Force Measurements
The SI unit of force is the newton, N, which is derived from the SI base units of mass (kilogram), length (metre) and time (second). Newton is the force that, when applied to an object with a mass of one kilogram, would give it an acceleration of one metre per second per second.
In practice it is not convenient to create an acceleration of one metre per second per second on an object having a mass of one kilogram in order to realise the measurement standard of force of one newton. Instead the practical realisation of the unit of force makes use of known masses (deadweights) which will produce a known force when subjected to the effect of local acceleration due to gravity. Machines which generate force in this way are called deadweight machines.
The Force Laboratory is equipped with four force measurement standard machines. The two smaller machines are deadweight machines of rated capacities of 5 kN and 60 kN respectively. The two larger machines, which work on hydraulic amplification principle, generate forces up to 3 MN by amplifying the force generated from the 60 kN deadweight machine.
The Force Laboratory provides calibration services for force measuring instruments such as proving rings and load cells in the measuring range from 0.5 kN to 3 MN. Calibration of force measuring instruments of measuring ranges from 0.5 kN to 60 kN will generally be performed by using the two deadweight machines which are capable of generating forces with an measurement uncertainty of ¡Ó0.002%. For force measuring instruments of measuring ranges from 60 kN up to 3 MN, calibration will normally be performed by using the two hydraulic force measurement standard machines. The measurement uncertainties of the forces generated by these machines are ¡Ó0.02% and ¡Ó0.05% respectively. The measurement uncertainties quoted on the calibration certificate will include the measurement uncertainties due to the instrument under test itself during the calibration.
Calibration of force measuring instruments would normally involve the following tests :
Compression and/or Tension Tests
Calibration is based on clause 6.4.2 of BS EN 10002-3 : 1995, such that four series of calibration forces are applied to the force measuring instrument for each operation mode (i.e. tension or compression). For each measuring series, the forces are applied in sufficient number of equal steps (normally 10) distributed evenly throughout the usable range of the force measuring instrument.
Overloading Test
Prior to the calibration, an overloading test will be performed in accordance with clause B.1 in Annex B of BS EN 10002-3 : 1995, such that the force measuring instrument is subject 4 times in succession to an overload of about 110% of its capacity. The charge for the overloading test is included in the charge for the tension or compression test.
Bearing Pad Test
The bearing pad test and variable voltage test may also be offered upon request from the client. The bearing pad test, which applies only to compression force measuring instruments, is for assessing the interaction between a force measuring instrument and its support on the force measurement standard machine. This test will be performed in accordance with clause B.2 in Annex B of BS EN 10002-3 : 1995, and is recommended only for new instrument or instrument after repair.
Variable Voltage Test
The variable voltage test is applicable to force measuring instruments requiring electrical supply for operation as mentioned in clause 6.1.3 of BS EN 10002-3 : 1995. The test is to verify that a variation of ±10% of the supply voltage has no significant effect on the force measuring instrument. In this test, the change in the readings of the force measuring instrument under test will be reported when the supply voltage is varied between ±10% of the rated voltage. The applied force during this test will be the rated capacity of the force measuring instrument under test.
Calibration charges
For force measuring instruments in measuring range up to 60 kN
| Fee in HK$ | |
|---|---|
| Tension test including overloading test. | $4080 |
| Compression test including overloading test. | $4080 |
| Bearing pad test. | $2380 |
| Variable voltage test. | $1020 |
For force measuring instruments in measuring range above 60 kN up to 3 MN
| Fee in HK$ | |
|---|---|
| Tension test including overloading test. |
$4760 |
| Compression test including overloading test. | $4760 |
| Bearing pad test. | $3060 |
| Variable voltage test. | $1360 |
Enquiry Telephone: (852) 2829 4835, (852) 2798 7347
e-mail: tkchan@itc.gov.hk
