Re: How accurate are ALL depthfinders?
Walleyehed,<br /><br />There are only two things that keep a fish finder from reading in water of less than two feet and they are related in a sense.<br /><br />The first thing that prevents them from reading shallow is simply a timeing problem. Transducers are required to do two utterly opposite things and to alternate which one they are doing. The two things are; first, to convert an electrical signal from the display unit into sound. Second; the convert sound energy into electrical energy to send back to the display unit for processing. No transducer can do both things at once. <br /><br />With that in mind you have to consider two properties of the signal sent to the transducer by the display unit. The first property is the pulse repetition rate, which is simply the number of times if fires in a period of time. The second property is the pulse length, which is to say for how long is the signal being sent. Both of these parameters vary by the depth range the finder is set to. The deeper the maximum depth in the range the slower the pulse repetition rate and the shorter the pulse duration.<br /><br />Just like any person no transducer can talk and hear at the same time and every time they talk that ping they make has a life span, then there is a blank period in which the transducer waits for a return echo, and then at some time governed by the pulse repetition rate it will fire again. The point is that with the speed of sound in water constant at 4,500 feet per second the very beginning of an outgoing sound wave travels some distance while the sound is still being made, and because we're talking about an echo here it means that whatever that time is divided into 4,500 will give you how far and dividing that number by 2 will give you the minimum depth the fish finder is capable of measuring. Its really that simple. It can't hear the return echo because its comming back to the transducer while the transducer is still making a noise. <br /><br />To understand the second problem, which is similar to the first, you have to understand that the sound that is made by a trasducer is not very clean. By that I mean that it does not have a distinct beginnning and end. What actually happens (and remember this is very minute) is that when the pulse of electricity from the display unit hits the piezeolecectric element in the transducer it not only makes a distinct ping (its actually more of a click) at the same time the transducer will continue to ring just a little bit, like a bell would. Unfortunately for a transducer that ringing sound (which is measured by a value which is normally labeled "Q") is not different from anyother sound - and it can't hear when its talking, or in this case ringing. It is that ringing that blocks out the top part of the screen, usually just a couple of inches. On the other end of the stick all return echos also produce a bit of ringing following the true echo. It is this return echo that causes a bit of fuzziness in the display of small hard targets. For whatever its worth the Q value for most standard transducers is in the high 20's to the low 30's. If you really want extremely clear pictures in shallow water it is imperative that you use a transducer with a low Q value. For whatever its worth the transducer I use has Q values of 8 at 50 kHz and 10 at 200 kHz. There are other features about it that contribute to this besides the size of the transducer element but overall it has about 50 times the sensitivity (17 dB) of the standard transducer. Even at that it still won't shoot shallow targets all that well, simply because my maximum pulse repetition rate is 1,500 pulses per minute with a minimum pulse duration of 0.13 ms. It you take those numbers and do the math you see that the absolute minimum depth I could possibly shoot at would be half of a little less than three and a half feet, and then you have to add the outgoing and incomming ring to it (that pesky old Q factor).<br /><br />So, that's generally what stops you from shooting shallow, not a buffer.<br /><br />Now there is another feature that almost all decent fish finders have that is called the keel offset. What it allows you to do is add or add to the indicated depth to make up for the placement of the transducer relative to the water surface. Some guys like to set theirs so that what is indicated is the depth below their running gear, most folks never touch it at all. About the only time that haveing been reset will ever get you into trouble is if you dont' know about it having been done and are in an inboard boat. But even at that the depth limitation below the transducer will be limited to the things I mentioned above. The display unit, and all of its intricate software, will still read down to zero if a suitable signal can be processed to give that result.<br /><br />By the way, I do understand how transducer's work, at least to the extent that a nonprofessional interested amateur can.<br /><br />Thom
<br /><br />About 10-years ago a retired captn of nuke subs explained how satellites were used to map the oceans floor to within ¼-in for use in sub navigating. Didnt say if they actually use that accuracy.<br /><br />Boatist Simrad, w/10-in SunView display. It has no competition.
