Fill - Form 1: CALIBRATION OF HYDRAULIC FORCE MACHINES REQUIREMENTS,

XIX IMEKO World Congress

Fundamental and Applied Metrology

September 6-11, 2009,Lisbon, Portugal

CALIBRATION OF HYDRAULIC FORCE MACHINES – REQUIREMENTS,

CONCEPTS, PROBLEMS, SOLUTIONS

Boris Katz

, Peter Kornhauser

, Shlomi Bitas

P. K. Calibration & Consulting Labs, Tefen, Israel, boris.katz@pk-labs.com

P. K. Calibration & Consulting Labs, Tefen, Israel, peter.kornhauser@pk-labs.com

Beton Daruch, Natanya, Israel, betondar@bezeqint.net

Abstract – Modern building technologies (for example

prestressed concrete to produce beams, floors, bridges, or

installation, testing and monitoring of ground anchors)

require the use of accurate force machines (hydraulic jacks).

One component of successful building works is a correct

and accurate calibration procedure of hydraulic jack for

tensioning cables or anchors. This article analyzes two

different calibration methods: By means of hollow load cell

and thread bar and calibration in the closed frame.

We analyze these methods using different accessories

and their influence on the uncertainty of calibration results.

As a very important part of calibration procedure is a correct

choice of the calibration mathematical model (interpolation

curve force-hydraulic pressure and hydraulic pressure-

force), there is a need to use a special software. Such is the

developed software FORCE-401-S which permits to

communicate the measurement line “load cell-amplifier-

computer”; to choose the correct interpolation polynom;

enable to compute the errors and uncertainty values and

build the table of calibration results.

The calibration procedure is performed according to

the standard ISO 7500-1 "Verification of static uniaxial

testing machines - Part 1: Tension/compression testing

machines - Verification and calibration of the force-

measuring system". We recommend to widen the definitions

of this standard and to include calibration of non-force units

scaled devices (as an example pressure gauges).

Keywords: force calibration, interpolation, uncertainty.

1. FORCE MEASUREMENT-METHODS,

ACCESSORIES

Building technologies requires very wide range of

hydraulic jacks both in terms of capacity (from 2 ton force

to 500 ton force) and working prestressing technology (one

steel cable, some steel cables, thread bar).

Calibration of jacks with loading force capacity up to

20 ton performed by means hollow Load Cell C6A HBM

and set accessories: steel cable

2 as a loading element,

spherical cap

3 and centering elements 4 (Fig. 2). For the

calibration of jacks with loading capacity up to 100 tons

used the same measurement method, but instead of steel

cable, the thread bar 5 used as a loading element.

The spherical cap and two accessories 4 ensure coaxial

loading, except non-coaxial forces and reduce the

measurement uncertainty. According to our research the

uncertainty values are in Table 1.

Fig. 1. Hydraulic Jack.

Pressure gauge

(reading device)

Hydraulic pump

Hydraulic jack for one steel cable

Hydraulic jack for some steel cables

244

Fig. 2 Calibration by means of steel cable. Fig. 3 Calibration by means of thread bar.

Table 1. Uncertainty values.

Calibration with spherical cap Calibration without spherical cap

Uncertainty

%6.0%2.0

%2.1%5.0

*of reading value

T Calibration of jacks with loading capacity of up to 500

tons performed by means different methods:

Loading and measuring in horizontal closed frame (Fig.

4):

1 is an horizontal frame, 2 is an hydraulic jack, 3 is a

Load Cell;

Loading and measuring by several steel cables (Fig. 5):

are steel cables,

2 is an hydraulic jack, 3 is a hollow Load

Cell;

Loading and measuring in vertical closed frame (Fig. 6):

is a vertical frame, 2 is an hydraulic jack, 3 is a Load Cell

21 321

Fig. 4. Loading and measuring in horizontal closed frame. Fig. 5. Loading and measuring by several steel cables.

245

Fig. 6. Loading and measuring in vertical closed frame.

(a) Disadvantages of this method are:

• A long preparation time of calibration system;

• A special form of spherical cap is required;

• Difficulties of coaxial mounting of all elements

(jack, Load Cell and spherical cap);

• Low measurement uncertainty values.

• Due to that, this the method is practically unused.

(b) Substantial advantages of this method are:

• The calibration process is similar to prestressing

working process;

• The hollow form of Load Cell allows for

performance calibration without special accessories

and ensures coaxial loading of all elements of

measurement (calibration) system.

• The disadvantage of this method is a low accuracy

of the hollow Load Cells.

• A short preparation time of calibration system;

• The possibility of using relatively high accuracy

Load Cells;

• Relatively high measurement uncertainty.

246

2. CALIBRATION RESULTS-

INTERPRETATION

One feature of prestressing machines (hydraulic

jacks) is their inability to read the force value (the reading

device is pressure gauge) –due to that, the calibration results

presented in an "applied force-reading" form. In this

situation, error accuracy estimation is problematic (we

cannot compare of force values with pressure gauge reading)

and the classical formulae of deviation value is not used.

For solve this problem, P.K. Labs developed the

software FORCE-401-S which according to calibration

results grants the "Least Sum of Squire" method to build the

analytical curve

)(RfF = , where F -is the applied force

and

- reading of the pressure gauge and computes the

relative interpolation error

q as an accuracy estimation:

%100⋅

−

where

F - is a theoretical (computed) force value from

interpolation equation;

F - is an applied (measured by means of Load Cell)

force.

The software includes four options of choice of the

interpolation curve:

1. Interpolation polynom of

1 degree: baRF

;

2. Interpolation polynom of

2 degree:

cbRaRF ++=

;

3. Interpolation polynom of

3 degree:

dcRbRaRF +++=

;

4. The software automatically chooses the degree of

interpolation according to minimal interpolation

error.

It is understood that each calibrated device has

different technological and technical deviation, such as

friction between piston (rod) and cylinder, high temperature

of oil in the hydraulic system, roundness deviations of

piston and cylinder and its fitness. All of these factors

influence the values of interpolation coefficients. Our

experiments confirm that calibration curve doesn't intersect

the "zero point" of coordinate system "force-pressure".

Fig.7 demonstrates different forms and characteristics of

parabolas.

The character of interpolation curve and his location

requires:

1. Defining the correct lower limit of the

measurement range;

2. Adding to the calibration table of two

interpolation equations:

(

)

RfF = and

inverse function

()

FfR = .

Fig. 7. Interpolation curves.

247

3. CALIBRATION CERTIFICATE -

STANDARDIZATION

The International standard ISO 7500-1:2004 defines

the calibration requirements of force testing machines, as a

scale reading device which is expressed in units of force.

Based on our research and work experience we recommend

to include to the International Standard ISO 7500-1:2004

also requirements for prestressing machines (hydraulic

jacks), where the reading scale device is expressed in units

of pressure

and to classify accuracy classes.

The following tables (as an examples) demonstrate the

calibration table according to this standard (accuracy class 1

on Fig.8 and accuracy class 3 on Fig.9).

Fig. 8 Calibration results (example).

Fig.9 Calibration results (example).

248

4. CONCLUSIONS

1. P.K. Calibration laboratory has the technical,

metrological and mathematical base to perform

calibration of hydraulic jacks of different load ranges.

2. Calibration certificate of hydraulic jacks contains the

measurement results table and two Interpolation

equations:

• force as a function of pressure;

• pressure as a function of force.

3. The calibration results table is built according to the

definitions and requirements of the International

Standard ISO 7500-1:2004.

4. Our recommendation is to extend the International

Standard ISO 7500-1:2004 for non–force units scaled

devices (as an example pressure gauges) and to classify

prestressing machines (hydraulic jacks) for accuracy

grades according to this standard.

REFERENCES

[1] ISO 7500-1:2004 "Verification of static uniaxial testing

machines - Part 1: Tension/compression testing machines -

Verification and calibration of the force-measuring system".

[2] B. Katz, L. Anavy, I. Nehary "The calibration system of

force measurement devices – conceptions and principles",

Proceedings of the 19

International Conference "Force, Mass

and Torque Measurements; Theory and Application in Laboratories

and Industries", Cairo, 2005 .

[3] L Anavy, B. Katz "Uncertainty and Interpolation", Proceedings

of the 19

International Conference "Force, Mass and Torque

Measurements; Theory and Application in Laboratories and

Industries", Cairo, 2005.

249

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