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 find the temperature break
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choogenboom
Junior Member

43 Posts

Posted - 07/16/2010 :  07:56:43 AM  Show Profile Send choogenboom a Private Message  Reply with Quote
Curious about how you find temperature breaks. I have been relying on my chartplotter's temperature graph - a Garmin 3010C (display) + GSD20 (sounder module) + Airmar P66 (transducer + speed + temperature). Before I go to far into this topic I should share that I design temperature sensors (for air, not water) for a living so I have a decent handle on the physics of the problem. What I have noticed is that best I can tell the Garmin GSD20 is doing a fairly long time average of the temperature data. I am sure that I have crossed many temperature breaks but on my temperature chart I have never seen a sudden steep rise, or a step change, in temperature. If you dig around Airmar's site you find they spec the temperature accuracy at +/-0.2C and that it will respond to within 63% of a step change in 22 seconds so the problem is NOT the transducer. Garmin does not spec the accuracy of the GSD20 nor do they mention the integration time constant used for the time averaging. Bottom line is I am certain the GSD20 is doing the opposite of what you would want to find a temperature break and I am guessing that's the norm for other brands of equipment as well. Anyone out there have similar experience with their electronics? Anyone have a temperature plotter that DOES work the way you would want it to? Or is relying on your temperature graph not done and you should look for color changes, rips, weed lines etc to find the breaks?

Grady White 330 Express

Sirecks
Senior Member



1262 Posts

Posted - 07/16/2010 :  12:00:03 PM  Show Profile  Visit Sirecks's Homepage  Send Sirecks a Yahoo! Message Send Sirecks a Private Message  Reply with Quote
I'm a real noob at this myself, but my first question is what part of the season are you talking about? If you go out early in the season (April or May or so), I've watched the temperature rise quickly as I go through eddies or fingers of the GS. I have a smallish Garmin and it picks it up pretty quickly. Like, less than a half mile I've watched from 69 to 72 under power going to my fishing spot. This time of year, the water is so warm, I've not witnessed a dramatic temperature difference. In fact, leaving the harbor is always warm, it cools off a degree or two as you head out and then getting towards the GS it goes back up a degree or two. That's about 50 miles where it only varies a degree or two from the rocks to say 200 feet.

Again, I've only been doing this a short while. I'm sure that someone with much more expierence and a bigger brain than me can answer this better.

If you look at this temp map from last Tuesday you'll see what I mean. There is only about 2 to 3 degrees difference from the body of water between the beach and the GS. There is a big area of 81 between the 80 and 83, so that would stop you from seeing a big temp break, until you crossed into the GS and saw the jump from 81 to 83.

http://www.sstcharts.com/sc_10_07_06_06_00_AM_H.shtml

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2003 Sea Pro 220
"Reelly Blessed"
2002 Yamaha 250 OX66
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choogenboom
Junior Member



43 Posts

Posted - 07/16/2010 :  1:06:16 PM  Show Profile Send choogenboom a Private Message  Reply with Quote
I have been out between april and october as well as fished solid weed lines and visible rips. In that time I have not see my temperature plot do what I would expect which is show a steep ramp up or down as I cross what I think is a meaningful temperature break. As an experiment last night I dropped a digital meat thermometer into the water off the back of my whaler which has a lowrance 525C and the temepratures matched with +/- 0.1F. I then took the meat thermometer and hung it off the back of my Grady right at the same location as the transducer (I actually spooked a sheephead that was right up against the transom between the motors while I was doing this) and verified that (1) my Garmin was reading 1 degree F low (a problem I can live with) and (2) short term 0.5 degree temperature changes that were registering on my meat thermometer were not showing up on my Garmin temperature graph. So I suppose its best to look at my initial post as a two part question:

Part one is - does anyone else have experience similar to mine that indicates that Garmin (and/or other manufacturers) time average their temperature data to smooth out short term fluctuations?

part two is - if we all have similar equipment that does similar things in terms of time averaging the temperature data how do you all find the tempertaure breaks?

My experience with my garmin is that by the time the graph shows a temperature rise the edge of the break is probably miles behind me.

Grady White 330 Express
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billin
New Member



8 Posts

Posted - 07/16/2010 :  1:46:58 PM  Show Profile Send billin a Private Message  Reply with Quote
the temp breaks you are seeing visiable in the forms of a rip are due to deep ocean upwellings you are measuring SST so once the water hits the top the temp levels out but underneath thats a different story. As far a dramtic SST changes I have only seen those in the early spring late dall and winter on the edges of the florida current(gulf stream) due to the current involved the two masses of water have a hard time mixing and you get very little radiate heat from the sun that time of year so SST will be more consitant with whats going on a few feet down as oppsed to now there are hugh changes in temp below the thermoclines you can go to the Rutgers website and they offer much of this information and offer sst charts and sub surface temp charts that give you an idea of thermal changes below 30 feet etc.
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Courtland
Senior Member



2222 Posts

Posted - 07/16/2010 :  1:48:29 PM  Show Profile Send Courtland a Private Message  Reply with Quote
This is on my Northstar.

I can adjust the temperature to whatever it is at the dock. So I could call Joe on the radio before we left and say "hey what temp you got?" I could then adjust mine to read the exact same as his. I suspect you can do this in the calibration section/setup on your plotter.

I can also adjust the frequency/time it takes a reading in the same setup menu. Take a reading every second, 2 seconds, 1/10 of a second, etc..

My unit is 3 years old and NOT that hightech. I would assume you could do the same.

You can also get a dedicated temperature display with a seperate transducer on it, they can read to hundreths of a degree at varying times and continually adjust.

In closing- rarely will 2 machines have the same temp., but you should be able to calibrate them. You should be able to run 30kts. or whatever and see immediately the change in temperature on your display. My guess is you need to change the time frame of how often you take a reading in your setup menu.
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brianh
Senior Member



549 Posts

Posted - 07/16/2010 :  2:16:20 PM  Show Profile Send brianh a Private Message  Reply with Quote
I think Courtland hit on the problem, how often it takes a reading. I have a Garmin and a Raymarine. These 2 units were almost 4 degrees apart. I didn't worry too much about that, I just set the Raymarine -3.8 degrees and now they read pretty close. If you fish with another boat, be sure and calibrate them to read the same.

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choogenboom
Junior Member



43 Posts

Posted - 07/16/2010 :  3:06:06 PM  Show Profile Send choogenboom a Private Message  Reply with Quote
The garmin 3010C does not have a temperature offset OR a reading frequency. It only has a graph time span and temperature span settings. The graph time span does not seem to impact the time averaging. After googling "garmin 3010 temperature offset" I cam across many disgruntled Garmin 3010 owners complaing that (1) it reads up to 3 degrees low (atrocious, IMHO) and there is no user settable correction or offset. Garmin tech support is aware of the problem but to date has not offered a software update to correct it. Note that I measured the termistor output directly with a precision ohmeter and confirmed the error is introduced in the GSD20 sounder module. The 3010 just displays what its told by the GSD20 module so does not play a part in accuracy but can/should play a part in allowing for user settable offset and time average. I ran across an airmar T42 (thru hull temperature sensor with NMEA183 output) as a possible solution although at $200 its no bargain. I might just stick a $20 digital meat thermometer from Walmart into my livewell - it will work better than my $2000 garmin!

Grady White 330 Express
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choogenboom
Junior Member



43 Posts

Posted - 07/20/2010 :  11:05:30 AM  Show Profile Send choogenboom a Private Message  Reply with Quote
Tested my system's response to a step change in temperature this morning and turns out I was very wrong. It responds instantly. I set it up so I could jumper a resistor in parallel with the thermistor and simulate a step change in temperature from 86 to 112 degrees and thats exactly what my graph showed with no delay and no averaging. That tells me that in practice a temperature break is not going to show up as dramatically as I thought it would.

Grady White 330 Express
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