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What should an ideal weigh off room look like? In this photograph we see a good example of
what a weighing area might resemble. From left to right, notice the small scale for items to be
weighed in grams, the marble tablet for the intermediate scale, the intermediate scale from 500
gm to say 20 kg., the floor scale and the magnehelic gauges monitoring the air pressure in
different parts of the room and in relation to the hallway outside.
The room has minimum horizontal surfaces for dust to settle on. The floor is a sealed material
non reactive to potential materials to be weighed. The walls are stainless steel for ease of
cleaning. The room is very well lighted for easy reading of numbers posted on the digital
readouts. Let us examine now what can go wrong in a weighing room, even if we weighed
materials in this modern weighing area.
Even if you had that ideal weigh room to perform your weigh offs, you still face the possibility of
a weigh off error. Here is a list and discussion of some common root causes of weighing errors.
Some errors occur primarily due to the conditions in the weighing room itself.
** Wrong scale used: All scales have limits on the quantity of materials to be weighed. For
companies that do not have SOP’s for scale use, the scale chosen for weigh off by the operator
may in fact not be capable of accurately weighing the material.
** Objects near the scale: Take only what you need into the weighing area. Take a few pair
of gloves; not boxes and boxes. Do not take tool boxes into the weighing area. You should have
only the tools you need to weigh the material. Before weighing, make sure there are no objects
under or near the scale to be used.
** Wind currents: Are not really a factor with floor scales but are a critical factor when
weighing small quantities of materials. In parts of the country where it gets warm the use of an
electric fan in the room can produce a weighing error
** Objects too large for scale: Even if the object is within the correct weighing range of the
scale, it may have a bulk density so low that its mass is too large for the scale. Good examples
of this are the flow enhancers used in solid dosage manufacturing.
** Weighing off-center: The object to be weighed should be placed in the center of the tare
vessel. Again, this is especially true of the smaller objects to be weighed. Never place the tare
vessel or the material to be weighed toward the side of the weighing pan.
** Tare vessel too small for its content: Plastic “Weigh boats” commonly used as tare
vessels come in a variety of sizes. Never use a tare vessel that is too small for its content. Doing
will produce a weighing error if the material touches a non scale area.
** Excessive vibration: A commonly overlooked and under appreciated weighing factor.
Scales need to be placed on a solid object free from vibration. Modern scales will post an alarm
if it cannot come to rest with a zero reading but older scales depend upon sound operator
judgment for accurate read out.
** Weighing material with an electrical charge in plastic containers: Some materials, such as
natural fibrous materials or steroid like components, will generate electric currents in the
weighing process. This causes some materials to stick to weighing containers, plastic bags etc.
When the operator goes to transfer the materials, he or she should verify that there is no
residual material left in the container.
** Confusion between unit of measure: gm. Vs mg: Clear weigh off instructions in the SOP
and batch record help prevent this type of error. Inadequate lighting in the scale areas may
produce conditions where numbers and units are difficult to read. Always have one operator as
the weigher and a second person as the checker. In most companies the checker bears ultimate
responsibility for weighing errors, not the weigher!
By reviewing these simple tech tips for the Weighing Area, you should be able to avoid some of
the common errors seen in the weighing operation.
| Tech Tip for the Weigh Room: Common Causes of Weighing Errors |
| Solid Dosage Training, Inc. FREE TECHNICAL HELP The Weigh Room |
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