C z e c h  M e t r o l o g y  I n s t i t u t e  
Okružní 31,  638 00  
Brno  
 
1. ------IND- 2010 0779 CZ - EN- ------ 20101224 --- --- PROJET 
 
The Czech Metrology Institute (CMI), as the competent local body for stipulating metrological and 
technical requirements for a specified measuring device and for the stipulation of test method s for type 
approval and for verification of a specified measuring device pursuant to Section 14(1) of Act No. 
505/1990 Coll., on Metrology, as amended, and pursuant to provisions of Section 172 et seq. of Act 
No. 500/2004 Coll., the Administrative Code, as  amended (hereinafter the "AC"), has on 30 July  2010 
commenced ex officio proceedings pursuant to Section 46 of the AC, and based on su pporting doc u-
ments issues this:  
 
 
I. 
M E A S U R E  O F  A  G E N E R A L  N A T U R E  
number:  0111 -OOP -C012 -10 
referenc e no. 0313/005/10/Pos.,  
publication date: 11 October 2010  
 
stipulating metrological and technical requirements for specified measuring devices, 
including test methods for verifying specified measuring devices:  
"laboratory hydrometers with increments less t han 1 kg • m -3 except for hydrometers 
for the measurement of soil granularity (Casagrande)"  
 
1   Basic definitions  
For the purposes of this measure of a general nature, terminology and definitions contained in the 
VIM1) and the following apply:  
1.1   hydro meter with constant mass : a measuring device with constant mass whose depth of imme r-
sion in the measured liquid determines its density or concentration of one of its comp onents  
NOTE   The hydrometer may or may not include a thermometer.  
1.2   laboratory hydrometer with increments less than 1  kg · m-3: a glass hydrometer with constant 
mass intended for the precise measurement of solutions with an expected density between 600  kg · m-3 
and 2 000 kg · m-3 under laboratory conditions  
1.3    laboratory hydrometer for general measurement: a laboratory hydrometer that is not expli c-
itly intended for the measurement of the density of a specified type of liquid  
                                                      
1)   International Vocabulary of Metrology - Basic and General Concepts and Associated Terms (VIM).  2 2   Metrological requirements  
2.1   Stipulated service conditions  
A hydrometer must meet metrological requirements for service temperature range specified by the 
manufacturer depending on the type of liquid being measured, for example +10  °C to +30  °C for 
aqueous solutions, or –10 °C to +50  °C for petroleum and petroleum products.  
The reference temperature for testing hydrometers is 20  °C or 15  °C, the reference temperature for 
alcohol is 20  °C. The reference pressure is normal atmo spheric pressure.  
2.2   Measurement interval  
The measurement  interval of a laboratory hydrometer for general -purpose density measurements b e-
tween 600  kg · m-3 and 2 000 kg · m-3 or the measurement interval for measuring the density of petr o-
leum and petroleum products is specified by the manufacturer, while meeting the requirements of T a-
ble 1.  
The measurement interval of a laboratory hydrometer for the measurement of density of alcohol must 
be (780 to 830) kg  · m-3 or (980 to 1 000) kg  · m-3 while meeting the requirements of Table 1.  
Table  – Laboratory hydrometer mea surement interval requirements  
Values in kg  · m-3 
Liquid for which it is intended  for general purposes  alcohol (a solution of 
ethanol and water)  petroleum 
and petr o-
leum pro d-
ucts 
Measurement interval range  20 50 201) 502) 60 
Increment size  0,2 0,5 0,2 0.5 0,5 
Total indication interval  22 55 20.4 51 63 
1) Applies for a measurement interval of (980 to 1000) kg  · m-3. 
2) Applies for a measurement interval of (780 to 830) kg  · m-3. 
 
NOTE   Hydrometer graduated scale numbering can be in g  · cm-3 if requireme nts of Table 1 are met.  
If the laboratory hydrometer includes a thermometer, its measurement interval must correspond to the 
service temperatures specified in Article 2.1.  
2.3   Maximum permissible error  
2.3.1     Maximum permissible alcoholmeter error  
The maximum permissible error of a laboratory hydrometer is ±  0,5 kg · m-3. 
2.3.2    Maximum permissible thermometer error  
The maximum permissible error of a thermometer that is part of a laboratory hydrometer is ±  0.5 °C. 
3   Technical requirements  
3.1   Hydro meter design  
A laboratory hydrometer is composed of a glass tube, which may have a built -in thermometer, and 
whose lower portion contains a conical or hemispherical weight. Connected to the upper part is a c y-
lindrical stem that is sealed at its upper end.  Measure of a General Nature No. 0111 -OOP -C012-10 
3 The entire outside surface of the hydrometer must be symmetrical around its main axis. A circular 
cross -section must not exhibit any sudden changes, and transitions of all parts of the hydrometer must 
be smooth and rounded.  
The taper ratio of the stem must  not exceed 0.1 mm per 100 mm. The maximum permissible deviation 
from circularity of any stem cross -section along the entire length of the scale is 0.10 mm.  
The total length of the hydrometer must not exceed 360 mm, except for an alcoholmeter with a mea s-
urement interval of (780 to 830) kg  · m-3, whose total length must not exceed 420 mm.  
No part of the hydrometer may contain any unattached material.  
3.2   Hydrometer graduated scale  
A hydrometer may have only one graduated scale, which must fit tightly again st the inside wall of the 
hydrometer casing, must not exhibit unevenness and must not be helically twisted. The grad uated scale 
of the hydrometer must not have been damaged by flame during sealing of the stem.  
The graduated scale markings must be perpendic ular to the axis of the hydrometer, parallel to each 
other, and clear. All hydrometer graduated scale markings must have the same thickness, which must 
be no more than 0.2 mm.  
If hydrometer graduated scale markings have differing lengths, then the length o f short markings must 
be at least one fifth of the circumference of the stem, medium markings at least one third of the ci r-
cumference of the stem, and long markings at least half of the circumference of the stem. All grad u-
ated scale markings must be right -aligned along one imaginary perpendicular line.  
The total length of the graduated scale for a measurement interval range of 20  kg · m-3 must be (110 to 
130) mm and for a measurement interval range of 50  kg · m-3 it is (120 to 140) mm, except for an 
alcoho lmeter with a measurement interval of (780 to 830) kg  · m-3, whose total graduated scale length 
must be (170 to 190) mm.  
The highest graduated scale marking on the hydrometer must be at least 15 mm below the top of the 
stem, and the lowest marking at least  5 mm above the location where the stem widens to meet the cy l-
inder.  
Hydrometers are calibrated to be read at the lowest part of the meniscus (for transparent liquids) or at 
the upper edge of the meniscus.  
To check that the hydrometer graduated scale is co rrectly located in the stem, the stem surface must 
have a verification line that aligns with the highest numbered mark on the graduated scale.  
The mutual positioning of the hydrometer graduated scale and thermometer must be such that the va l-
ues of both gra duated scales can be read without turning the hydrometer, and the right ends of the h y-
drometer graduated scale lines must pass behind the profile of the stem.  
3.3   Hydrometer thermometer  
The capillary must be capable of withstanding an upper temperature l imit of +50  °C or the upper limit 
of the upper measurement limit of the thermometer's graduated scale in accordance with  Article 2.2 
plus 10  °C, whichever is greater.  
The divisions of the thermometer's graduated scale must be 0.5 C.  
The total length of the  thermometer's graduated scale must be at least 65 mm.  
To check that the thermometer's graduated scale is attached correctly, a verification line is placed on 
the backside of the cylinder at the location of the reference temperature marking during hydromet er 
calibration.  4 3.4   Working position of the hydrometer  
The hydrometer must float in liquid with its vertical axis perpendicular to the surface. The deviation of 
the hydrometer's axis to the side from vertical when looking at the hydrometer's graduated sc ale must 
not be greater than 1º 30´.  
When meeting the previous requirement, stability must also be maintained, where the hydrometer is 
deflected to the opposite side and then must return to its original position.  
3.5   Material  
The glass used in the manufa cture of hydrometers must be transparent, colourless and must permit 
easy reading of the graduated scale.  
4   Markings  
4.1   Markings on the measuring device  
A laboratory hydrometer must be legibly and permanently marked with the following information:  
– the manufacturer's name or mark,  
– measuring device type,  
– serial number and date of manufacture,  
– unit of measurement (kg  · m-3 or g · cm-3); 
– reference temperature (20  °C or 15  °C); 
– the type of liquid for which the hydrometer is intended, if appropriate,  
– how to read data (read at upper edge of meniscus or the lowest point of the meniscus),  
– type approval mark.  
4.2   Verification mark  
The verification mark is etched onto the backside of the body of the hydrometer below the stem a t-
tachment location.  
5   Type approva l 
5.1   In general  
The type approval process for the measuring device includes the following tests:  
a) external inspection,  
b) shape and dimension check,  
c) test for stability and that the axis of the stem of the hydrometer is perpendicular to the su rface 
of the li quid in which it is floating,  
d) hydrometer accuracy test,  
e) thermometer accuracy test.  
5.2   External inspection  
The following is assessed during an external inspection:  
a) that prescribed tec hnical documentation is complete, including the user manual,  
b) that metro logical and technical characteristics specified by the manufacturer in documentation 
conform to requirements of this regulation listed in sections 2 and 3,  Measure of a General Nature No. 0111 -OOP -C012-10 
5 c) the appearance and completeness of information in accordance with  prescribed technical 
document ation. 
5.3   Hydrometer shape and dimension check  
The shape and surface of the hydrometer must not exhibit any sudden changes and transitions.  
The deviation from circularity and the taper of the hydrometer stem are checked at each marked point 
of the hydrometer 's graduated scale with a micrometer to an accuracy of ±  0.01 mm, and the devi ation 
from circularity of any stem cross -section must not exceed values listed in Article 3.1.  
The total length of the hydrometer is measured with a length measurement device wit h an accuracy of 
± 0.5 mm. 
The prescribed dimensions of the hydrometer's graduated scale markings are checked with a length 
measurement device to an accuracy of 0.01 mm.  
5.4   Functional tests  
5.4.1    Assessment solutions  
5.4.1.1    In general  
For testing, assessment solutions needed to assess the hydrometer graduated scale are used. Prior to 
testing, solutions must be tempered to laboratory temperature with a maximum deviation of ±  5 °C 
from the reference temperature, and temperature changes during measurem ent must not exceed 
± 0.5 °C · h-1. 
Assessment solutions pursuant to 5.4.1.2 or 5.4.1.3 are used depending on the nature of the measured 
liquid for which the manufacturer has stipulated the measuring device (see Article 4.1), i.e. depending 
on whether it i s an oily liquid or aqueous solution.  
5.4.1.2     Hydrometers for oily liquids  
For hydrometers for oily liquids, the following solutions are used in accordance with  the measurement 
interval: 
– (620 to 650)  kg · m-3  a mixture of i -pentane and n -pentane,  
– (650 to  680)  kg · m-3  a mixture of n -pentane and petroleum ether,  
– (690 to 890)  kg · m-3  a mixture of petroleum ether and transformer oil,  
– (900 to 1080)  kg · m-3 a solution of 96% vol. sulphuric acid and 80% vol.  
  finely refined a lcohol.  
5.4.1.3     Hydromet ers for aqueous solutions  
For hydrometers for aqueous (non -oily) liquids, the following solutions are used in accordance with  
the measurement interval:  
– (730 to 810)  kg · m-3 a mixture of ethyl ether and ethanol,  
– (800 to 1 000)  kg · m-3 a mixture of ethanol  and distilled water,  
– (1000 to 1130)  kg · m-3 a solution of 96% vol. sulphuric acid and 80% vol.  
  finely refined a lcohol.  
– (1130 to 1850)  kg · m-3 a mixture of sulphuric acid (96% vol.) and water.  6 5.4.2    Test for stability and to ensure that the axis of  the stem of the hydrometer is perpendic u-
lar to the su rface of the liquid in which it is floating  
The perpendicularity of the stem axis is tested by submerging the hydrometer into a suitable calibr a-
tion liquid up to the lower edge of the hydrometer's gradu ated scale. Using an angle template placed 
behind the hydrometer, the deviation of the hydrometer's stem from vertical is determined. The proje c-
tion of the outline of the stem is observed from a distance of at least 250 mm.  
The subsequent stability test is  performed by tilting the hydrometer opposite to its stem's lean angle.  
This test must meet the requirements of Article 3.4.  
5.4.3    Hydrometer accuracy test  
Prior to the test, the hydrometer and all necessary aids must be thoroughly degreased by dipping i nto 
chromic sulphuric acid for at least 30 minutes, rinsing with running water and rinsing with di stilled 
water.  
NOTE   Degreasing is particularly important prior to testing in aqueous solutions, where greasiness can cause a 
significant difference in hydro meter readings.  
Accuracy is tested by comparing measured values of the tested hydrometer against the values of a 
reference hydrometer in calibration liquids for the given hydrometer measurement interval in acco r-
dance with  Article 5.4.1.  
Measurement is perf ormed at least three times for at least five points of the graduated scale.  
During the test, it is important to ensure that the hydrometer and thermometer (that is part of the test 
bath) are at least 10 mm apart and away from the walls of the container wit h the calibration liquid.  
When testing in volatile assessment solutions, care must be taken that during lengthier measurements, 
drops do not form on the hydrometer stem close to the solutions surface, caused by the condensation 
of saturated vapours. Prior to reading data from the hydrometer graduated scale, any drops must be 
wiped off with a strip of filter paper.  
After a test in non -oily liquids, both the reference and tested hydrometer are rinsed with distilled w a-
ter, after a test in oily liquids, they ar e rinsed with petroleum ether.  
The measurement error at each point of the graduated scale must not exceed the maximum permissible 
error in accordance with  Article 2.3.1.  
5.4.4    Hydrometer thermometer accuracy test  
The accuracy of the thermometer (that is part of the hydrometer) is tested by comparing measured 
values of the tested thermometer with values of a reference thermometer, for at least three points of the 
scale: at the hydrometer's reference temperature and at both endpoints of the thermometer's sc ale. 
The measurement error at each point of the graduated scale must not exceed the maximum permissible 
error in accordance with  Article 2.3.2.  
6   Initial verification  
6.1   In general  
The following is performed during initial verification:  
a) external inspe ction,  
b) shape and dimension check,  
c) test for stability and to ensure that the axis of the stem of the hydrometer is perpendicular to 
the surface of the liquid in which it is floating,  
d) hydrometer accuracy test,  Measure of a General Nature No. 0111 -OOP -C012-10 
7 e) thermometer accuracy test.  
6.2   Visual inspecti on 
The following is assessed during visual inspection of laboratory hydrometers:  
a) that the measuring device is the same as the approved type;  
b) appearance (including description) and condition of the measuring device.  
6.3   Hydrometer shape and dimension chec k 
Shape and dimensions of the hydrometer are checked in accordance with  Article 5.3.  
6.4   Functional tests  
6.4.1    Test for stability and to ensure that the axis of the stem of the hydrometer is perpendic u-
lar to the su rface of the liquid in which it is fl oating  
The test for stability and that the axis of the stem of the hydrometer is perpendicular to the su rface of 
the liquid in which it is floating is performed in accordance with  Article 5.4.2.  
6.4.2    Hydrometer accuracy test  
The hydrometer accuracy test  is performed in accordance with  Article 5.4.3.  
6.4.3    Hydrometer thermometer accuracy test  
The accuracy test of the thermometer as part of the hydrometer is performed in accordance with  Arti-
cle 5.4.4.  
7   Subsequent verification  
These measuring devices a re not subject to subsequent verification.  
 
8   Notified standards  
To specify metrological and technical requirements for measuring devices and to specify verific ation 
test methods stemming from this measure of a general nature, the CMI notifies Czech tech nical sta n-
dards, other technical standards or technical documents of international or foreign organizations or 
other technical documents containing detailed technical requirements (hereinafter "notified sta n-
dards"). The CMI shall publish a list of these no tified standards, with their assignation to relevant 
measures, along with the measure of a general nature, in a manner available to the public (on the we b-
site www.cmi.cz).  
Compliance with notified standards or parts thereof is considered to be, to the exte nt and under the 
conditions stipulated by a measure of a general nature, to be compliance with those requirements 
stipulated by this measure to which these standards or parts thereof apply.  
 
 
II. 
J U S T I F I C A T I O N 
The CMI issues, towards the implem entation of Section 24c of Act No. 505/1990 Coll., on metrology, 
as amended, this measure of a general nature, stipulating metrological and technical requirements for 
specified measuring devices and test methods for verification of these specified measurin g devices.  8 In Decree No. 345/2002 Coll., specif ying measuring devices for mandatory verification and measuring 
devices subject to type approval, as amended, the annex "List of Specified Measuring Device Types" 
classifies " laboratory hydrometers with increm ents less than 1 kg • m -3 except for hydrometers for the 
measurement of soil granularity (Casagrande)"  under item 7.1.1 as measuring devices su bject to type 
approval and verific ation. 
The CMI thus issues, towards the implementation of Section 24c of Act No . 505/1990 Coll., on m e-
trology, as amended, for the measuring device type specified as "  laboratory hydrometers with incr e-
ments less than 1 kg • m -3 except for hydrometers for the measurement of soil granularity 
(Casagrande)" , this measure of a general nat ure, stipulating metrological and technical requir ements 
for laboratory hydrometers with increments less than 1 kg • m -3 except for hydrometers for the mea s-
urement of soil granularity (Casagrande)  and test methods for type approval and verif ication of thes e 
specified measuring d evices.  
 
III. 
N O T I C E  
A measure of a general nature cannot be appealed (Section 173 (2) of Act No. 500/2004 Coll., the 
Administrative Code, as amended (hereinafter the "Administrative Code").  
Pursuant to provisions of Section 172  (5) of the Administrative Code, a decision on objections cannot 
be appealed in any manner.  
The conformance of a measure of a general nature with legislation can be assessed in review procee d-
ings pursuant to Section 94 to 96 of the Administrative Code. A p articipant may submit a request for a 
review to the administrative body that issued this measure of a general nature. If the administrative 
body does not find reasons for commencing review proceedings, it shall notify the applicant within 
thirty days of th is fact along with its reasons. A decision on commencement of review proceedings 
can, in accordance with  Section 174 (2) of the Administrative Code, be issued within three years of the 
effective date of the measure of a general nature.  
 
 
 
IV. 
E F F E C T I V E  D  A T E 
This measure of a general nature takes effect on the fifteenth day following the day of its publication 
(Section 24d of Act No. 505/1990 Coll., on metrology, as amended).  
This measure of a general nature shall be published for a period of 15 days.