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The barograph perfected

Mar 4, 2015
While a barometer displays current atmospheric pressure, a barograph records pressure over time, allowing a voyager to see the trend in pressure, revealing clues to future weather. The Mintaka Duo from Starpath is an electronic barograph.

While a barometer displays current atmospheric pressure, a barograph records pressure over time, allowing a voyager to see the trend in pressure, revealing clues to future weather. The Mintaka Duo from Starpath is an electronic barograph.

In the age of weather GRIB files delivered by satphone, there can be something reassuring about the polished presence of a brass barometer by the nav station. No matter what the forecasts say should be happening with the weather around you, a barometer will tell you what actually is happening. Now a versatile electronic pressure-sensing tool, the Mintaka Duo Dual-Sensor Precision Barograph from Starpath combines the best of both the electronic and traditional barometer worlds. 

The classic analog barometer with the round, clock-like dial is called an aneroid barometer. It has a collapsible metal chamber pumped free of air. The chamber is prevented from fully collapsing, however, by an internal spring pushing it apart. As atmospheric pressure increases, air pressure pushes on the chamber, compressing the chamber and the spring. When air pressure drops, the spring pushes the chamber apart and it expands. Via a series of shafts and gears, this expansion and contraction of the chamber is translated into the movement of the barometer’s indicator needle. 

One great aspect of this ingenious little machine is that it requires no outside power source; atmospheric pressure supplies the force to move the needle.

What if you want a record of how barometric pressure changes over time? French physicist Lucien Vidi invented the first practical recording barometer, or barograph, in 1844. 

A traditional barograph uses a stack of four or five aneroid chambers that drive an arm equipped with an ink stylus that records on a paper-wrapped drum driven by a clockwork or an electric motor. The result is a record of changing atmospheric pressure, making it easy to spot the trend of pressure change. The downside to one of these analog devices, however, is that they require a bit of a regular maintenance: replenishing the ink for the stylus, changing the paper on the drum and either winding the clockwork or changing the batteries. 

David Burch, the director of the Starpath School of Navigation in Seattle (www.starpath.com) and the author of numerous books on navigation and weather, knows the value of having a good barometer on board. So Burch and Starpath decided to create an electronic version of the recording barometer. Called the Mintaka Duo Dual-Sensor Precision Barograph, it can handle the recording task without the need for ink, styluses or paper.

According to Burch, this unit has been a few years in the making. He wrote in an email: “I have been working on the ideal user interface for marine applications for many years … and in fact wrote The Barometer Handbook in 2009 anticipating this device, so the job of that book was to show folks how important accurate pressure can be.”

Burch spent some time looking for the right person to team up with and found him in a live-aboard MIT PhD in electrical engineering and computer science named Jerry Barber. Burch designed the user interface, including what functions to display and how to compute them, while Barber designed the circuit board and wrote the software for the unit. 

The Mintaka Duo barographs are assembled at Starpath from components ordered by Burch and Barber. After programming, the barographs are calibrated over the full range from 600 to 1,080 millibars, after which each unit receives a custom calibration curve.
   
As its name indicates, the unit uses two independent pressure-monitoring chips made by Bosch in Germany. “Having two sensors (i.e., dual-sensor) we get increased accuracy and dependability,” Burch writes. “With computer access to the device (Mac or PC) you can read the raw data from each sensor, which provides a way to monitor any long-term drift of a sensor.”

While you could use the Mintaka Duo as a simple barometer by setting the screen to just read out the current pressure, the real usefulness of the device is in its barograph mode.
   
The Mintaka Duo shows a graph of successive pressure readings so you can see the trend of pressure change, just like a barograph ink trace on paper. A great feature of an electronic unit compared to paper is how easily you can change the way the data is displayed. Users can choose a wide range of time frames: from 30 minutes to three hours, six hours, 24 hours, two days, four days, 12 days, 30 days, 60 days and finally 120 days.

Another of the many features is the display of something called pressure tendency characteristic code. This a way for the National Weather Service (NWS) to evaluate the quality of data it receives from ship reports. Burch explains: “It is a number between zero and 8 that characterizes the shape of the barograph trace during the past 3 hours, such as steady then rising (code 3), or increasing then decreasing with pressure same now as it was (code 0), or increasing then decreasing pressure now lower than before (code 8).” 

These characteristic codes are plotted by the NWS on surface analysis charts. They provide mariners with additional information on barometric trends. 

Since the graphical representation of barometric trends is so useful, Starpath has an output capability that allows you to hook up a Mintaka Duo with a Mac or PC and look at the data on a larger screen with more detail than available on the unit’s LCD screen. Users can also control the unit via their computers or update the Mintaka Duo with the latest firmware via an Internet connection.
 
The Mintaka Duo is a modern, electronic marine barograph that can display barometric trends going out to 120 days, connect to your computer via USB and has a pressure-averaging option to compensate for semi-diurnal variation in the tropics. And it can do all that without any ink, stylus or rolls of paper.

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