Monitoring dew point temperature at your helm, why and how

Allan Seymour wants to see the current Dew Point and outside Air Temperatures at the helm of Sally W because fog is often about to shut in when the two temps become similar. And there are at least three marine sensors that claim to deliver dew point over NMEA 2000, two of which Allan owns. But actually getting the value to show on his instrument display turned out to be quite a challenge. I helped with the troubleshooting, and came to better appreciate the value of dew point in the process.

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“Dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%” and so the temps you’re seeing above on Allan’s Garmin GMI 20 indicate air almost completely saturated with water. In fact, the fog was already thick around Sally W in Camden Harbor that August 20 morning, though air temp increasing faster than dew point would soon forecast a burn-off.

That morning the fog also lifted at my house a mile inland and l was later able to graph the divergence between air and dew point temps thanks to the excellent Tempest Weather Station I installed on my roof last February. And, yes, the fog closed back in that night and hung with us for days. (You can see that graphical history and/or real-time conditions at my public Tempest webpage or my associated Weather Underground Station.)

But if fog and dew occur as atmospheric conditions approach 100% relative humidity, why not just monitor humidity? The National Weather Service explains why dew point better indicates how “humid” it will feel outside, and let’s note that it’s much easier to compare (and graph) dew point and air temps simply because they’re in the same denomination. Plus, as I finally explore the subject, I’m impressed with how important dew point is to airplane pilots.

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Some old salts who already value dew point use an analog tool called a sling psychrometer to measure it, though the process is a bit of work (as shown in this good explainer video). However, dew point can also be calculated using air temperature and relative humidity, both of which are measured electronically by the three NMEA 2000 marine sensors above. I wish I could say that all three devices also calculate dew point and output it on N2K, but the truth is more complex.

Actually, the situation on Sally W was doubly complicated when I came on the scene. The manual for Allan’s Airmar 220WX WeatherStation indicates dew point output — because he had also purchased the humidity sensor option — and his elderly Garmin GMI 10 was able to display a data window labeled Dew Point. But there was no number in it, and we eventually learned that both devices were guilty.

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I brought along a Maretron DSM410 on my first troubleshooting visit and it showed Dew Point as soon it was attached to Sally W‘s N2K network. So the problem was only the GMI 10? Well, we couldn’t tell for sure because Maretron displays can calculate dew point if they see air temperature and relative humidity data on the network, as suggested in the description of their WSO100 ultrasonic wind and weather station (now discontinued, I don’t know why).

Then Allan remembered that he had once purchased a Yacht Devices YDHS Humidity Sensor, but he hadn’t been able to see the dew point (he so desired) on the GMI 10, and so he stuck it in a drawer. Like the Airmar WX, the YDHS purportedly does its own dew point calculations, and its manual describes output using two standard NMEA 2000 temperature PGNs that support Dew Point as a “Source” (or type).

So the GMI 10 really seemed to be the problem. Moreover, Garmin tech support apparently agreed that there was dew point display issue and told telling Allan that if sent the GMI 10 in, they would either fix it or send him GMI 20. And when I tried Allan’s YDHS sensor with the GMI 20 installed on Gizmo Junior (as shown at right above), and the 20 nicely displayed all the YDHS data, dew point included.

Allan got so excited that he bought a GMI 20 himself (figuring that Sally W could use two if Garmin came through). But when he installed it on his N2K network with the Airmar 220WX, a dew point number did not show up. Doh!

So it was time to get serious about the troubleshooting, and this time I showed up with a Windows laptop and NMEA 2000 to USB gateways from both Maretron and Actisense. That way I could diagnose the Airmar WX’s output using either N2KAnalyzer or NMEA Reader, each of which can be more informative, depending on the circumstances (and in my experience).

The Actisense NGT-1 also meant that I could run Airmar’s WeatherCaster display and configuration software, which turned out handy. Because when we could not detect a dew point temperature PGN coming from Allan’s 220WX, Airmar tech support said we could enable it with WeatherCaster. But that only turned out to be partially true.

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The only 220WX PGN with dew point that I could activate with WeatherCaster — by increasing the transmission interval to more than 0.0 times per second, as shown at left above — is called “Additional Weather Data” and was created by Airmar. So, yes, a Dew Point gauge started to work in WeatherCaster on my laptop and the value was going out to Allan’s N2K network, but no other company’s display is going to show it unless special programming has been done.

The Garmin GMI 20 definitely can not display the dew point temp contained in proprietary PGN 130880, and Airmar tech support doesn’t know of a display that does (aside from the WeatherCaster PC software). They were also apologetic about their initial claim that the WX outputs dew point over the standard Temperature PGN 130312 (like it does with Air Temperature), and said that the Airmar engineering department will consider a firmware update.

If that happens, the WX output will probably look a lot like what I saw coming from the YDHS using N2KAnalyzer (upper right). Yacht Devices uses both 130312 — for backward compatibility with older N2K displays — and the newer “Temperature, Extended Range” PGN 130316. This should put Dew Point (plus Humidity and Air Temp) on most any N2K display, though there’s never a guarantee and let’s hear from anyone who has had trouble with this.

Meanwhile, Allan installed the YDHS in the stack of his Lord Nelson Victory Tug and set up his GMI 20 to display its data instead of looking for it from the WX. So he finally has the current Dew Point and Air Temperatures in Sally W‘s wheelhouse, though the shortcomings of the GMI 10 and the 220WX were a bit discouraging along the way. (At least both tech support teams were responsive, and both products do a whole lot well.)

But those shortcomings also suggest that many boaters are unaware of how dew point can forecast the arrival or departure of fog. I certainly was, but I’ve seen it work now — thanks, Allan — and plan to have it on Gizmo when she hopefully heads Downeast next summer. What are your thoughts on this subject, and are there other sensors that can put dew point on a boat display?