subject: How to Properly Clean and Care for Your Analytical Balance [print this page] How to Properly Clean and Care for Your Analytical Balance
As with all precision instruments, the final objective of care and maintenance is to ensure that errors in measurement are obviated to the extent possible. Errors can be either systematic or random. Systematic errors can either be caused by a defect in the scale itself, or by the way it is operated. A systematic error results in readings that always deviate in one direction. For example, an incorrectly calibrated scale may continuously give readings that deviate, however slightly, above the true values.
Systematic errors may be difficult to spot. The trouble with them is that they do not average out. Random errors deviate in both directions and a large number of readings will average out this type of error. For example, when the same sample is weighed over and over again, it produces slightly different results. Using statistical methods will minimize the impact of random errors.
Caring and maintenance of analytical balances involve the following major steps.
Keep the weighing plate and weighing area clean. This usually entails nothing more sophisticated than using a fine brush (generally provided with the scale) to dust away any spills or residual material in the weighing chamber.
Regularly confirm that all control functions are working smoothly.
Test and confirm Repeatability. When the same load is placed more than once onto the weighing pan of a lab balance, it should give identical readings; but there is usually a very small variability. The difference between the largest and the smallest result for a specified number of measurements is known as the standard deviation. To carry out the test, select a single test weight of nominal value between 50% and 100% of the weighing capacity of the instrument. Start with the weighing pan empty and reset the reading to zero. Alternately you can place and remove the test weight ten times. If the variation between the upper and lower readings is less than the allowed tolerance, then the instrument passes the repeatability test.
Test and confirm Cornerload . You can use the same test weight as for the Repeatability test. First, place the test weight in the center of the pan and adjust the reading to zero. Then move the test weight so that its center is half way from the center of the pan to the periphery at the front, right, rear, and left.
Test and confirm Linearity. Select two weights of standard nominal value close to one-half the weighing capacity of the balance. Label the test weights as A and B. Take and record a reading with weight A on the weighing pan. Then, replace weight A with B, and adjust the reading back to zero. Take one more reading with both weights A and B on the weighing pan. The readings with just weight A on the pan and subsequently with both weights should match within the linearity tolerance.
Finally, as has been mentioned in related articles, always keep in mind the basic precautions. Remember to recalibrate your balance every time you move it to a new location, whenever it is subject to a significant change in ambient temperature and whenever it has been disconnected from the mains supply for a significant period.
