Tuesday, April 12, 2016

Connecting a Barometer to Expedition

There is much virtue in having an accurate barometer integrated into our echart programs. This can be accomplished with just an accurate sensor without displays of its own, or we can interface a full function barometer—the only requirement is a NMEA output of sentence  WMD or XDR.   Atmospheric pressure and its trend remains our key indicators of wind and weather changes, so having this displayed prominently at all times as we navigate or carry out route selection is always helpful. We typically rely on numerical forecasts that start with our present or recent positions, so seeing the true pressure we measured with an accurate barometer compared to the predicted pressures is a fundamental check on the forecasts… rather like comparing the depth sounder to the chart and tide tables as we enter shallow waters. 

Programs like Expedition add to the interpretation of what we see because they offer versatile graphic displays. Some barometers  have a display of their own but they are rarely as versatile or convenient as the Stripchart display in Expedition.

Connecting a barometer to a computer takes place in two steps, first prepare the barometer to output a NMEA sentence the program can read, and then configuring the program to read that input. This is essentially the same process for any barometer capable of this interfacing. See the barometer instructions on how to export the NMEA sentence needed. Either of the above will work with Expedition. Other echart programs might require one or the other.

Typical  sentences look like:


Other weather information could be included in either one of them. The last 3 characters are a checksum.

It is also valuable to check the barometer manual to see how often the signals are put out in seconds.

Configure Expedition to display pressure in a Number Box and Stripchart.

1. Launch Expedition

2. On the top LEFT of the menubar, Click "Expedition" drop-down

3. Select:  Instruments / Serial and Network Ports

4. The COM port used for the barometer should show up on the left. Select that one. (If it does not show up, then check Device Manager to see that you are connected.  A terminal reader such as CoolTerm—a neat and useful app—could help with this.)

5. Set these Connection Settings: NMEA 0183 Instruments; 4800 Baud rate; 8 bits; None Parity; 1 Stop bits; Boat 0 Boat (0 def)

(Note there is a default baud rate of the barometer output. 4800 would be standard, but the barometer manufacturer may use something different. We have seen them set at 57600, for example.)

6. Leave unchecked: Redirect incoming data and Command.  Leave checked: Use position fix; Validate checksums

7. Then click Apply and then click RAW DATA button and then recall the output frequency from the barometer and wait that long for data stream to show.  After data confirmation, close all set-up windows.

8. On the top LEFT of the menubar, Click "Expedition" drop-down, once again.

9. Select: Settings / Channels and set  Barometer “Damping (seconds)” to 0 on the left, and Barometer “Display precision” = 2 on the right.

Now we need to set up a Number Box to display the  barometric pressure "Baro" as follows:

10. On the top RIGHT of the menubar, Click "Window" drop-down and check or confirm "Number boxes" is checked. You can use any of the three boxes to show the baro data. You can display more than just Baro in one of the boxes.  

11. Click anywhere in the Number box window to display the "Edit" number boxes button, add Barometer, and configure the box to taste (i.e., Bold and with yellow background). Click the small + next to Barometer in the left panel to open the display options.  That baro display will then be persistent till you change it. It should be reading the same pressure you see on your barometer, except Expedition will now be showing two decimals, whereas the barometer itself might show only one on the front display, or maybe none, ie whole mb only.  This then would be another asset of having your barometer connected to Expedition.

12. Launch Stripchart by clicking "Expedition" drop-down (top LEFT of the menubar), then “Applications" / "Stripchart"

13. At this point you can either add the baro to a chart layout already in use or if this is the first one you will see Bsp (boat speed) as  the default.

14. To add baro to an existing chart, click "View" / "Number of channels" and increment the number by one to add a new data strip.  It will be added as "Bsp" by default at the bottom of screen.

15, Right-click "Bsp" and click "Edit this stripchart" to access Settings and choose "Baro".  Click OK and the pressure feed will begin tracing across the stripchart.

16. Do menu Save to save the new baro strip chart template (This step saves the template, but not related to saving the data).  The video below shows a few ways to use the chart for reading pressure and tendency, including:
— use of tool tip
— note middle pressure shown is not a scale element, it is actual pressure at the moment.
— use of grid option
— use of wands and computed sd for storm warning in the tropics.
— recalling past chart data from the log file.

Video illustration of the points above.

A special thanks to Andrew Haliburton for itemizing the steps of this process, which was the starting point for the lists above.

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