Laboratory balances are precise instruments that are capable of measuring items weighing less than one ten-millionth of a gram. With such small tolerances, it is important that users understand the need to provide stable operating conditions; otherwise, the benefit of using highly precise balances is lost. Below is how to ensure that your balance is providing you with the most accurate readings possible:
Provide a firm foundation
The finest laboratory balance in the world will do you little good if it doesn't rest on a solid, immovable surface. If you are going to spend thousands of dollars on a precise balance, you should also invest in a balance table. Balance tables are also known as anti-vibration tables, and their job is simple--reduce as much vibration and movement as possible. They are constructed of heavy materials, such as marble or granite, and they provide an extremely stable platform for your balance.
Shield from air currents and drafts
Very precise lab balances capable of measuring in the thousandths of a gram are sensitive to air currents within the immediate environment. You can easily counteract this problem by placing the balance within an enclosed container or one that has at least all sides protected. Be careful where you place the balance so that it does not rest under a vent or near a source of moving air such as a computer fan.
Keep your balance clean
As with air currents, lab balances that measure the mass of tiny objects are capable of being influenced by the weight of dust, moisture and other contaminants. Keep your balance shielded within a dry, dust proof case when not in use, and be sure that the weight pan is completely clean before using. A non-lint cloth such as a microfiber towel is a good choice to use for gently wiping down your balance.
Be sure your balance is accurate
Once you know that your lab balance is stable, clean and clear of drafts, you can conduct an accurate test of its capabilities. You must be completely confident in its measurements; these dimensions represent the areas that you will need to evaluate to ensure a correct reading:
- Span - the first dimension of balance accuracy is determining if an instrument is finding the true mass of a given object. For example, if you measure the mass of an object at 24.32 grams, but you know the actual value is 23.78 grams, then there is a problem with the span of the balance.
- Linearity - this dimension measures if balance readings are consistent across the scale range. An an example, if you are able to measure an object with true accuracy at 0.005 grams, but you find that the measurement is off when you weigh an object of 2.25 grams, then the linearity of the balance is suspect. A well-functioning balance should accurately measure the mass of an object at any point between the minimum and maximum ends of the scale.
- Repeatability - this dimension measures the consistency of measurements from various points in time. For example, if you weigh an object three times and the balance provides answers of 5.23, 5.26. and 5.19 grams, this inconsistency is going to be problematic. Even if the true value is 5.26 grams, the amount of variation makes it impossible to know if your balance is trustworthy.
You should evaluate the span, linearity, and repeatability by using a set of test weights. Test weights must be stored in a protected case, kept immaculately clean and not damaged in any way. To evaluate a lab balance, simply take a series of repeated measurements of test weights, and compare the measurements with the known mass values of the weights. You will need to conduct these measurements at all weight ranges to ensure the linearity is correct. If gaps in any of these areas emerge, you will need to contact a professional for assistance in repairing or calibrating these highly-tuned instruments.