How An Out-of-Level Balance and Other Factors Can Cause Major Problems in Weighing Accuracy
The importance of proper leveling
If you own a balance, mass comparator, or high-resolution industrial scale, it is important that the device is not only calibrated in its final location but also leveled properly. There are many new features available on today’s weighing equipment that make users aware of when a balance is out of level via display prompts, graphics, and audible alarms. Out-of-level situations are so detrimental to the results of an experiment that balances can now be equipped to document levelness via an “alibi” memory, which will allow managers and external auditors to see when weighing results could possibly be tainted.
Any of us who are regularly involved in the use of a scale or balance know that the infamous level (spirit) is usually located on the rear of the balance in a difficult place to find and inconspicuously positioned as if it weren’t really that important. In fact, it is critical in the overall accuracy of the balance. The more expensive your balance, the higher the resolution (analytical, semi-micro, micro, etc.), the more significant this article is for you.
Example of leveling a balance utilizing an assisted leveling display
New technology now available to properly level balances utilizes an internal assisted leveling display. This display allows the user to level a balance quickly, accurately, and safely with minimum effort.
A comprehensive look at a balance not in level
Figure 1 Diagram showing the new-generation Sartorius Cubis balance with assisted leveling on the display.
There are few manufacturers that produce balances above the 1 microgram level due to the technology needed to accurately display a repeatable result. When these manufacturers calibrate a particular unit, they first ensure that the level vial is perfectly centered by a process of positioning the balance on a jig that is mounted to a calibrated table. This table is calibrated to be perfectly level (within an extremely tight tolerance, primarily checked with laser positioning systems), so that each balance is manufactured at exactly the same reference point
Therefore, when setting up a balance at your facility in its final position, it is critical that each balance be:
- Parallel to the direction of acceleration due to gravity
- Perpendicular to the weighing system
At an angle a = 0.3°, the following applies:
A=W-cosa = W · 0.9999875
200 g x 0.9999875 = 199.9975
This means a 200g sample would weigh 0.0025 g less on a tilted balance. Always level the balance using the level indicator as a guide! Understanding the effects of gravity in weighing accuracy
Before we continue, a basic discussion of the fundamentals of weighing is needed in order to understand how the effects of gravity on a balance can cause potentially large errors in overall accuracy. Weight, as we all know, is defined as follows:
The weight force that we actually sense is not the downward force of gravity, but the normal force (an upward contact force ) exerted by the surface we stand on, which opposes gravity and prevents us falling to the center of the Earth. This normal force, called the apparent weight, is the one that is measured by a scale or balance. (http://en.wikipedia.org/wiki/Weight )
It can be represented in a simple equation:
Weight (w) = Mass (m) x Gravity (g) 9.80665 m/s2 or 32.174 ft/s2
The gravity listed in the above equation is an average for the planet Earth and shouldn’t be assumed for any specific location where a balance is being used. Gravity can in effect be up to 0.5% different from location to location; even moving a balance from a basement to a third floor can cause enough moderation in the gravitational effect to result in an error on very accurate balances. Therefore, a balance needs to be located in its final position prior to an initial calibration, as the simple act of moving a balance after calibration will, in all probability, cause the balance to be out of tolerance.
Moving an analytical balance to a location that is only 4 m higher really makes a noticeable difference: Instead of 200.0000 g, only 199.9997 g is displayed, which means 0.0003 g lighter than the actual mass. Each time a balance is moved to another location, it must be calibrated (adjusted).
Not to digress, but it is critical to realize that not only the levelness but also the final location of a balance will have a direct impact on its performance. A balance is always calibrated in the factory where it’s produced and may even come with a “certificate of calibration.” However, this certificate simply reflects a manufacturer’s overall quality system; therefore, a balance always needs to be recalibrated with certified weights at its final resting place.
Other factors that affect accuracy
Many other factors can affect the overall accuracy of your weighing equipment, including temperature and humidity fluctuations, vibration, static charges, magnetically charged samples, and air buoyancy. Scale manufacturers have taken significant steps to help dampen and sometimes alleviate environmental factors; however, it is important to take special care in determining where a precision balance is located. Remember, balances don’t like change, so always keep them in the same location and under the same ambient conditions, and especially keep them level.