Airmar DST810 Multisensor, really smart

Airmar trail blazed the NMEA 2000 “smart” sensor about fifteen years ago, building a microprocessor right into a transducer so that a single rugged N2K cable can both power the sensor and deliver Depth, Speed, and Water Temperature to almost any display, regardless of brand. And while the company deservedly dominates the world of water-related smart sensors today, they were a tad slow to adopt the now-common technique of including a Bluetooth app for the detailed calibration that many of the display manufacturers fail to provide.

But after initial testing, I think that the relatively new DST810 Smart Multisensor is a thoroughly modern N2K device. The free CAST Bluetooth app works well and offers about every level of calibration possible. So even the most accuracy-obsessed boater can get maximum precision from a DST810, which also has improved raw sensing features compared to its predecessor, and with only a modest cost increase.

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In fact, the DST810 Smart looks just like the venerable DST800 Smart Triducer. Both are typically kitted with a valved casing — also available in bronze or stainless — and a blanking plug (mainly because the paddlewheel has to be kept clean). Both also claim depth performance from 1.6 to 600 feet, absolute water temperature accuracy to about two degrees Fahrenheit (with a much finer change response), and a paddlewheel pulse rate of 5.6 per second per knot.

But while the 800 recalculates its three main senses once per second, the 810 updates STW 5 times per second for “visibly smoother speed-through-water reporting” and adds a Trim/Heel sensor (also known as Pitch/Roll). While that’s all good, the big news is the app that can perfect these values for a specific boat install.

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Connecting the Airmar CAST app — either Android or Apple iOS — turned out to be simple and reliable. Like many current Bluetooth Low Energy (BLE) devices, the app itself makes the connection, no messing with the system settings on your phone or tablet. You do need to confirm the DST810 serial number for the initial connection — to avoid transducer confusion in a marina — but after that, it just works. Moreover, as shown upper left, on first connect you can give the 810 a custom name and pick the type of boat icon you want to see on that CAST graphic data display page.

I don’t picture many boaters regularly opening the app to see Depth, Speed, etc., but that is surely the obvious first troubleshooting step if one or more of the DST810 data values stops showing, or acting odd, on a boat’s N2K displays. And, full disclosure, please note the zero knot Speed on my test screen. Although I’ve had the 810 for over a year — and even managed to freeze it solid in a bucket outside my shop, an unintended test it passed with flying colors — I never got big Gizmo underway last season. So consider this DST810 review part one, though I’m already confident that I’ll be able to calibrate it quite well.

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First off, control of an N2K sensor’s individual data PGN (Parameter Group Number) output is a rare feature, but it can be useful. For instance, if the paddlewheel failed beyond calibration or cleaning, I’d rather turn off the output than have badly inaccurate STW showing on my displays and messing with calculations like “Sailors” True Wind, Fuel Economy, and Current Set/Drift.

(Note that I used similar Airmar PGN control when troubleshooting Sally W‘s dew point temperature issues last summer, but that required a laptop and a USB-to-N2K gateway. If and when other Airmar sensors like that 220WX WeatherStation get CAST-style Bluetooth app configuration, it will be most welcome.)

Note too that a PGN refresh rate is different from a sensor update rate, so setting the former faster than the latter won’t get you much besides increased data traffic on your N2K network. And turning off redundant PGNs, like the older Temperature 130312, can reduce traffic unless you have an older display that can’t use 130316 (see this NMEA PDF).

Meanwhile, most any MFD or instrument system will let you set a Depth offset — probably Sea Temp, Roll, and Pitch, too — but that middle screen above shows one of the slickest Offset implementations I’ve seen. For instance, CAST knows that a negative Depth offset means you’re correcting for Depth Below Keel — see proper labeling on the data display screens further up — while a plus offset is meant for Depth Below Surface. In my experience, many MFDs are vague in this department.

Unfortunately, that type-of-Depth labeling doesn’t travel over N2K, and neither do the preferred units you can set in CAST (last screen). But at least you can clearly set everything the way you want it in the app and then reference its data as you setup your N2K displays.

Oh, and note the two levels of CAST Trim/Heel calibration: Enter them manually like Depth and Temp offsets, or, if your boat is sitting right, just hold down the app button for about 30 seconds to zero out the raw values. Which also foreshadows the amazing multitude of speedo calibration levels…

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Calibrating STW is quite tricky because water often flows at a different rates close to the hull than the whole boat’s overall Speed Through the Water, and those differences can change with overall speed and/or heel, with the latter rate changes often quite asymmetrical. Yike!

Now, I know that sail racing instruments from the likes of B&G and others support deep STW calibration — I was once quite impressed with how well a Raymarine i70s can do it — but I don’t recall seeing all the choices CAST offers, and using them seems about as clear and easy as possible. (Good PDF manual here.)

So what CAST terms “Basic” is a single percentage — like +10% means that 10 raw measured knots will show as 11 knots — which you can simply enter. But that little “magic wand” icon leads to dual methods of auto calculating the percentage, either by comparing the DST800 STW to your phone’s GPS SOG (Speed Over Ground) or by running a known distance.

In both cases, CAST prompts you through reverse runs to detect and average out possible current flow, and in distance mode you can take as many runs as desired, even throw out messy ones. For many powerboats, one of these calibrations well executed at typical cruising speed is probably all that’s needed for a usually solid STW reading, but sailors will likely add the Heel mode illustrated in the middle screenshot.

Speed at Heel calibration can also use either SOG or distance but it is best done without current because separate port and starboard corrections are calculated, not averaged, because they can be truly different (unless the transducer is on the centerline). It’s also best done at a sailboat’s average heel as CAST will interpolate the heel correction against the main speed correction for other angles.

Still not precise enough for you? The third screenshot suggests how you can build a table with different STW corrections for up to 10 different boat speeds and two heel angles port and starboard. This is a manual operation presumably prefaced by lots of data logging and analysis, but I know avid racers willing to do the work to get an edge (and hope that we hear from ones who try it with a DST810).

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I did stumble on a minor bug in the iOS version of CAST: the values on the Offsets page are actually in Meters though shown as Feet. Hence the “5.1 ft” raw Depth versus “15.2 ft Depth Below Keel” display (in lefthand screen collage), and so my -0.5 foot offset is actually -0.5 meter. While I suspect that most users will figure this out eventually, Airmar should fix the app.

In the righthand collage, you can see that the Android CAST version of the same half-meter Depth offset is correct at 1.6 feet, and also how Actisense Reader shows that both transducer depth and offset are included in the N2K Water Depth PGN (which might further help with realizing the iOS bug).

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Finally, here is the DST810 connected to Gizmo’s Simrad GO5 (though the boat is still in a shed). Sadly note that only two bits of device configuration are offered for the DST810, Depth Offset and Device Instance.

I was pleased to see, by opening CAST, that the +1.5 ft Offset I’d just entered into the GO5 was written into the 810 transducer (and wasn’t just a data overlay only available to same brand displays, which is how some MFD sensor configurations work). So redundant sensor access is another plus for having a separate Bluetooth configuration app.

Then again, CAST is missing Device Instance configuration, though that remains perhaps the most confusing aspect of NMEA 2000. In some cases, Maretron particularly, two sensors outputting the same PGN need different Instances if you want to see both values at once. But look at the dual Depth, Roll, Pitch, and SOG values that I’ve set up to show on the GO5 instrument screen, no Instance manipulation required (and useful custom labeling included).

At any rate, the screen should be a useful reference when I finally get to experience DST810 raw accuracy plus CAST calibration on the water, hopefully in May or June. But given this bench testing — plus the wide success of the original DST800, usually despite limited calibration — my expectations are high. And maybe some readers can share their on-the-water DST810 experience in the meantime?