DI versus SI Water column coverage

[Hollada]

What is the coverage difference between the DI beam and the SI water column?

[Wayne P.]

Start with page 45 for the spec coverages:

HELIX Series Operations Manual (widen.net)

[Hollada]

Might have missed it, but don’t see anything on the water column cone angle.

[SLYDoggie]

Might have missed it, but don’t see anything on the water column cone angle.

No such thing as water column Angle. There is only the cone/beam angle. The water column is simply a representation of the water depth. The deeper the water, the wider or deeper the water column. Both SI and DI will be the same at any instant in time.

Maybe These 2 Humminbird tutorial videos will help. I find that the "folding the SI image" part on the SI video really helps with visualizing SI water column.

Si --->

DI --->

[Hollada]

I got into this debate with someone on the HB tips group on FB which is why I am asking. He swears he sees things in the water column that won’t show up in DI. I assumed they had the same coverage, though with the DI having a 75 degree cone perhaps the higher in the water the fish are, the less you see on DI compared to the SI water column. This is why I am searching for info on the shape of the water column “cone” and I use quotes to avoid a debate over the word cone lol.

Is the water column derived from the DI piezos? That’s probably a question I should have asked.

With either the water column or DI, the deeper the water the wider the “cone”.

[Wayne P.]

I got into this debate with someone on the HB tips group on FB which is why I am asking. He swears he sees things in the water column that won’t show up in DI. I assumed they had the same coverage, though with the DI having a 75 degree cone perhaps the higher in the water the fish are, the less you see on DI compared to the SI water column. This is why I am searching for info on the shape of the water column “cone” and I use quotes to avoid a debate over the word cone lol.

Is the water column derived from the DI piezos? That’s probably a question I should have asked.

With either the water column or DI, the deeper the water the wider the “cone”.

The spec coverage is different between 2D and DI. DI is larger so stuff can be displayed on DI and not 2D.
A Down Imaging unit's transducer has two piezos--one for 2D and the other for DI, they are separate and in no way the same other than 2D ranges DI (depth scale).
DI has a water column and so does 2D.
The DI water column is generated by the DI frequency and shows what that frequency detects.
2D is generated by the 2D frequency and shows what that frequency detects.

When either are recording sonar echoes, they are recorded one vertical pixel row at a time on the right side of the display. Everything within the coverage is recorded in that first row of pixels and then that row is moved to the left and then the first row records again. That first row is the only real time data--everything to the left is recorded history, and the further left, the older the history.

Stuff is within the 2D coverage a lot longer than in the DI coverage since the 2D pulse is a somewhat inverted cone shape. The DI pulse is very thin front to back (inches) and wide to the sides.
When 2D records a fish as an arch, it is due to the TIME the fish is within the sonar's coverage. The TIME within the DI's coverage is not long enough to create the arch shape, most of the time a fish will just be displayed as a dot.
DI and SI use the thin sonar pulse to create the picture-like images==think CAT scan.

[Wayne P.]

This is the same fish on 2D and DI

[Hollada]

Never mentioned 2D. The discussion is about the SI water column versus the DI “cone”.

[Wayne P.]

Never mentioned 2D. The discussion is about the SI water column versus the DI “cone”.

The spec coverage for each side of SI is 86 degrees.
The actual SI total coverage is from the water's surface to past vertical under the transducer on both sides==180 degrees +
The spec DI coverage for 455 kHz and 1.2 MHz is 75 degrees left to right, its actual coverage is somewhat more.

In function, it is three different sonars that are pinged in a sequence. What all three do is completely separate from each other.
Four completely separate sonars when 2D is included.

The water column is the same for both, just get two water columns for SI since there are two SI pulses. The physical display size of the water column is directly related to the SI Range setting.
The physical display size of the water column with DI is directly related the Depth range setting that 2D sonar determines.
Both SI and DI do Slant Range, not distance like 2D sonar.

SI has more coverage to the sides since the piezos are tilted at an angle so it can do Side Imaging. Do not remember the actual angle of tilt. It was discussed extensively many years ago for positioning a SI transducer to do real Down Imaging with 1/2 of the transducer.
The DI piezo is positioned horizontal so its pulse is Down.
That would be why subjects can be recorded on SI and not DI, and a subject can be determined if it is left or right with Side Imaging.

[Hollada]

Thanks Wayne!. That’s exactly what I was looking for.