Testing Your Laboratory Balance (Page 2 of 6) in pdf

Preliminaries

Handling test weights

The weights used to test lab balances are precision devices and need to be handled

accordingly. When handling weights, avoid direct hand contact with weights by using clean

gloves. Avoid sliding weights across any surface, especially across the stainless steel

weighing pan of the balance under test.

Environment

In order to pass any test of reproducibility an instrument must be operating in an acceptable

environment. A poor environment will degrade the results of a standard deviation (SD) test

and falsely suggest that the performance is substandard. There are several aspects of the

environment which impact the performance of a lab balance.

Temperature

The accuracy and overall performance of any lab balance is affected by the room temperature.

For best stability and performance the room temperature should be regulated to within one

degree Fahrenheit without interruption. The instrument should remain with power ON

continuously.

Air Drafts

In the cases of measurements with resolution of .001 gram and less, the force exerted by

moving air is readily detectable. A shroud or enclosure around the weighing pan will shield

the pan from these effects. Avoid plastic materials for draft shields.

Static Electricity

Static electricity exerts a mechanical force which is readily detectable by analytical and

microbalances. An example of static electricity exerting a mechanical force would be lint

sticking to clothing. Static will be a problem when it exists on the object being weighed, on

the person using the balance, on draft shields, or on weighing vessels. Sources of static are

carpets, Vibrum shoe soles, plastic draft shields, plastic weighing vessels, and melamine

(Formica) table tops. Low ambient humidity exacerbates static problems.

You can test for a static problem easily. On an analytical balance place a metal enclosure (a

coffee can works well) over the weighing pan, so that the pan is enclosed by the can but NOT

touched by it. If the weight readings stabilize with the can in place, then static may be the

cause of the instability. Notice that the coffee can provides an effective draft shield too.

Floor vibration / Table Instability

Many lab balances are extremely sensitive to vibration or movement. If the weight readings

change as you walk around the instrument, or if the readings change as you lean on the table

or move objects on the table, then the table and floor are affecting weight readings. You can

minimize these effects by using an especially sturdy table and minimizing movement. Users

of microbalances often need specially built marble tables on concrete floors.

IES Corporation

Repair of Laboratory Balances

(800) 541-0852

www

Testing Your Laboratory Balance Page 2