OK. Not to be didactic or long-winded (which means I'm going to be both), here's my primer on electronic scales, why and how to use them, and what to watch out for.
First off, I've worked in and around the sensor industry for over 25 years.
Here's what I know about electronic scales:
Electronic scales have three major parts: a strain gage (or strain
gages arranged in a Wheatstone Bridge), signal conditioning electronics, and control/readout features. Technically speaking, a strain gage is a transducer. A transducer has an electrical output that changes in proportion to a physical stimulus (it also works the other way - a transducer can be a device that changes a physical attribute based on an electrical stimulus - but that's not we're talking about here). When force is applied to a strain gage, it changes an electrical characteristic - resistance, inductance, capacitance, etc - (usually resistance), that change is interpreted by the signal conditioning electronics, and is displayed by the readout.
You put a force on a strain gage (the additional mass caused by dumping powder in a pan) and its resistance changes. This change in resistance is "read" by the signal conditioning electronics, modified/interpreted by the control algorithms, and displayed on a digital readout.
There are three specs that indicate the accuracy of these devices. These are - in order of importance for cheap (less than $1000) weighing devices - Thermal effect, linearity, and repeatability. I'll explain them in reverse order:
- Repeatability - Specifies the device's ability to display the same weight the same way time after time. This is very good, even on very cheap devices.
- Linearity - Specifies the device's ability to proportionally "track" the physical input (weight) through its specified range accurately. On cheap devices, this is great through the middle of the range, but breaks down when you're trying to weigh on the extreme ends (lowest or highest weight) of the device's range. This is inconsequential for us because we don't weigh full powder charges of .1 grains or 7000 grains.
- Thermal Effect - OK. Here's the one we've got to worry about. changes in temperature affect the readings. Temperature changes affect the strain gages (a lot), as well as the signal conditioning electronics (enough to worry about, but not so much). High-end manufacturers spend big money on components to minimize this effect - that's why they're high end. Some go as far as to install heaters and thermostats to ensure that everything is at the same (elevated) temperature, every time. You're not going to get this kind of engineering in any device that costs you less than $1000.
So, how do you maximize the accuracy of your "cheap" device? Here's how:
- Leave it on all the time. This minimizes the thermal effect. By leaving the device powered up, everything "warms up" and stabilizes. Also, If you have a "tare" button, press it before each reloading session. Again, this will cancel out any drift caused by temperature changes, as well as any changes in electrical or other factors (battery drain, etc.) that can affect weighing.
- Make sure your scale is level, and that there are no mechanical forces (breezes, etc.) that can skew your readings. The Laws Of Physics are not suspended just because you're using a digital scale.
- Eliminate electrical noise. This can be tricky. Electrical noise - radio frequency interference (RFI), electro-magnetic interference (EMI), as well as transients (voltage spikes) can be hard (even for experts) to diagnose and prevent. If your scale plugs into an electrical outlet, make sure that there are no inductive loads (motors, fluorescent lights with ballasts, solenoids, etc) on the same circuit. These can cause the signal conditioning electronics to misinterpret the signal from the transducer. This is especially important in "cheap" devices (less than $1000).
Follow these guidelines and even the cheapest $20 eBay scale from China can be used to weigh powder charges.
If you're careful about it, almost any electronic scale will outperform a beam balance, both in speed and accuracy.
Whew!