Where to find A B C calibration factors for polynomial equation, G factor for Linear equation & K factor for Period units.

1. Introduction

There are various different equations available to achieve the required engineering units from a Vibrating Wire Piezometer. To complete these equations require to use different factors that can be found on the instruments calibration certificate (see Figure 1.).

Figure 1. Where to find various values on a calibration certificate

2. Formulas and the factors required for data reduction

The following three formulas will allow you to convert the readings into a kPa measurement:

• Calculation using ‘Period’ units.

 E = K(10^7/P0^2–10^7/P1^2)

Where,
E is the Pressure in resultant Engineering units,
K is the Period Gauge Factor for units of calibration from the instrument’s calibration sheet,
P0 is the Period ‘base’ or ‘zero’ reading
P1 is the current Period reading.

• Calculation using Linear units.

E = G(R0–R1)

Where,
E is the resultant Engineering unit,
G the Linear Gauge factor for the units of calibration from the instrument’s calibration sheet,
R0 is the Linear ‘base’ or ‘zero’ reading
R1 is the current Linear reading.

• Linear unit calculation using a Polynomial equation.

Linear units maybe applied to the following polynomial equation, for calculation of Engineering units to a higher order of accuracy.

E = AR1^2+BR1+C

Where,
E is the resultant Engineering unit,
A, B and C are the Polynomial Gauge factors A, B and C, from the instrument’s calibration sheet,
R1 is the current Linear reading.

The value C is an offset value and relates to the atmospheric pressure experienced by the Piezometer at the time of calibration.
This figure will have changed at the time of installation due to changes in altitude or barometric pressure, so C should be re-calculated at the installation time as follows:

C = -(AR0^2+BR0)

Where,
  A and B are as above,
R0 is the Linear ‘base’ or ‘zero’ reading.

Please note that the sign of the re-calculated value of C, should be the same as the original value of C, so if the original is negative then the recalculated value should also be negative.

For further information on the formulas for Vibrating Wire Piezometers please refer to the Vibrating Wire Piezometers manual:

http://www.itmsoilsupport.com/manuals/Man106_Standard_Vibrating_Wire_Piezometer.pdf

http://www.itmsoilsupport.com/manuals/Man033_Vibrating_Wire_Heavy_Duty_Piezometer.pdf

If this has not solved your issue please submit a ticket through the support site.

Checked JK, v1.0, 18/02/12