Reliability of test results of BDV tester
Note There are five key areas that directly affect an instrument's ability to produce accurate test results. These are cleanliness, the method of breakdown detection, the speed at which the instrument switches off the test voltage after a breakdown, the accuracy of the voltage measurement and the accuracy of the electrode gap setting mechanism. The issues of cleanliness and output voltage verification have already been discussed; the remaining issues are dealt with here.
(1) Breakdown detection. Ensure that the instrument complies with the requirements of the test standard(s) that will be used.
(2) Voltage switch-off time. The faster the voltage is switched off after breakdown, the better. This is because arcing continues in the oil until the voltage is switched off, and arcing results in decomposition of the oil to produce by-products that can affect subsequent test results. For this reason, the IEC 60156 standard requires that the test voltage is switched off in less than 10 ms for mineral oil and in less than 1 ms for silicone oil. The ASTM standards require that no more than 20 mJ of energy is discharged into silicone oil, and a fast switch-off time is needed to achieve this.
(3) Electrode gap setting. An accurate electrode gap is extremely important for obtaining accurate test results. If the gap is too large, the test result will be too high and conversely if it is too small, the test result will be too low. The easier it is for users to adjust the gap, the more likely it is that it will be accurately set.
(4) Ease of gap adjustment. The most popular method for adjusting the electrode gap is with a thumbwheel. With this method, no tools are needed and it is easy to adjust the gap.
(5) Unwanted movement of electrodes. A common problem is that the electrodes move while the vessel is being handled or cleaned, resulting in an inaccurate gap setting. This is particularly true of instruments that have thumbwheel electrode gap adjustment as thumbwheels are easy to move accidentally. In addition, backlash in the adjustment mechanism can make movement of the electrodes inconsistent, which means that they are difficult to adjust accurately, and marked graduations are rendered inaccurate. Because of this, many test set users resort to checking the electrode gap every day or even before every test. Some older instruments provide screw clamp locking that is effective in preventing electrode movement, but it is hard to see and use.
