Here's my own take on the subject. Feel free to pick it to pieces.
As this thread demonstrates, everybody's going to have their own requirements for an ideal multimeter. As such, the thing should be made modular so the end user can swap out parts to fit their needs, rather than trying to create a
one size fits all meter as everybody and their brother has done.
In that case, the base meter would be a rack-like case with some basic infrastructure that all designs would need: Physical structure for the meter, battery holder(s), a processor, slots for modules (consisting of mechanical supports and a standard set of electrical connections) and such. The modules would be in two or three from factors. A small, simple input module that could handle basic functions like voltage, current, resistance, etc. These modules would ideally interlock horizontally so 3 or 4 of them could be lined up rack style (no dead space between modules) at the bottom of the meter, resulting in the multi-input design dave is after. Measurement circuitry requiring more space could be built in double or triple-wide modules.
Display modules would cover the full width of the meter and as I said above and possibly be designed around a bus so they could be stacked as necessary.
Controls could be integrated into these display modules, exist as a dedicated display module, or perhaps a third control panel type module is in order.
Point being, people can easily assemble a meter to meet their own needs and leave out the extra stuff.
This would turn the open-meter into a set of standards… The mechanical specs of the modules, communications between each module and the processor, current demands and such. Once you've got that, anyone can create their own modules to expand the meter's abilities. To facilitate that, blank modules (just the plastic housing and maybe standard connectors) could be sold, as well as populated V/I/R/C, LCD and control modules.
Since the modules and services would be standardized, alternative form factors for the meter "rack" could be created as well. Rather than the standard meter housing with lots of module slots, a compact probe-style meter could be made with one or two input slots and a display slot. In that case, the input could be either banana jacks, a point probe with common jack, or tweezers like those obnoxiously expensive LCR meters. A bench-top model would also be perfectly reasonable… Power supply module maybe? That would allow disposable batteries, rechargeable or plug-in sources.
The battery is going to be user replaceable.
Is that particular battery a standard battery that will be in production x number of years from now?
Use something that is ubiquitous like the 18650 Li-ion cells. They're cylindrical, have AA-like proportions (though with larger dimensions) and typically provide around 2300 mAh @ 3.6V. In that case, AA-style battery holder designs (snap in, cradle with cover, caped tube, whatever) would be easy to adapt and would eliminate the need for device-specific battery packs. While they don't quite make the run-out-to-a-drugstore cut, they're used as the base cell in a lot of consumer devices. My older Acer netbook uses them. Judging from the shape, dimensions and specs of the battery pack, my dust buster uses them, and my cordless drill probably does as well. Point being that even if they become hard to find new, it should still be fairly easy to scrounge them out of old battery packs. Using individual removable cells means one can carry around a few extra if spending a lot of time in the field.
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