April 14th, 2009

Tips For Using Sensitive Electronic Scales

Some of our forum members have observed issues with the Acculab VIC-123, an 0.001g precision electronic balance made by Sartorius. (The Denver Instrument MXX-123 is very similar.) The two main complaints seem to be sensitivity to drafts, and instability of zero, causing weight read-outs to “drift” over time. We have seen the latter problem in less expensive scales such as the PACT. (Read PACT report).

Acculab VIC-123 digital scaleForum member Ronemus, who lists his profession as “instrumentation scientist”, offers the following advice:

“It is necessary to isolate the scale from drafts and vibrations. Laboratory scales with this sort of resolution (.001g) generally have a housing around the pan with sliding doors for access and vibration isolators in the feet. Those scales cost thousands of dollars, and some features must be cut to reach a price we’re willing to pay. Unfortunately, the instruction manuals accompanying our scales generally aren’t very good at spelling out the steps necessary to have them operate to our satisfaction.

A small draft (one you can barely feel) can easily shift the reading a few tenths of a grain, so some sort of enclosure is needed. I use a cardboard file box with one end cut out, so 3 sides and the top remain, and that’s good enough for 0.1 gr (6 mg) stability; however, that may not be sufficient for 0.01 gr.

For stable zeros it’s necessary to warm up for at least a few hours (they’re generally left on continuously to avoid drift) and keep the room temperature fairly constant (within a few degrees).

Inexpensive scales are also susceptible to electrical noise, either riding the power line or through the air. Power line noise can be eliminated with a good filtered power strip (I recommend a Tripp-Lite Iso-Bar), not just a surge suppressor. Cordless and cell phones, fluorescent lights, wireless computer networks, baby monitors, etc. can cause problems at short range, so they should be kept away from the scale as much as possible.”

We have also observed that these scales MUST be properly leveled. They just won’t work right if they are tilted even a litte bit. This editor recently worked with an Acculab VIC-123. Its owner was complaining that it would drift and the read-out was constantly flickering. Well the scale was mounted on a plastic folding table with a top that was warped and bowed in the middle. It was virtually impossible to level the scale properly. The scale was also directly under a large ventilation fan. Once we transfered the scale to a bench with a solid flat surface, leveled the scale front to rear AND side to side, and ensured the scale was isolated from air currents, the scale worked perfectly.

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