Hi Boys,
For me the most important is the quality, such as Tanita.
second of usability,
third of the price.
Dezso Orsovai
On Saturday, February 1, 2014 5:58 AM, "leop_at_lyradev.com" <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 - 01:08:40 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:48 CET