ISO 18976:2026

International
Standard
ISO 18976
First edition
Testing of refrigerant compressors
2026-01
Essais des compresseurs pour fluides frigorigènes
Reference number
© ISO 2026
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Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions .1
3.2 Symbols .3
4 Uncertainty of measurement and test conditions . 5
4.1 Uncertainty of performance data .5
4.2 Uncertainty of measurement .5
4.3 Refrigerant circuit state points .6
4.4 Test conditions .6
5 General requirements . 7
5.1 Calculation methods .7
5.1.1 Principle .7
5.1.2 Specific enthalpy .8
5.1.3 Refrigerant mass flow .8
5.1.4 Power input .8
5.1.5 Basic formulae .8
5.2 Requirements for the selection of a test method .10
5.2.1 General .10
5.2.2 Second concurrent test .10
5.3 Test period .11
5.3.1 General .11
5.3.2 Steady state conditions . .11
5.3.3 Recording of measured data .11
5.4 Pressure and temperature measuring points .11
5.5 Oil circulation .11
5.6 Fractionation . 12
5.7 Calibration of calorimeters . . 12
5.7.1 Heat leakage . 12
5.7.2 Reference temperature . 12
5.7.3 Calibration procedure. 12
5.8 Source of refrigerant data . 13
5.9 Kinetic refrigerant properties . 13
6 Test methods .13
6.1 General . 13
6.2 List of test methods . 13
6.2.1 Calorimetric methods . 13
6.2.2 Flow meter methods . 13
6.3 Choice of test methods for test X and test Y .14
6.4 Method A: Secondary fluid calorimeter .14
6.4.1 Description .14
6.4.2 Calibration . . .17
6.4.3 Test procedure.17
6.4.4 Requirements .17
6.4.5 Additional information .17
6.4.6 Determination of refrigerant mass flow .17
6.5 Method B: Flooded system calorimeter .17
6.5.1 Description .17
6.5.2 Calibration . . .18
6.5.3 Test procedure.18
6.5.4 Requirements .18
6.5.5 Additional information .18

iii
6.5.6 Determination of refrigerant mass flow .18
6.6 Method C: Dry system refrigerant calorimeter .19
6.6.1 Description .19
6.6.2 Calibration . . . 23
6.6.3 Test procedure. 23
6.6.4 Requirements . 23
6.6.5 Additional information . 23
6.6.6 Determination of refrigerant mass flow . 23
6.7 Method G: Water-cooled condenser/gas cooler on the discharge side .24
6.7.1 Description .24
6.7.2 Calibration . . . 25
6.7.3 Test procedure. 25
6.7.4 Requirements . 25
6.7.5 Additional information . 25
6.7.6 Determination of refrigerant mass flow . 25
6.8 Method D: Refrigerant gas flow meter . 25
6.8.1 Description . 25
6.8.2 Requirements .27
6.8.3 Additional information . 28
6.8.4 Determination of refrigerant mass flow . 28
6.9 Method F: Refrigerant flow meter in the liquid line . 28
6.9.1 General . 28
6.9.2 Description . 28
6.9.3 Test procedure. 29
6.9.4 Requirements . 29
6.9.5 Additional information . 29
6.9.6 Determination of the refrigerant and oil mass flow . 30
6.10 Method M: Energy balance on compressor . 30
6.10.1 General . 30
6.10.2 Description . 30
6.10.3 Calibration . . . 30
6.10.4 Test procedure.31
6.10.5 Requirements .31
6.10.6 Additional information .31
6.10.7 Determination of refrigerant mass flow .32
7 Determination of the power input by the compressor .34
7.1 Measurement . 34
7.1.1 General . 34
7.1.2 Measurement for externally driven compressors . 34
7.1.3 Measurement for motor compressors . 34
7.2 Calculation . 34
8 Test report .35
8.1 General . 35
8.2 Test results . 35
Annex A (normative) Conversion of measured performance data to specified test conditions
for compressors with intermediate pressure port .37
Annex B (informative) Estimation of errors .40
Bibliography .43

iv
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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Any trade name used in this document is information given for the convenience of users and does not
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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 86, Refrigeration and air-conditioning,
Subcommittee SC 4, Testing and rating of refrigerant compressors.
This first edition of ISO 18976 is a technical revision of ISO 917:1989, which was withdrawn in 2015.
The main changes are as follows:
— addition of two stage and economized compressors;
— Clause 2 was updated;
— Clause 3 was updated, additional terms defined because of, for example, refrigerant blends with
temperature glide;
— addition of the new Clause 4 “Uncertainty of measurement and test conditions”;
— deletion of the list of measuring devices;
— extraction of calibration of calorimetric methods into a separate clause;
— addition of transcritical application;
— addition of test requirements for inverter driven compressors;
— addition of cyclic capacity control;
— reference point numbering, symbols and indexes revised to allow for economised compressors and to
simplify formulae;
— former Annex B regarding the list of symbols was moved under Clause 3;
— the text content of former Annex C was revised and is now Annex B “Estimation of errors”;
— document was editorially revised.

v
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.

vi
International Standard ISO 18976:2026(en)
Testing of refrigerant compressors
1 Scope
This document applies to single stage, two stage and economised refrigerant compressors. Selected test
methods are described for the determination of the refrigerating capacity, the power input, the isentropic
efficiency and where possible the volumetric efficiency. These test methods provide results of sufficient
accuracy to permit consideration of the suitability of a refrigerant compressor to operate satisfactorily
under any set of basic test conditions required for a given application.
NOTE Tests on complete refrigeration installations are dealt with in ISO 916.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and symbols
For the purposes of this document, the following terms and definitions 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 Terms and definitions
3.1.1
refrigerating capacity
Q
product of the refrigerant mass flow at the compressor inlet port and the difference between the specific
refrigerant enthalpy at the compressor inlet port and the specific enthalpy of fluid entering the evaporator
expansion device
3.1.2
subcooling
difference between the bubble point temperature of the refrigerant corresponding to its pressure and the
temperature of the liquid refrigerant
3.1.3
power input
P
power demand to drive the compressor
Note 1 to entry: The determination of the power input is specified in Clause 7.
3.1.4
coefficient of performance
COP
R
ratio of the refrigerating capacity to the power input
Note 1 to entry: Both, refrigerating capacity and power input are at the specified test condition.

Note 2 to entry: The test report should make clear whether the power input is referring to the electrical power or the
shaft power.
[SOURCE: EN 13771-2:2017, 3.1.6, modified — “power absorbed” was changed to “power input” and Note 2 to
entry was added]
3.1.5
subcritical operation
operating condition with discharge pressure below the critical pressure of the refrigerant
3.1.6
transcritical operation
operating condition with discharge pressure above and the suction pressure below the critical pressure
3.1.7
part load operation
operation with active capacity control at reduced capacity for compressors with capacity control mechanism
Note 1 to entry: On/off cycling of the compressor motor is not considered as capacity control.
[SOURCE: EN 13771-2:2017, 3.1.9]
3.1.8
fluid
liquid, gas or vapour including the state of appearance close to and above the critical pressure
3.1.9
volumetric efficiency
η
v
ratio of the actual volume rate of flow under compressor inlet conditions, at the requirements specified in
4.4, to the displacement of the compressor
3.1.10
isentropic efficiency
η
i
ratio of total isentropic compression power to the power input
Note 1 to entry: Total isentropic compression power is the sum of individual product of mass flow multiplied by the
isentropic change in enthalpy across the corresponding compression stage.
Note 2 to entry: The value according to this definition is only used as reference value for the necessary correction
calculation specified in this document.
3.1.11
oil circulation ratio
x
oil
ratio of the measured oil mass flow to the mass flow of the circulating oil/refrigerant mixture
Note 1 to entry: Oil circulation ratio can differ at the different compressor ports.
3.1.12
calorimeter
component of the test system refrigerant circuit intended to determine the refrigerant mass flow via an
energy balance
Note 1 to entry: To limit the uncertainty of the mass flow determination caused by the heat loss, calorimeters are
usually heat insulated.
Note 2 to entry: The compressor or the economiser heat exchanger can be utilised as calorimeter.
3.1.13
speed of rotation
number or rotations per unit time

3.1.14
economiser heat exchanger
heat exchanger in a system in which a partial mass flow is evaporated at intermediate pressure and lead to
the respective compressor port, to subcool the main mass flow after the condenser
Note 1 to entry: With this system design, the refrigeration capacity and the coefficient of performance increase. At the
same time, the discharge temperature falls.
Note 2 to entry: There are also other economiser systems possible without heat exchanger, e.g. with flash tank.
3.1.15
zeotropic refrigerant
blend composed of two or more refrigerants whose equilibrium vapor and liquid phase compositions are not
the same at any pressure below the critical pressure
[SOURCE: ISO 817:2024, 3.1.47, modified — The term “zeotrope” was replaced by the term “zeotropic
refrigerant” and the references to 3.1.7 and 3.1.37 were removed.]
3.2 Symbols
For the purposes of this document, the symbols of Table 1 and the indexes of Table 2 apply.
Table 1 — Symbols
SI
Symbol Designation
unit
A area m
a speed of sound m/s
c specific heat capacity of heating and cooling liquid J/(kg K)
d diameter m
f electrical frequency Hz
F heat leakage factor W/K
F flow factor for centrifugal compressor m
F
F head factor for centrifugal compressor —
H
F mass flow ratio —
m
h specific enthalpy J/kg
specific enthalpy of refrigerant gas at the compressor outlet pressure (2) having the
h same entropy as the refrigerant gas at the compressor inlet pressure (1) for calculation J/kg
i1-2
of the isentropic efficiency (specified test conditions)
specific enthalpy of refrigerant gas at the compressor outlet pressure (2) having the
h same entropy as the refrigerant gas at the compressor intermediate pressure port (7) J/kg
i7-2
for calculation of the isentropic efficiency (specified test conditions)
−1
n compressor speed of rotation s
P power input W
p absolute pressure Pa
m refrigerant mass flow as determined by the test kg/s
a
M refrigerant mass flow at the specified test conditions kg/s
m mass flow of heating or cooling liquid kg/s
f
m oil mass flow kg/s
oil
m mass flow of liquid refrigerant oil mixture kg/s
x
V refrigerant volume flow m /s
V volume flow of refrigerant oil mixture m /s
x
T absolute temperature K
TTaabbllee 11 ((ccoonnttiinnueuedd))
SI
Symbol Designation
unit
difference between fluid outlet of economiser heat exchanger and bubble temperature
ΔT K
eco
corresponding to intermediate pressure
t temperature °C
t mean surface temperature of the calorimeter °C
cal
t bubble point temperature of the refrigerant °C
b
t bubble point temperature of the secondary fluid °C
bs
t reference temperature °C
x
t inlet temperature of heating or cooling liquid °C
s1
t outlet temperature of heating or cooling liquid °C
s2
U electrical voltage V
V theoretical compressor displacement at declared speed m /s
sw
oil circulation ratio in the refrigerating system, expressed in mass of oil per mass of
x kg/kg
oil
mixture
η isentropic efficiency —
i
η volumetric efficiency —
v
v specific volume m /kg
density of refrigerant corresponding to pressure and temperature at which the flow rate
ρ kg/m
a
is measured
Q heat input to the calorimeter W
i
Q electrical input to the heater W
n
Q refrigerating capacity at the specified test conditions W
Table 2 — Indexes
Index Designation
a actual
amb ambient
1 refrigerant at the compressor inlet
2 refrigerant at the compressor outlet
3 refrigerant at the inlet of the condenser/gas cooler
4 refrigerant at the outlet of the condenser/gas cooler
5 refrigerant at the inlet of the expansion device
6 refrigerant at the outlet of the evaporator
7 refrigerant at the intermediate pressure port (connection to the compressor)
8 refrigerant at the inlet to flow meter
9 refrigerant at the inlet of expansion device C7
10 liquid outlet of refrigerant at the economiser heat exchanger (HX)
liquid refrigerant at bubble point corresponding to the pressure at the compressor outlet according
f2 to the specified test conditions, for subcritical operation or fluid refrigerant at the compressor outlet
pressure and the temperature of the gas cooler outlet at the specified test condition
liquid refrigerant at bubble point corresponding to the pressure at the compressor intermediate pres-
f7
sure port according to the specified test conditions, for subcritical operation
liquid refrigerant at bubble point corresponding to the pressure at the compressor intermediate pres-
f10
sure port plus the temperature difference of the economiser HX
b bubble
d dew
TTaabbllee 22 ((ccoonnttiinnueuedd))
Index Designation
i isentropic
cal calorimeter surface
crit critical point of refrigerant
f fluid
oil oil
s secondary fluid
x refrigerant/oil mixture
4 Uncertainty of measurement and test conditions
4.1 Uncertainty of performance data
All measuring instruments in the test setup shall be selected and calibrated so that the final result is within
the maximum uncertainties of the measured value as indicated:
— refrigerating capacity: ±2,5 %;
— electrical power input: ±1 %;
— mechanical power input: ±2,5 %.
4.2 Uncertainty of measurement
Uncertainty values are considered to cover a 95 % confidence interval, i.e. ±2 times the standard deviation.
This is also known as extended uncertainty. Except where otherwise stated in the particular clauses,
measurements shall be carried out within the maximum uncertainty of the measured value as indicated.
Where a percent value is stated, it is the relative uncertainty of the directly measured value.
— absolute pressure: ±1 %;
— electrical:
— current: ±1 %;
— frequency: ±0,2 %;
— power: ±1 %;
— voltage: ±1 %;
— refrigerant mass flow: ±1 %;
— refrigerant volume flow: ±1 %;
— speed of rotation: ±0,07 %;
— temperatures:
— temperature for differences: ±0,05 K;
— temperature differences measured directly: ±1 %;
— other temperatures: ±0,3 K;
— time: ±0,1 %;
— torque: ±1 %;
— water flow: ±1 %.
Adherence to these limits does not ensure the requirements of 4.1 are obtained automatically.
For the combined uncertainty of the performance data, see Annex B.
4.3 Refrigerant circuit state points
Figure 1 illustrates the state of the refrigerant as it passes through the system.
This is a general illustration showing conditions occurring in typical subcritical single stage systems, which
are not all relevant for compressor performances (e.g. the shown pressure drops), but still important for
inherent calculation. p-h diagrams relating to specific test methods do not show the various pressure drops
in order to keep diagrams as clear as possible.
a) Circuit diagram b) Pressure vs enthalpy diagram
Key
1 refrigerant gas at the compressor inlet
2 refrigerant gas at the compressor outlet
3 refrigerant gas at the inlet of the condenser or gas cooler
4 refrigerant at the outlet of the condenser or gas cooler
5 refrigerant fluid at the inlet of the expansion device
6 refrigerant gas at the outlet of the evaporator
A compressor
B condenser
C expansion device
D evaporator
Figure 1 — Refrigerant circuit
4.4 Test conditions
The specified test conditions under which the test is to be performed and their allowable deviations are
given in Table 3.
Table 3 — Specified test conditions and the allowable deviations
Allowable deviation during
Specified test conditions
the entire test period
a
Absolute pressure , compressor inlet, p ±1,0 %
a
Absolute pressure , compressor outlet, p ±1,0 %
a
Absolute pressure , compressor intermediate pressure port, p ±1,0 %
Refrigerant temperature at the compressor inlet, t ±3,0 K
Refrigerant temperature at the compressor intermediate pressure port, t ±3,0 K
Nominal compressor speed of rotation, n ±1,0 %
Nominal electrical voltage, U ±1,5 %
Nominal electrical frequency, f ±1,0 %
Ambient temperature, t ±3,0 K
amb
a
Dynamic deviations due to pulsations are not considered.
The values for the relevant specified test conditions shall be recorded.
For the calculation of refrigeration capacity, additional values can be necessary:
— gas cooler outlet temperature in case of transcritical operation;
— temperature difference at the liquid subcooler or aftercooler in case of two stage operation (flash tank
without temperature difference). The value of the pressure p shall be adjusted according to Annex A to
reach the requested temperature difference.
For cyclic capacity control the deviations of Table 3 refer to the average values per control cycle.
For operation with constant capacity, the deviations can be applied to one of the following:
— every single recorded value;
— the average values per minute;
— the average of the values for the test period, ±2 times standard deviation of the measured values.
For wet operation at point 7, intermediate pressure port [see Figure 3 a)], the correction of Annex A can still
be used. In this case the heat balance at the economiser heat exchanger can be used to determine the specific
enthalpy of the refrigerant at point 7.
For compressors with factory assembled economiser heat exchanger, where the temperature at point 7
cannot be measured, and thus the state of appearance, cannot be determined exactly, the same assumptions
as for the wet operation apply.
5 General requirements
5.1 Calculation methods
5.1.1 Principle
The determination of the refrigerating capacity at the specified test conditions comprises:
— the evaluation of the actual mass flow rate of the refrigerant, obtained for each test method used by
means of an apparatus which is inserted into the outer part of the test circuit, between the outlet and the
inlet of the compressor or by an energy balance on the compressor according to method M, as described
in Clause 6, when operating according to Table 3;

— the correction of this mass flow to the mass flow at the specified test conditions using the ratio of the
actual specific volume (v ) of the refrigerant gas at the compressor inlet to the specific volume of the gas
1a
at the specified test conditions (v );
— the product of the corrected mass flow and the difference between the specific enthalpies at the specified
test conditions of the refrigerant gas at the compressor inlet h and the fluid (refrigerant) entering the
evaporator expansion device h for single stage expansion cycles; for multiple stage expansion cycles
f2
h is used. The refrigerant at the inlet port(s) is superheated above the dew point temperature to the
f10
stated value.
NOTE For the purposes of this document, it is assumed that the volume flow rate is constant when the compressor
is operating according to Table 3.
5.1.2 Specific enthalpy
The value of the specific enthalpy is taken from recognised data of the thermodynamic properties of the
refrigerant used. The determining parameters are the temperature and pressure of the refrigerant at the
relevant cycle points.
NOTE Up to condensing temperatures of 0,95 × T , the liquid enthalpy depends on temperature only. Above
crit
0,95 × T , the enthalpy is determined by the temperature and pressure.
crit
5.1.3 Refrigerant mass flow
The refrigerant mass flow is either measured directly or calculated from measured values of temperatures,
pressure and heat flow (see the methods in Clause 6).
5.1.4 Power input
The power input only considers power to drive the compressor, see also Clause 7.
Power input for auxiliaries and accessories that are necessary for the operation of the compressor shall be
recorded and documented.
NOTE For compressors with a factory assembled or factory specified frequency inverter, the power input is the
electrical power at the input terminals of the inverter, see also Clause 7.
5.1.5 Basic formulae
5.1.5.1 Mass flow
Any refrigerant mass flow m entering or leaving the compressor as determined by measurement shall be
converted to the specified test conditions using Formula (1):
v n
a
mm  (1)
a
v n
a
For motor compressors, the correction factor n/n is replaced by f/f .
a a
5.1.5.2 Compressor refrigerating capacity
The refrigerating capacity for compressors is calculated using Formula (2):
Qm hh (2)

12f
For compressors with an intermediate pressure port Formula (3) and Formula (4) apply:
Qmhh (3)
11 f10
ttT (4)
ff10 7 eco
The value ΔT is a given value for the temperature difference at the economiser heat exchanger. If this
eco
value is 0, the performance data are for economiser operation with a flash tank.
NOTE The index f10 is used for the theoretical value corresponding to the pressure at the intermediate port plus
a temperature difference if an economiser heat exchanger is used (see also Table 2). The index f7 corresponds directly
to the pressure at the intermediate port.
Alternatively, to Formula (3) the capacity can be calculated as follows:
Qmhh mhh (5)
11 ff27 72
5.1.5.3 Volumetric efficiency
The volumetric efficiency η is calculated using Formula (6):
v
m
 v (6)
v 1
V
sw
NOTE Within the limits specified in this document, it is assumed that the volumetric efficiency is constant.
5.1.5.4 Power input and isentropic efficiency
The power input is converted from the measured power input to the specified test conditions using
Formula (7) to Formula (10).
The isentropic efficiency η is calculated using Formula (7):
i
P
ia
  (7)
i
n
P 
a
n
a
For motor compressors, the correction factor n/n is replaced by f/f .
a a
The isentropic power is the sum of isentropic powers for each mass flow rate entering the compressor.
Calculation of isentropic power demand at specified test conditions is done using Formula (8):
Pmhh mhh (8)
ii1121 77i 27
NOTE Single stage compression m = 0.
The isentropic power demand at actual test conditions is calculated using Formula (9):
Pmhh mhh (9)
ia 11ai 21aa 77ai 27aa
The isentropic efficiency is assumed to be constant for conversions from actual (measured) conditions to
the specified test condition (p , p , p , t , t also including n or f ). Conversion is only allowed within the
1 2 7 1 7
deviations defined in Table 3. The power P at the specified test condition is calculated using Formula (10):
P
i
P P (10)
a
P
ia
For compressors with an intermediate pressure port see Annex A for correction to specified test conditions
and evaluation of the pressure at the intermediate pressure port.
The coefficient of performance COP is defined in 3.1.4.
R
5.1.5.5 Characteristic factors for centrifugal compressors
The performance of a centrifugal compressor can be described via the flow factor and head factor per
compression stage. See Formula (11), Formula (12) and Formula (13) and the overall isentropic efficiency as
in 5.1.5.4.
Flow factor for first and second stage compression:
m
F  (11)
F
 a
Head factor for first stage compression, from point 1 to point 2:
hh
i12 1
F  (12)
H1
a
Head factor for second stage compression, from point 7 to point 2:
hh
i72 7
F  (13)
H2
a
5.2 Requirements for the selection of a test method
5.2.1 General
Generally, two test methods as specified in Clause 6 shall be used at the same time for determining a mass
flow, a test X and a test Y. The results of the two methods shall correlate within 4 %. The test result is the
mean value of the two methods.
Where testing devices are in constant use and are subject to perio
...