Watching the change in barometric pressure is a time-honored way to predict the weather.

Yet barometers don’t make that easy to do. Barographs, which chart those changes, are obviously far more useful. But traditionally they have been large, delicate mechanical devices unsuitable for anything except ships.

The Weems & Plath Electronic Barometer 4002 is a digital barometer with an LCD current-pressure display and a separate LCD barograph display. Retailing for $170 to $205, it’s an excellent weather instrument, ideal for home use and for consultation before a day sail. Yet it has numerous design shortcomings, and I’m not sure it’s weatherproof enough to be used on board. I do not recommend it as a timepiece, moon phase indicator, yacht-race timer or anything else it purports to be.

The 4002 is something of a mystery. It comes with only the most perfunctory manual, displays obscure icons that appear and disappear for no apparent reason, offers a USB jack but doesn’t say why, and includes a switch on the left-hand side for an undisclosed and non-obvious purpose.

Little additional information about the 4002 can be found online. Seattle’s Starpath Institute appears to have worked hard to make it more usable, providing online both the manual and supplementary instructions. Starpath sells the 4002, as do Captain’s and Amazon.com. My attempt to write a brief manual for it can be found on Amazon.com.

On the “pro” side of the ledger, the 4002 is extremely accurate. It remained within 0.2 millibars of my nearest NOAA station during most of the five-day test period I gave it, and within its stated accuracy of 0.5 mb the entire time.

In the lower window is displayed a line graph of pressures over your choice of six time periods: the past 2, 4, 6, 12, 24 or 48 hours. You can momentarily change among those time periods. You can also change the time period that remains on display. I typically keep it at 48 hours, to show the long-term trend. If interesting things are happening, I cycle through all six periods.

The graph does not display the pressures numerically—only their trend. But you can review the pressures numerically over any time period you choose. Four AA alkaline batteries lasted five months, even with several backlight illuminations each day.

The “con” side is a bit longer.

•  The current-pressure readout is large and bold, but the graph of pressures is hard to see without using the backlight. The label for the time period currently on display—for example, “2H” for two hours—is tiny.

•  There’s no dedicated button that momentarily illuminates the backlighting. Pushing the “ZOOM/-” button illuminates the screens for about 20 seconds.

•  There’s no single button to push to convert hPa (millibars) into inches of mercury or vice versa. You have to bring up several menus to do that, and the change persists until you go through the procedure again.

• As noted above, the 4002 has icons whose function is not apparent and not explained. For example, the word “IN” appears under the moon-phase indicator, as does the word “TEMP.” Nothing I do causes them to go away. But something I did caused a crossed-out gray laundry basket to appear onscreen. I still don’t know what it means.

• There’s an optional alarm that warns of a user-selected pressure change over a user-selected period of hours. There’s also a gale warning, which the manual says sounds an alarm when there’s a drop of three mb over a three-hour period. Is that a different alarm, or is it the same as the optional pressure-change alarm? If it’s separate, can it be turned on and off? The manual yields no clues.

The 4002′s plastic mounts are fairly substantial, but to keep it safe, I mounted it to a piece of marine plywood, then suspended that from hooks using shock cord. I’m still not sure how shockproof it is, or how well it will resist salt air.

I’m pretty sure water would wreck it, since its plastic-and-metal case has open vents. This may limit its use onboard except in the most sheltered locations. If you’re willing to risk its life, and you bring a trustworthy back-up, the 4002 could be a real aid underway.

Dan Richman, a career journalist now covering efforts to modernize the nation’s electrical system, sails a 1979 Montgomery 17 in Puget Sound and beyond.