We got a great question recently from our friends at Nevada Weighing asking about the importance of testing environments. The environment surrounding a balance is vital if you are to maintain the manufacturers stated performance ratings. Really, the manufacturer doesn’t matter in this regard, as a bad environment will adversely affect any balance. There are many influences to consider and to remove, as far as is possible, from the area surrounding the balance.
The air currents and changes in temperature from direct sunlight will play havoc with the performance of a balance. Keep all balances out of an area that will be covered by direct sunlight, yes, even in Northern climes (for our Canadian friends).
The changes in air pressure and air movement caused by the opening of doors, people passing by, air conditioning etc. can all cause balances to give unstable and incorrect weights. All these should be avoided, even if the balance has a draft shield.
Even subtle vibration can affect the performance of balances. The table wobbling or the table not being strong enough can make the balance vibrate and the weight displayed unstable. You must have a stable and strong platform. Machinery vibration, people walking past on a springy floor, trains going past the building within 1000 yards, all these issues will be reflected in poor balance performance.
To a sensitive balance such as a 4 place or above (0.0001 g or higher sensitivity) the temperature change caused by the radiated heat from your hand is enough to upset the balance. This is one reason why people often wear gloves when dealing with balances (not just balance calibrators). With all balances, direct heat of any sort should be avoided.
No this is not a joke! With the most sensitive of balances, your breath is more powerful than you think. Very often when we calibrate balances that are analytical or above, we cover our mouths and breathe in a shallow fashion, to make sure we do not disturb the balance reading. Even though the doors are closed on the weighing chamber, you will be amazed how a little air current will disturb things.
Keeping the doors closed
It is always imperative to keep the doors open for the shortest time possible on analytical balances, especially if the balance has a conventional force restoration mechanism that will generate heat and is under the platter. The heat builds up in the weighing chamber over time. If the doors are opened for long, the warm air will escape and cold air will fill the chamber, leading to air disturbance in the chamber, right over the platter. Here is a good reason to consider “crossover” doors, that allow for holding the sample in one hand and opening the door with the opposing hand. This dramatically cuts down the time the doors are open.
We, as human beings generate static every day. It builds up in our bodies, our clothes and is also generated by machinery in laboratories. Parts of most balances have some ferrous material in them and as such are affected by static. The electrical field set up by static can radically affect results by attracting balance parts to each other, the attraction causing a force that the balance will see as weight (or lack of it). Use of an Ionizer will help with this problem, as it will dissipate the static that builds up in the vicinity of the balance.
When all is said and done, the balance is one of the most sensitive pieces of equipment that you will use in the laboratory. Consider that a digital thermometer may read in 0.1 °C, perhaps 1000 divisions of the scale. (100 ÷ 0.1 = 1000) while the 200 g balance reads in 0.0001 g that is 2,000,000 divisions of the balance range. It is no wonder that the balance needs to have a safe and secure environment (or perhaps I am just biased!).