# Force measurement

**BS 8422**-2003 pdf free.Force measurement 一 Strain gauge load cell,systems- Calibration method.

7.1.1 Position the loading fittings to ensure axial force application.

7.1.2 Carry out a pre-load in accordance with 4.7.

7.1.3 Record the zero force output.

7.1.4 Apply a series of at least five, substantially equally spaced, increasing forces from zero force up to and

including the maximum calibration force.

7.1.5 At each force, record the output after a period of not less than 30 a.

7.1.6 Were decreasing forces are to be applied, reduce the applied force to zero force, pausing at the values of force which had been applied during the increasing series and recording the output as specified in 7.1.5. 7.1.7 Record the final zero force output.

7.1.8 Repeat the operations specified in 7.1.3. 7.1.4. 7.1.5, 7.1.6 and 7.1.7 twice to give three series. Between successive series, rotate the load cell and its associated fittings through 120° about its principal axis or, if this is not possible, through 1800. After each rotation. pre-load the load cell at least once to its maximum calibration force.

7.2 Shunt resistor output

NOTE If the load cell is provided with a built-in shunt calibration facility, this may be used to determine its efitet on the output when activated.

7.2.1 If shunt resistor output measurement is required by the customer, carry out the operations specified in 7.2.2. 7.2.3 and 7.2.4 immediately after carrying out the operations specified in 7.1.

7.2.2 Record the zero force output.

7.2.3 Activate the shunt calibration facility and record the output.

7.2.4 Report the difference between the output with the shunt calibration activated and zero force output.

NOTE 1 This difference is sometimes known as the Caicheck figure.

NOTE 2 This difkrenct’ may also be used to compute the force that would produce the output.

7.3 Calculation of non-linearity

7.3.1 Calculate the mean deflection (3.1.7.1) for each increasing calibration force and record these values

on the calibration certificate (see Clause 23).

7.3.2 Using the method of least squares, compute the coefficients of a polynomial equation giving deflection as a function of applied force and record these values on the calibration certificate (see Clause 23).

NOTE I This is generally a second order equation, but higher orders arc permitted if statistically supported by the data (e.g. by use of the F test: see ASThI E74). A linear fit may also be used if requested by the customer.

NOTE 2 Many c1.stomers will find the coefficients of an equation giving force as a function of measured deflection equally valuable. and ii is recommended that these coefficients are also recorded on the calibration certificate.

7.3.3 For each calibration force, calculate the difference between the actual load cell deflection and the value computed from the equation.

7.3.4 The non-linearity shall be taken as the maximum of these differences, expressed as a percentage of maximum deflection, and shall be recorded on the calibration certificate (see Clause 23). together with information as to the order of fit and whether it has been forced through zero.

7.4.1 Calculate the maximum difference between the three deflections at each increasing calibration force.

7.4.2 The reproducibility shall be taken as the maximum of these differences, expressed as a percentage of maximum deflection, and shall be recorded on the calibration certificate (see Clause 23).

NOTE lithe load ccli has previously been calibrated in accordance with this British Standard, it is permissible tocarry out the three loading runs at the same orientation. The above calculation will therefore result in the maximum repeatability for the calibration forces (as specified in Clause 8). rather than the reproducibility, and this should be recorded on the calibration certificate. together with the previously obtained reproducibility figure.

8 Supplementary calibration A — Repeatability

8.1 Position the loading fittings to ensure axial force application.

8.2 Carry out a pre.load in accordance with 4.7.

8.3 Record the zero force output.

8.4 Apply the selected force to the load cell and maintain for a period of not less than 30 s. Record the output. Calculate the load cell deflection.

NOTE For the determination of repeatability, the selected force is normally the maximum calibration force, but the repeatability may also be determined at any other force.

8.5 Repeat 8.3 and 8.4 to give three applications of the selected force.

8.6 Calculate the maximum difference between the three load cell deflections obtained at the selected force.

8.7 The repeatability at this force shall he taken as this maximum difference, expressed as a percentage of maximum deflection. It shall be recorded on the calibration certificate (see Clause 23) together with the selected force value.

9 Supplementary calibration B — Hysteresis

9.1 Carry out the standard calibration procedure specified in 7.1, with the exception of the rotation of the load cell between calibration series as specified in 7.1.8 (see Note).

NOTE The rotation of the load cell between calibration series may be carried out but is not essential for the purposes of determining the hysteresis.

9.2 Calculate the average value of load cell deflection for each increasing and decreasing calibration force.

9.3 For each calibration force, subtract the load cell deflection for increasing force from the deflection for decreasing force.

9.4 The hysteresis shall be taken as the maximum of these differences (when expressed as absolute values), expressed as a percentage of maximum deflection, and this value shall be recorded on the calibration certificate (see Clause 23).

10 Supplementary calibrations C — Creep and D — Creep recovery

NOTE 1 A graphical representation of creep and creep recovery measurement is shown in Figure 1.

NOTE 2 In order to obtain reliable data from creep and creep recovery measurements, it is important to have determined the calibration machine loading characteristics and to take the outputs of the load cell at precisely defined time intervals This is of paramount importance if the data is to be used for comparison purposes.

10.4 Apply the maximum calibration force, immediately record the initial force output, and record the reading thereafter at intervals not exceeding 5 mm. over a period of not less than 30 mm. Subtract the initial force output from each subsequent output. Express these differences as percentages of maximum deflection and record them on the calibration certificate (see Clause 23). The creep shall be taken as the maximum difference (when expressed as absolute values) obtained within the first 30 mm.

10.5 Remove the force, immediately record the initial zero force output, and record the reading thereafter at intervals not exceeding 5 mm, over a period of not less than 30 mm. Subtract the initial zero force output

I from each subsequent output. Express these differences as percentages of maximum deflection and record them on the calibration certificate (sec Clause 23). The creep recovery shall be taken as the maximum difference (when expressed as absolute values) obtained within the first 30 mm.**BS 8422 pdf free download**.Force measurement