Indian Journal of Engineering & Materials Sciences
Vol. 3(,!June 1996, pp. 131-132
Note
Calibration of reference hydrometers using
solid density standards
o C Sharma & S Sinha
National Physical Laboratory, Dr K S Krishnan Road,
New Delhi I ro 012, India
Received 29 Septemher 1995; revised 29 January 1996
This note suggests the use of solid density standard to
calibrate reference standard hydrometers instead of conventional method of a calibrated open scale hydrometer.
This ensures better accuracy of measurement, ease of
operation and traceability in density measurement.
Reference standard hydrometers are calibrated by
most of the National laboratories using Cuckow
method I. These hydrometers are used to calibrate
laboratory grade hydrometers for routine applications. The process of calibration involves weighing of
the hydrometer in air and in a liquid of known density,
immersed to the level of graduation in question. The
hydrostatic weighing in the liquid is carried out in
temperature controlled bath which is normally
maintained at 20°C. The density of calibration
medium used for hydrostatic weighing of hydrometer
is determined by using an open scale hydrometer. The
use of solid density standard for determination of
density of calibration medium instead of an open
scale hydrometer is more practical. The open scale
hydrometer has to be calibrated in a medium of
known density by using a sinker whose volume is
determined by taking water as standard for density
measurement. Use of solid density standard would
remove all these anomalies and the density of the
calibration
medium is determined directly by
knowing mass of the solid density standard in air and
in the medium.
Solid density standard-Water
presents some
limitations as reference standard
for density
measurement and accuracy achieved? is about 3 ppm.
Solid density standards have been fabricated- whose
accuracy of density measurement has relative
uncertainty of 7 x 10-7• Solid spheres made of
quartz, Zerodur, silicon low expansion glass are used
as solid density standards':". National Physical
Laboratory, New Delhi maintains solid density
standard like other standards of physical measurement. Quartz serves as primary standard and
accuracy of measurement up to 2 ppm has been
achieved. All measurements are carried out at 20°C.
From primary standard, secondary standard of
quartz or low expansion glass in various sizes are
generated. One of the secondary standards of
diameter 25 mm was used as solid density in
calibration of reference standard hydrometers. A
special arrangement was developed for loading and
unloading the sphere using a suspension system for
hydrostatic weighing.
Experimental set-up-- The experiment set-up
consists of a thermostatic bath in which distilled
water.is filled. Xylene or some other liquid is taken in a
cylindrical jar and dipped in thermostatic bath which
has both heating and cooling arrangements.
HOOJED
TO
BALANCE
11
\
10
Fig. I-Suspension
system for solid density standard and
hydrometer [1. Micrometer head, 2. L N screw, 3. top plate,
4. transfer pan, 5. lock skew, 6. clamp for SOS, 7. solid density
standard, 8. support for SOS, 9. reference standard hydrometer,
10. clamp for hydrometer, and II. up-down motion assembly]
132
INDIAN 1. ENG. MATER. SCl.,JUNE
Temperature within the jar is maintained at 20°C with
a variation of ± 0.0 I°C using temperature regulator.
Mass measurement is carried out with a Mettler
balance H-315 in which arrangement for loading
below the pan has been provided. The resolution of
balance is 100 ug and capacity I kg. A suspension
system fixed on the cover of the jar helps to load and
unload the density standard on the balance. A
support which carries the solid density standard may
be moved up and down by operating a micrometer
head and places the density standard on a triangular
support connected to the balance. When operated
down, the solid density standard is made to rest on
triangular support, thus loaded on the balance and
mass measurement may be carried out. After
measurement is over, it may be lifted up thus
unloading the balance. The bath is movable as it is
mounted on parallel horizontal rails. The top of the
stem of hydrometer is gripped in a specia\1y designed
clamp to sink the hydrometer to a desired graduation.
The whole assembly is given in Fig. I.
Measurements-First of all bath position is set and
triangular support system is connected to balance.
The standard is loaded on the balance and its mass is
measured in the calibration medium. The standard is
then unloaded from balance. The balance is now
disconnected with suspension system and bath is
moved and hydrometer is connected to balance
through its own suspension arrangement. The
hydrometer may be immersed to any defined mark
using this special suspension device which may be
moved up or down. Thus mass of the hydrometer up
to the given mark of immersion in the medium is
determined. From mass of solid density standard in
air and calibration medium, the density of calibration
1996
medium is first determined. From known density of
medium the value of density corresponding to the
mark of hydrometer up to which ithas been immersed,
may be calculated as per standard procedures.
Conclusion-The
concept of using solid density
standard to evaluate the density of liquid for
calibration of reference standard hydrometer is more
promising as solid density standards provides much
higher accuracy of measurement than water as
density standard. The calibration of open scale
hydrometer makes use of sinker and density of the
calibration medium thus determined have an
accuracy of only few parts in IOs. By using solid
density standard, one can determine the density of
calibration medium directly with accuracy in ppm
range. Besides, it provides ease in operation of system
and whole calibration process is less time consuming.
Traceability which is now-a-days a much sought
after requirement, results directly by the use of solid
density standards and eliminates the need for
preparation and maintenance of very pure samples of
water as density standard.
Acknowledgement-The
authors wish to thank
Prof. E S Raja Gopal, Director, NPL, for his
continuous encouragement and interest in this
work.
References
1 Cuckow F W. J Sac Chern Ind, 68 (1948) 44.
2 Sharma D C, BhamraS S& MathurB G, MAPAN. 7 (1992) 3.
3 Bowmon H A. Schoonover R M& Carroll C L. J Res Nat Bureau
Standards. 78 (1974) 13.
4 Spickweck F. Kozdon A. Wagenbreth H. Toth H & Hoburg D.
PTB Mitt, 100(3)(1990) 169.
5 Sharma DC & Sinha S, Indian J Eng Mater sn, 2 ( 1995) R9.