4.Data Reduction

4.1Load Calculation When Reading Digits on the GK-502

The basic units utilized by geokon for measurement and reduction of data from Model 3000 Load Cells when read using the GK 502 readout box are 'digits'. The calculation of digits is based on the following equation:

Digits = mV/V × 4000

equation 1: Digits Calculation

Where:

mV is the output of the Wheatstone bridge circuit in millivolts.

V is the excitation supplied to the Wheatstone bridge circuit in volts.

Load is calculated from digits by determining a change in reading (in digits) and then multiplying by the appropriate calibration factor. See the following equation:

L = (R₁ − R₀) × G × CF

equation 2: Load Calculation Using Linear Regression

Where:

L is the load in lb or kg.

R₀ is the initial no-load reading.

R₁ is the current reading.

G is the linear gauge factor from the calibration sheet (see Appendix C).

CF is the conversion factor (optional) as listed in Table 1 below.

table 1: Engineering Units Conversion Multipliers

For example:

A Model 3000 load cell has an initial no-load reading (R₀) of 164 (regression zero reading can be found on the calibration sheet), a current reading (R₁) of 2050, and a linear gauge factor (G) of 148.7 lb per digit.

Inputting the values into Equation 2 gives:

L = (2050 − 164) × 148.7 = 280,450 lb

Note that the equations assume a linear relationship between load and strain readings, and the linear coefficient is obtained using regression techniques, which may introduce a substantial nonlinearity around the zero reading.

For greater accuracy, a polynomial expression to fit the data can be used:

L = AR₁² + BR₁ + C

equation 3: Load Calculation Using Polynomial

Where:

L is the load in lb, kgms, etc.

R₁ is the current reading.

A and B are the coefficients derived from the calibration data.

The C coefficient is derived by plugging the initial no-load reading, obtained in the field, into the polynomial equation.

For example:

A Model 3000 Load Cell has a current reading (R₁) of 2050. The polynomial coefficients, A, B, and C are −0.000286, 150 and −26,990, respectively. (C was derived by substituting the field no-load reading (R₁) = 180 into the polynomial equation while setting P to zero.)

Inputing the values into Equation 3 - Load Calculation Using Polynomial gives:

L = −1200 + 307,500 − 26990 = 279,300 lb

4.2Load Calculation When Reading the Output in mV/V on a Model P3 Readout Box

The load is calculated using the following equation:

Load = (M₁ − M₀) × J × CF

equation 4: Load Calculation Using mV/V

Where:

L is the load in lb or kg. or units chosen.

M₀ is the initial no-load mV/V reading.

M₁ is the current mV/V reading.

J is the mV/V gauge factor equal to the linear gauge factor G * 4000.

CF is the conversion factor (optional) as listed in Table 1.

In the example in the calibration sheet shown in Appendix C, J = 148.7 x 4000 = 594, 600 lb/mV/V.

Alternatively, the value for the Full Scale mV/V can be used following the P3 instructions.