Fishfinder depth calibration

Taniwani

Furuno Super Fan
I noticed since a while that my TZT2 fish-finder seemed to show a different depth when compared to my old sounder (a 20 year old ST60). The TZT2 shows a constant 0.5m less.This is only of interest when it gets tight and I haven't been so close to the ground lately, but yesterday, moored in about 3m of water (with 2m draft) I went with mask and yardstick to find out the truth. The result, the old sounder is about right and the TZT2 shows about 0.5 to 0.6 meters less.

I have checked everything. Both transducers are 0.8m below the water line and on both sounders the transducer depth is set to this value. When I checked, bottom was level and the two transducers are about a foot apart, left and right of the centerline. The transducer I use is a airman B117 two frequency standard transducer that I already had from a previous fish finder. The TZT2 transducer setting is for 520-PLD. It is properly set to salt water. I get the same depth reading on HF or LF.

I have 'corrected' the problem by setting the transducer depth to 1.3m and now have the proper depth displayed. But I'm wondering if somebody may have an explanation for this behaviour.
 
You will normally get better and faster responses if posting sounder type questions in the fishfinder/sounder topic area. The units calculate based on the speed of sound through water. I haven't heard of any reported differences. Maybe you have some variance due to beam angle and hull deadrise.
 
Thanks. Don't think it is a speed sound issue. The difference is the same at nay depth. Always 60cm less. AT 100m it doesn't really matter, at 2m or 3m it does. As it is constant, I can get it to read correctly by setting transducer depth deeper than actual. So it is easy to fix, just curious if anybody else observed this.
 
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