It takes very little skill or technology to know that youre in the midst of a storm. It takes considerably more to know that a storm is coming. That information, however, can be a lifesaver if youre on the water.
Prior to the early 17th century, a mariners only tools for predicting weather were his senses and a collection of rules on the order of, Red sky at morning, sailors take warning. If a crew member had rheumatism, an aching limb might presage wet weather. The reliability of all these methods left a great deal to be desired.
Torricellis invention of the mercury barometer changed all this. Now it was possible to predict an impending storm with some accuracy many hours before it struck. And the barometer, to this day, remains a fundamental tool for weather prediction (see sidebar on page 22.)
The barometer, in most of its forms, is a simple gadget. Early ones were all made of glass tubes with one end sealed and evacuated and the other immersed in a pool of liquid-usually mercury, because its dense enough to permit the tube to be short. Liquid from the pool, which is open to the atmosphere, is forced up into the tube by the ambient atmospheric pressure until the downward pressure exerted by the liquid exactly equals the atmospheric pressure forcing it up into the tube. A scale-just an ordinary ruler attached to the side of the tube-made it easy to read how far the liquid level had dropped (or risen). This type of liquid barometer is still the most accurate one available.
If the level in the glass rose, atmospheric, or barometric, pressure was increasing; if it fell, pressure was decreasing. Sailors around the world learned very quickly that when the glass fell, foul weather was on its way.
Over the next 350 years, only a few changes were made to this very basic instrument. To help overcome the fragility of the glass tube barometer, variations on the theme were developed that used thin curved sealed tubes or bellows. When the pressure dropped the bellows expanded or the tube straightened. The movement of these aneroid (the word means no liquid) devices was generally transmitted to an indicating needle through a geared mechanical linkage that converted the sealed containers small movement to a larger, easier-to-read needle. This type of barometer is still the most popular one today.
A further refinement was the barograph-a gadget that made it simple to observe pressure changes without having to remain at the barometer. The traditional barograph consists of a pen or similar writing implement attached to the needle of an aneroid barometer, and a rotating paper-covered drum that the pen writes on. If you look at the squiggles left by the pen, you can observe just how the barometric pressure has changed over a prolonged period of time.
In the fast few decades, electronic barometers have appeared on the scene. The pressure-sensing element of these devices works on the same principle as the aneroid barometers bellows, but consists of a thin membrane formed onto a silicon chip over an etched, sealed recess in the chip. Strain gauges on the membrane sense any movement of the membrane, and display it as electrical signals.
Microprocessors take the electrical signals and convert them to a readable form, usually on an LCD display. Because microprocessors have memories and computational capabilities, these electronic barometers also can remember former readings and keep track of pressure changes, as well as use other on-the-chip sensors to monitor such items as date, time, temperature and relative humidity, in addition to air pressure.
What We Tested
We were particularly curious to see how the newer electronic barometers stacked up against the classic aneroid barometers and barographs, so we concentrated our product search on electronic models. We found four: two older versions-the Perception II and the Weather Monitor II-from Davis, who has been making weather instrumentation for quite some time now, and a couple of newcomers: the Altitude Electric Barometer and the Speedtech WeatherMate Electronic Barometer. We compared them to two representative instruments from Weems & Plath, a well-known manufacturer and distributor of quality marine instruments, an Atlantis aneroid barometer model #200700 , and a Dampened DeLuxe Quartz aneroid barograph.
We didnt bother including a liquid barometer in our tests; theyre sold as laboratory instruments, but arent often used out in the real world. Nor did we obtain a lot of conventional barometers, since that technology is very well established, and we wouldnt expect to find significant differences among them.
How We Tested
Essentially, we tested all the devices by lining them up side-by-side and using them. We adjusted each to a sea level zero setting, and noted their readings, comparing them to NOAA radio reports from a local station.
We ran them for a period of several weeks, noting any differences in readings and/or other aspects of performance. We checked to see how useful each unit was for predicting weather and noted how easy each was to use.
Finally, since we knew that temperature sensitivity is a possible problem with electronic devices, we tested all the units in a large fish tank, using ice, then a heater, to create temperature extremes.
What We Found
We found that all the units are inherently accurate to within .02 inches of mercury-or precise enough. All could be adjusted for changes in altitude, although there were differences in the convenience of making the adjustment. This isn’t an adjustment that youll make frequently, unless you trailer your boat between the seashore and mountain lakes. Further, its not critical in predicting weather-actual pressure readings are important but not as important as accurately tracking the pressure changes.
All the units tested can give some indication of trends in barometric pressure changes; there are major differences in how conveniently they do this, and how much operator attention is required.
All operated consistently over a wide temperature range.
Weems & Plath Atlantis Barometer
This is a fine example of a classic aneroid barometer. Its a cylindrical instrument in a brass case with flanged face and back, measuring 5-1/4″ in diameter and 3″ deep. It has a nicely engraved face calibrated in both millibars and inches. There is a black indicating needle and a hand-adjustable brass one. It discounts for $125.
The black needle accurately points to the current barometric pressure; you measure trends by setting the adjustable needle to coincide with the current reading, and then coming back in an hour or so to see if theres been a change. Altitude adjustment is made by turning a mechanical adjustment screw.
Bottom Line: The Weems and Plath is simply beautiful. It features a highly polished brass case and carries a lifetime warranty. Its also the least convenient weather forecaster of any of the devices we tested-though thats the fault of the class of barometers in general, not of this particular one.
Weems and Plath Dampened DeLuxe Quartz Barograph
This one doesn’t really belong with the other products tested. Not because it doesn’t work-it provides the most complete record of barometric pressure variation of any instrument we tested, recording minute-by-minute changes. Nor because its difficult to use; aside from weekly paper changes and occasional pen replacement, it ran without a hitch, and provided clear, easy-to-read records of what the weather was doing and had been doing for several days.
Its three drawbacks are that its large (11″ x 6-3/4″ x 6-1/2″), its fragile, and its expensive (it lists for $1,362). We talked to Bob Condon at Plath, and he agreed that a barograph is more apt to find a home ashore than aboard; we included it as a benchmark for excellence in trend tracking.
Bottom Line: Like the aneroid barometer, this barograph is a thing of beauty, especially to those who love tradition. Its wood, glass and gleaming brass make it a great choice for a clubhouse or nautically styled living room. Alhough it worked very well, we can’t recommend it for use on the average boat.
Davis Perception II
The Davis Perception II represents a sharp break with tradition. A black plastic case (8-1/2″ x 3-1/4″ x 5-1/8″) houses an instrument that reads out barometric pressure, indoor temperature, indoor relative humidity, and, in addition, the time and date. Inasmuch as our main concern is how it functions as a barometer, we wont go into all its other functions in great detail, except to say that they all work.
The $150/$115 (discount) Perception II, like all the electronic models we tried, isn’t water-resistant, but it probably would be mounted belowdecks in any case, which is a good thing, since the LCD display should be kept out of direct sunlight. It operates from a 9- to12-volt power supply (110V AC if you use the adapter thats supplied with the unit); theres an internal 9-volt battery that will keep it running for 24-48 hours without losing any data.
The Perception II has an odd display. On the left side of the screen, it always displays indoor temperature (to the nearest degree) and current barometer reading (to the nearest whole unit). You can pick the units you prefer to be displayed. On the right side of the screen, the display changes depending on which button you push. TEMP, BAR, HUM, TIME are fairly obvious. RECL allows you to store a barometric reading, and recall it at some future time, along with its time and date.
When displaying temperature or barometric pressure, the Perception IIs screen will have either two displays of temperature (the one on the right can optionally be set to read to the nearest tenth of a unit) or two barometer displays. In the lower right corner of the screen theres a horizontal arrow labeled Barometric Trend thats displayed no matter which button is pushed. It tilts upward (or downward) if an increase (or decrease) in pressure over the past hour exceeds .02. Data is updated every 15 minutes.
Aside from the trend arrow, the Perception II has two means of keeping track of barometric trends. One is the store-and-recall function described above, which is the electronic equivalent of the barometers adjustable pointer, and a settable alarm, where you can pick a pressure change over a period of one hour that you wish to set off an audible signal. The alarm is checked every 15 minutes. This alarm feature doesn’t distinguish between a rising pressure and a falling one; you have to look at the trend arrow for that information.
Davis optional Weatherlink is a $120 (discount) attachment that connects the Perception II to a computer, either Mac or PC, enabling you to record more about barometric pressure (and indoor conditions) than any but a dedicated hobbyist would ever want to know. The Perception II carries a one-year warranty.
Bottom Line: The Davis Perception II is a full-featured, flexible monitoring weather station. Its not ideal, we think, for keeping track of barometric trends.
Davis Weather Monitor II
The Davis Weather Monitor II is basically a Perception II with added functions. Its $395/$275 price includes an outdoor temperature sensor, a wind vane, and an anemometer for measuring wind velocity. Optional gear includes an external temperature/humidity sensor and a rainfall sensor.
The display is housed in a case the same size and shape as the Perception II, but the display and push-button arrangement are different. The left side of the screen has a permanent display of windspeed and direction. The right side of the screen has a permanently displayed barometer trend arrow, which works exactly like the one on the Perception II.
The screens right side is also used for displaying a wide range of weather-related data. You can get a readout of inside temperature, outside temperature, wind chill factor, barometric pressure, indoor humidity, outdoor humidity and rainfall (if you bought the sensors), time and date, as well as a duplicate display of windspeed and a digital display of wind direction. There are also recallable stored high and low values and programmable alarms for most of these functions. With all these features, the Weather Monitor II handles barometric data in precisely the same way as the Perception II does, with the same limitations. Same warranty, too.
Bottom Line: The Weather Monitor II is really aimed more at the weather hobbyist than the boat owner. It has lots of functions, and becomes even more powerful if you add the Weatherlink and a computer. As such, its a better buy than the Perception II. If youre looking for something to help you anticipate changing weather by monitoring barometric pressure, we think you should read on.
Speedtech WeatherMate Barometer
Altitude 8900 Electronic BarometerThese two devices are, as far as we can tell, identical in function and in circuitry. Though quite different in appearance, they are sisters under the skin, or subcutaneous siblings, if you prefer. The Altitude 8900 is a small wall-hung package measuring 3″ x 4″ x 1″. The Speedtech is larger but thinner, measuring 4-1/4″ x 5-1/4″ x 3/4″, and can be either wall hung or free-standing. Both have a pair of switches in the back of the case to select metric or English units; both have four push-buttons to adjust for altitude and to recall all readings made over the past 24 hours. The Speedtech mounts these push-buttons on the face of the unit, the Altitude mounts them on the back. The Altitude has a white case, the Speedtech a slate-gray one. These were the only differences we could detect. The Altitude sells for $98.50, the Speedtech for $99.95.
The display consists of a bar graph-or line graph, depending upon what you select-showing current barometer reading and readings taken one, three, six, 12 and 24 hours ago. The current reading is also displayed digitally in the lower righthand corner of the display. This arrangement, we found, is extremely convenient. A glance at the display presents a clear picture of pressure trends over the past day. All readings over the past 24 hours are stored, so the display can be turned back an hour at a time to see what has happened. The display is refreshed every nine seconds.
Setting the initial altitude is somewhat inconvenient. Pressing the Alt button displays the altitude (in meters) youre pre-set for. To change this setting, you must press the Alt button, then the up or down arrow buttons to change the altitude in increments of 10 meters. You must then wait 15 minutes to see if your pressure reading corresponds to the one youre standardizing against-NOAA, in our case. Luckily, most will have to perform this operation only once.
At the top of the display are digital readouts of indoor temperature, relative humidity and a rudimentary comfort index, that reads out as dry, wet, or normal (no display). Below this is a pictorial display that shows the forecast: A picture of the sun (for clear weather), sun plus clouds (for slightly cloudy), clouds alone (cloudy), and clouds with rain (rainy) for 12-24 hours ahead.
These units are self-contained and will run for about one year, were told, on four AAA batteries (theres a low-battery indicator). The display is not illuminated, which shouldnt present a problem if the unit is mounted in a lit cabin. Neither model is waterproof and, like all devices employing LCD displays, both should be kept out of direct sunlight for prolonged periods.
Bottom Line: A choice between the Altitude 8900 and the Speedtech Electronic Barometer really depends on which configuration, or color, you prefer. Both units admirably perform their job of tracking trends in atmospheric pressure, with a display system that needs virtually no attention.
Recommendations/Conclusions
A barometer, particularly one such as the Speedtech and Altitude that allows you to monitor pressure trends at a glance, makes an excellent backup to a dedicated weather radio.
If youre a weather hobbyist, Davis Weather Monitor II is much more versatile than the Perception II for not much more money. If you don’t opt for a Weatherlink and computer, youll find that the Weather Monitor isn’t as handy for tracking barometer changes as the less expensive Speedtech and Altitude instruments.
If you want to do it the traditional way, a nicely polished brass aneroid barometer will certainly do the job. And while we are certainly in favor of tradition, wed probably supplement one with either the plain-Jane Altitude or Speedtech for planning our next days cruise.
Contacts- Altitude 8900, dist. Atlantic Main Corp., 42 Great Neck Rd., Waterford, CT 06385; 860/701-3443. Perception II & Weather Monitor II, Davis Instruments, 3465 Diablo Ave., Hayward, CA 94545; 510/732-9229. WeatherMate, dist.Speedtech Instruments, 10413 Deerfoot Dr., Great Falls, VA 22066; 703/759-0511. Weems & Plath, C. Plath, North America, 222 Severn Ave., Annapolis, MD 21403, 410/263-6700.
