DST-800PSF - W Temp reads crazy high

J

JordanBigel

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Hello, I have the DST-800PSF NMEA2K Spd/Temp Transducer connected to my N2K network. On my instruments (FI-70) the "W Temp" reading started to read crazy high last week.

I installed it about 2 years ago and its been reading normal (reasonable) temps. Just 2 weeks ago the water temp was about 68F but then last week I noticed it shows 104F and since then it has been creeping up and now its reading 118.1F. Powering off the system and restarting it is the only thing I could think of so far.

The depth reading is correct - normal (14.6 ft at the dock right now)

Is this something I can fix, or do I need a new transducer?

Thanks,
Jordan.
 
I had this happen in the spring with a B64 transducer. The way it measures temperature is with a thermistor on the face of the transducer that changes its resistance as the water temperature changes. It is extremely simple.

Your problem can be one of two things. The thermistor could have gone bad, in which case you will need to replace the transducer. The other possibility is that the internal connection from the thermistor to the NMEA 2000 conversion circuitry inside the DST800 has failed. With a conventional transducer, it would be possible to check the thermistor by measuring the resistance on the connector pins. Sometimes it is just an issue of corrosion on the connector. This can't happen with the DST800 because the signal on its cable is NMEA 2000 PGNs. Still, it may be worth giving a thorough cleaning with contact cleaner to the plug.

If the error is consistent, you can try changing the temperature calibration on the FI-70 to offset the high readings. Does everything else on your NMEA 2000 network work properly? A voltage issue could cause a problem too, though not likely.
 
Thanks for the feedback. Visual inspection of the NMEA connections (which are in a dry place) look like new. The FI-70 at the nav desk shows 13.4vdc on the NMEA bus, but I'll check the bus at the connection to the transducer to see if it is the same.

I wonder, based on how you describe the thermistor (I know nothing about how those things work) - would it be possible for an over-vigorous bottom cleaning with a stiff brush could damage the thermistor?
Thanks.
 
I should add in response to your other question - everything else on the NMEA network is functioning normally. Also, when first powered up, the temp reads something in the mid 90s and then over the first 2 or 3 minutes the temp gradually rises to about 118F, so setting an offset I dont think that will work, and really, if I have to set a 48 degree offset, its not really a system I can rely on.
 
JordanBigel":ywdtwz79 said:
I should add in response to your other question - everything else on the NMEA network is functioning normally. Also, when first powered up, the temp reads something in the mid 90s and then over the first 2 or 3 minutes the temp gradually rises to about 118F, so setting an offset I dont think that will work, and really, if I have to set a 48 degree offset, its not really a system I can rely on.
Click to expand...

Unfortunately that is just what mine did when the thermistor went bad. It would be close to normal on startup and then go up 30-40 degrees.

One last thing you might try is pulling the transducer out (assuming you have the blanking plug) and dunking it in a bucket with some ice to see how the temperature reading responds.

With mine the error was quite stable once it had been running a few minutes and the offset worked okay.
 
Wow, so today I turn on the instruments and "W Temp." reads: *99.9f - that's new.

I haven't tried to set an offset - I figure I need to get a real thermometer and drop it in the water first to find out the actual temp, so I haven't done it yet.
 
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