AdMet 2012 Paper No. PM 005
Latest Calibration Standards & Calibration Methods
for Pressure Balances
Nitin Kshirsagar
E-mail : nkshirsagar@wika.co.in
Abstract : This paper gives information on new developments of calibration standards
and modern equipment for calibration of Pressure Balances by primary (Cross Float
with Effective area calculation and Mass calibration together) method. Calibration
of Pressure Balance (Dead Wt. Tester) is always a tough task as the device itself is
influenced by various parameters such as environment conditions, local gravity. As the
pressure balances are manufactured to the tune better than 50 ppm uncertainties in
the Industrial Class now, calibration methods requires more precise set up to capture
the influences and better reference standards for calibration. European Union has
come out with a standard documents stating detail calibration method as described
above. Also basic standards by OIML for pressure balances and OIML standards for
mass calibration are considered in the method. Since such type of pressure balances
are used more and more by customers - need to calibrate these with use of precise
calibration methods will be a demand. WIKA as leading manufacturer of Pressure &
Temperature measuring Instruments has set up a concept of Center of Excellence (COE)
for Calibration Technology and continuing efforts to develop equipments for precision
calibration. WIKA has developed a special type Cross Float system with software to
capture the influences for this type of precision calibration. Features of the method and
equipment are described in detail in this paper.
Keywords: Pressure Balance, Cross-Float, EAL, OIML, uncertainty, DUC, REF, CrossFloat
1. INTRODUCTION
value of pressure, it is challenging to capture
the factors during calibration of Pressure
Balances as one pressure balance is under
calibration and standard used is also pressure
balance having better uncertainty or at least
equal to that of balance under calibration.
Principal of Pressure Balance
The pressure balance consists of a vertical
piston freely rotating within a cylinder. The two
elements of good machined quality define a
surface called ‘effective area’.
2. MANUSCRIPT
The pressure to be measured is applied
to the base of the piston creating an upward
vertical force. This force is equilibrated by the
gravitational downward force due to masses
submitted to the local gravity and placed on the
top of the piston. The piston is a part of the load.
Since lot of factors are affecting the corrected
Methods to apply
Two methods that follow are comparative
ones, consisting of comparing the balance
to be calibrated and the standard instrument
when both are submitted to the same pressure
and the same environmental conditions.
1
However, dependent on the presupposed
accuracy of the balance to be calibrated, and
according to customer requirement alternative
methods may be used:
Method A - Generated pressure method
The scope of this method is to determine
the bias error and the repeatability of the
calibrated pressure balance. This is done
by determining the generated pressure
corresponding to well identified weights. In
that method the weighing of the masses of the
instrument under calibration is optional.
Method B - Effective area determination
method
The scope of this method is to determine:
(a)
(b)
(c)
the value of the mass of all the weights,
including piston of the pressure balance
if removable.
4. ILLUSTRATIONS
For Pressure Balances, who are equipped
with piston-cylinder-systems for low and for
high pressure or with demountable pistoncylinder systems, should be conducted the
complete calibration process for each pistoncylinder system.
the effective area Ap referred to 20
°C of the piston-cylinder assembly of
the pressure balance as a function of
pressure. At high pressure, this area can
be expressed from the effective area
at null pressure A0 and the pressure
distortion coefficient.
Three to five measurement series must
be done with minimum six pressure point per
series, who are distributed over the complete
area of the dead weight tester .For the dual
area dead weight tester should be considered
minimum five pressure points for each area.
The pressure points should be chosen over
the area of the tested system (generally 1/10
and 10/10 of the highest pressure range).
The repeatability as a function of the
measured pressure.
Method A is usually not employed where
the smallest uncertainty is required.
3. EQUATIONS AND SYMBOLS
The measured pressure in-bar- from the
calibrating DWT can be calculated by using of
following equation.
Cross Float System
2
The effective area A0 of the piston will be
evaluated by the cross-float method (see pic
above), as the both Pressure Balances will
be by applying fine masses in balance. The
characteristics value of the profile is important
for the customer, so that he can choose the
application of the calibrated Pressure Balance
as reference standard at the accurate pressure
value in the system and with a high accuracy
of different calibration objects.
12. Software based uncertainty reporting
programs.
13. Comprehensive calibration reports.
4.1 Figures and Photographs
Effective area will be detected for each
piston of the Pressure Balance, which are of
different pressure range.
It is challenging task to capture the
influences as shown in the equations, as
smallest possible difference can cause
increase in the uncertainty.
Calibration Bench
WIKA Calibration bench is equipped
with following facilities to capture these
influences precisely during :
1.
Built in Pt 100 sensors for pistons of
DUC & REF
2.
Ultrasonic height sensors with display on
screen to indicate height of DUC & REF
side by side windows
3.
Mirrors for float level indication of DUC &
REF
4.
Digital indicators for atmospheric
pressure, Humidity & Room temperature.
5.
Patented software for automatic entry
of the influencing parameters as well as
mass values.
6.
Mass Comparator
4.2 Tables
Calculation of distortion coefficient λ and
effective are A0 at null pressure.
Special adaptors to mount the pistons of
DUC directly on the port so that height
difference is minimized.
7.
Count down within software for Spin rate
& piston Fall rate.
8.
Precise Fine
adjustment.
9.
Sate of the Art Mass calibration facility
up to F1 class masses.
control
for
The series measurements of pressure can
be fitted as linear – A(p)= A0(1+λp) and can
be calculated from regression of pressure
values p and area values Apj
pressure
10. Round the clock climatized calibration
rooms
11. Highly trained calibration professionals
with practical as well as theoretical
knowledge.
3
5. REFERENCES
Calibration and characterization is
complete after calculation of effective area A0
and distortion coefficient and final calculation
of uncertainty.
4
1.
EAL-G26Balances
Calibration
of
Pressure
2.
OIML R110- Pressure Balances
3.
OIML R111-1- Mass Calibration
4.
WIKA Laboratory- DKD-Germany COE
Literature
5.
Euramet cg-3