Fishfinder depth calibration

[Taniwani]

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.

 

[Johnny Electron]

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.

 

[Taniwani]

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.