With the advent of electronic balances, weight measurement would seem to be a straightforward task. However, poor weighing technique can greatly contribute to the “Uncertainty of Measurement,” (combined weighing errors), causing users to inadvertently use their balances incorrectly.
In order to improve your own weighing skills, here are some helpful hints that will improve your weighing accuracy:
Before weighing ensure your balance is leveled correctly. When the balance level is changed, the metrology of the balance is affected. Checking the level of the balance shows if the balance has been disturbed or moved.
Balances with automatic internal adjustment should have this function switched on. This will take care of any changes due to environmental factors, (drift). However, this does not excuse a user from the act of regularly calibrating the balance with a certified weight to prove traceability of measurement. These two actions, combined with regular preventative maintenance technical service, will give a sensible, three-pronged approach to accurate weighing.
Keep your balance clean at all times. Visually check for debris spillage prior to use and, if spotted, clean using a balance brush. A damp tissue can be used to remove encrusted spills if necessary. This is not just a courtesy to others nor conformance to regulations. Powder and debris can travel around the balance with the air flow. Cleaning keeps your balance working accurately by eliminating unwanted ingression that could damage the internal mechanical components of the balance.
When handling your sample, never use your hands to place tare weights or samples in the weigh chamber. The heat from your hand can cause weighing errors in the fourth and fifth decimal places of a gram. Use appropriately sized and shaped tweezers or tongs to handle your weighing vessels.
Use vessels of an appropriate size and material for your samples. For example, plastic weigh boats are a source of static, and a 180-g glass beaker has too large an uncertainty to be appropriate for weighing 100 mg of sample.
When placing items to be weighed onto the weigh pan, open the draft shield door only on the side to which you are weighing, i.e. if you are right handed, open the right door.
Understand how your particular model of balance indicates a stable weight (i.e., gives a weight that you can safely trust as a result).
All electronic balances give a visual indication of weight stability. Before you begin weighing on an analytical balance, close the draft shield completely and zero using the "0" or "Tare" key on your balance display. This will clear any previous tare or cancel any zero-drift. When beginning your sample weighing, it is normal to tare (zero out) your vessel or sample container. This should be done with the same precision as if you were taking the sample weight itself. Using tweezers, place your tare weight onto the weigh pan, close the door, press the tare button, and wait for the balance to give a stable zero.
Introduce your sample into your weighing vessel using the appropriate spatula, tweezers, etc. As before, weigh from one side only, keeping your hands out of the weigh chamber.
When placing objects on the balance weigh pan, aim for the same place each and every time, i.e. try to aim for the middle of the pan each time you weigh. This cancels out any effects caused by eccentricity (corner load error).
Be aware of how your balance is affected by your working environment. Modern, busy labs are not ideal places for the analytical balances we utilize. Even when your balance is correctly filtered for your environment, the balance will still be working hard to stabilize itself. The net result is that a five-place weighing may take as long as 45 to 60 seconds to reach stability in a large, modern laboratory.
Environment is also an important consideration when setting tolerances for checking and testing your balances. The specification of the balance is not the only factor in the uncertainty budget. Remember to allow for the uncertainty caused by your lab environment, as well as the uncertainty in your test weights. In reality, the balance tolerance in your place of use may be larger than the manufacturer’s specifications by a factor of 2 to 3.
Once again, when you finish weighing, check that the weigh chamber is clean and free of any spillage.
These simple tasks will help maximize your measurement scheme and make the most of an electronic balance.