I agree with all parts of Leo's post. Of course there are some who view
having the most expensive and highest quality tools available as
important. I'm not one of them. I will trust the results of two
inexpensive scales to be more trustworthy than one more expensive scale
and certainly more trustworthy (as well as much quicker) than a triple
beam balance or any other kind of balance beam scale.
I'm making very detailed records of my current build of a new F1M and in
this process I'm using two digital scales to weigh my parts. As I think
I said earlier there is rarely more than one milligram difference in the
readings. If I were a diamond buyer this might be a problem but for
indoor models I think it's very acceptable.
On 1/31/2014 11:57 PM, leop_at_lyradev.com wrote:
>
> I have both of the scales that Bill pictured plus an earlier 10g
> generic version of the Tanita scale Deszo posted about. They all work
> well and about the same. I take two of the Mini-Digital (generic AWS)
> scales with me to most major contests, as Kang has suggested. My
> advice is just to buy the inexpensive Mini-Digital scale ($15 with
> shipping on eBay) and keep a spare. It is also easier (and often less
> expensive) to spend another $15 than to return a scale for
> repair/replacement under warranty
>
>
> I wish the Mini-digital still was available in the 10g version rather
> than only in the 20g version. Both versions use a 14 bit analog to
> digital converter. This allows only one part in 16384 precision, not
> quite 1 milligram out of 20g. So, this 20g scale is really only good
> to 2mg accuracy at best. However, the other 20g scales are also seem
> have 14 bit AD converters so there is no advantage in buying them.
>
>
> These relatively inexpensive pocket scales use an aluminum beam with
> strain gauges (called a load cell) to measure the weight. The main
> failure mode, other than exhausted batteries, of these scales is the
> epoxy adhesive bond for the strain gauge to the aluminium failing. An
> early failure symptom is a scale that will not settle to a steady
> reading. A shock to the scale can cause the beam to vibrate putting a
> great deal of strain on the epoxied joint. My generic Mini-Digital
> scales came in a nice box with a form fitting foam holder and I always
> transport in this box. Any foam or other padding is pretty much
> required when transporting any load cell based scales. Also, bringing
> along and using the calibration weight will give one confidence that
> the scale is working correctly. Finally, the scale must be level to
> measure the weight as any deviation from level will reduce the force
> on the scale leading to an error in the weight.
>
>
> These load cell based scales all use similar electronics. The
> difference is in the firmware and the load cell. Theoretically, the
> measured weight is independent of the temperature and the position of
> the weight on the platform. All of the scales I have use four strain
> gauges on the beam which is the optimum configuration. This
> eliminates the load cell temperature based errors. This configuration
> can also eliminate the positional errors but the aluminum beams need
> be "tuned" so that the bending destortions (strains) are the same
> magnitude for all the strain gauges. This is easy for the load cell
> manufactures to do but the positional accuracy depends on the
> tolerances acceptable to the manufacturer. I find all of my pocket
> milligram scales to be good to better than 2mg in positional accuracy.
> However, the larger platform 100g/0.01g scales are often very
> position dependent.
>
>
> There are more precise and acccurate scales available but at greater
> cost. Most use a force balance system rather than a load cell and are
> neither as portable nor as rugged as the load cell based pocket
> scales. I use these more precise scales to make check weights (1200mg
> and 600mg for F1D flying) that I sometimes take with me to contests.
> Using these check weights increases my confidence in the output of my
> pocket scales.
>
>
> Finally, static forces are significant when weighing at the milligram
> level. I have used a hanging scale setup similar to what Bill
> pictured for a long time now. Not only are the static forces reduced
> (which makes for more accurate motor weights) but weight errors due to
> air current on plane are reduced. I would suggest something similar
> for all.
>
>
> LeoP
>
>
>
>
Received on Sat Feb 01 2014 - 09:44:41 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:48 CET