical vibrator to tap the thermometer is suitable for preventing the mercury column from
sticking; see 8.4. If this is not available, the thermometer can be tapped manually before reading the
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