Proving ring is a force measuring instrument whose shape changes as diameter deflects elastically under load which is used as a device for calibration/verification of Uni-Axial Testing Machines. A force proving system consists of a steel ring with a Dial gauge mounted so as to measure the vertical deflection when the loads are applied through the threaded blocks at the top and bottom. Applied load is known from its load and deflection characteristics.
For all these years, the proving rings are considered as standard of excellence for force calibration. However, they suffer from the following adverse characteristics:
- Creep – If, we maintain the same force and continue to measure the length in the dial gauge having a very high resolution, we will see the length continues to change over time
- Deflection measurement – when the forces are applied to the proving ring, it departs from its circular shape and becomes egg- shape or elliptical. The determination of the deflection of a proving ring depends on the subtraction of two large numbers, namely, the inside diameter of the proving ring and length of the Dial gauge measurement assembly. Since the difference is so small, any slight error in measuring either dimension leads to a large percentage error in the number at interest, the deflection
- Resolution – Any mechanical measurement system introduces errors which are difficult to control or overcome. The most obvious problem is resolution, which is limited by the fitness of the Dial gauge threads and the spacing of the indicator marks. Non-repeatability of duplicate measurements taken in the same direction depends mainly how much force is applied to the Dial gauge’s screw threads, while hysteresis measurements taken at the same points from opposite directions is dependent on the preload, friction, and looseness in the plates
- Temperature effect – Variation in the temperature of either the steel ring or the Dial gauge assembly will cause expansion or contraction, which will result in a change in the deflection reading. Hence, proving ring is susceptible to temperature effect. Also, spring constant changes with temperature, thus change in the calibration
- Response to extraneous forces – The construction of a proving ring does not lend itself to the cancellation of extraneous forces such as side loads and moment loads. Any load other than a pure force through the sensitive axis of the ring, can result in an extraneous output
- Specially trained personnel – the proving ring requires specially trained personnel for proper operation, because of the possibility of errors introduced by creep, and it also subject to errors due to temperature and extraneous loads. Normally proving rings are calibrated in terms of divisions for specific forces and hence, he should have knowledge to convert division to unit of force or vice versa