Tuesday, November 4, 2014

Choosing and Installing your Lowrance Transducer

The transducer connected to your Lowrance echosounder plays a critical role in producing quality map outputs.  Fortunately, the mechanics of producing quality hydroacoustic signals has been honed by 57 years of research and development by engineers at Lowrance.  Still, users play an important role in optimizing outputs by selecting the correct transducer and installing it correctly

Selecting a Transducer
There are several resources that can help you choose the correct transducer.  On one of our support pages, you can read about the types of Lowrance transducers, and common installs (e.g., transom, shoot-thru, bolt-thru).  Further, we have another interactive site that demonstrates the size of bottom that is scanned at different depths for different frequencies and transducer models.  The most common transducer for Lowrance HDS for inland applications (both fishing and mapping) is the dual frequency 83/200 kHz skimmer® broadband transducer (model HST-WSBL).  This is a popular transducer due to its small size, low cost, ruggedness, and reliable performance.

Still, Navico and our partners at Airmar Technology Corporation offer a wide range of compatible transducers for Lowrance and Simrad multi-function displays.  See the Navico Store for a full range of options.  Thus, the advanced user has a range of options to customize their setup to their use cases (e.g., inland vs offshore, small shallow ponds vs large deep lakes; Figure 1).
Figure 1. Airmar TM260 narrow-beam dual frequency transducer (left) and the much smaller Lowrance HST-WSBL dual frequency wide-beam skimmer® transducer (right) with a ping-pong ball as a reference to scale.
BioBase mapping outputs are currently optimized for the 200 kHz broadband frequency.  But users have the option of choosing narrow (6-deg), medium (12-deg), or wide (20-deg) beam angle transducers.  The beam angle determines the size of the acoustic footprint of each pulse and thus the local sample area (Figures 2 and 3).  Most Lowrance HDS Sounders for use in inland waters come equipped with the wide-beam HST-WSBL transducer.
Figure 2. Qualitative differences in signal return and simulated beam angle of the Airmar TM260 6-deg 200 kHz transducer and the Lowrance HST-WSBL 200 kHz 20-deg transducer.  Data were collected on Orchard Lake, Dakota Co., MN USA over a bed of dense aquatic vegetation (coontail and northern watermilfoil).
Figure 3.  Schematic demonstrating the overlap of acoustic beams across alternative 200 khz transducers given normal (10 pps) to fastest (20 pps) ping rates.  Beam angle is to scale, but actual cone overlap depends on boat speed.  For most inland mapping, overlap in the Y direction (i.e., in the direction of the boat path) across different beam angles is high and the actual difference in acoustic footprint can only be seen in the X direction (i.e., to port or starboard sides)

Although the TM260 and HST-WSBL represent both ends of the spectrum in terms of beam angle, both tracked bottom depth similarly (within 6") in aquatic plant environments in recent tests on a Minnesota lake (Figure 3).  Thus, in most common shallow-water circumstances we may expect similar map outputs with both narrow- and wide-beam transducers.
Figure 3.  Differences in depth declaration in an aquatic plant bed from a narrow beam transducer (Airmar TM260) and wide-beam transducer (Lowrance HST-WSBL) from repeated transects.  Data were collected by a Lowrance HDS-9 Gen2 Touch and analyzed by BioBase Automated Mapping System.  Dots are means from pooled samples along 3 repeated transects for each transducer.  Error bars represent 95% Confidence Intervals.  Overlapping confidence intervals mean that differences were not statistically significant.
Types of Mounts
There are a range of options for mounting your Lowrance or Simrad transducer that we discuss and demonstrate in a popular blog published in 2013.  In Table 1, we list the major types of mounts BioBase users deploy to survey aquatic habitats.

Table 1. Examples of 4 different transducer mounts: TransomPoleShoot-thru, or Bolt-thru mounts.

*Shoot-thru mounts with pliable, puddy-like duct-seal are portable, Shoot-thru mounts with epoxy are permanent.

**For this analysis, scupper-hole transducer mounts are considered "bolt-thru" but are not permanent

For the highest quality outputs and most reliable performance, we recommend the bolt-thru transducer mount, especially for high-use, dedicated survey vessels.  The transom mount is recommended as a high quality, flexible option where a permanent mount is not feasible.  However, misalignment of a transom-mounted transducer is a common issue that is often overlooked.

Importance of Proper Transducer Alignment
A transducer that is not parallel with the ground can result in inaccurate depths, vegetation detection, and bottom hardness estimates (Figure 4).

Figure 4.  Example BioBase output where a transom mounted transducer was slanted downward.  In this case, with the boat moving from SW to NE along shore, signal strength is much more diminished on the downslope than on the upslope where the signal is more direct.

Although properly aligning your skimmer® transducer seems simple to accomplish (Figure 5), boat transoms are often a vulnerable place for a transducer to be and can be knocked off alignment by a variety of unintentional actions (e.g., trailer loading, obstructions in water, etc.).  Often a misaligned transducer can escape the notice of even the most skilled operator.

Figure 5. Schematic of a properly installed skimmer transducer on a boat transom.

Monitoring your SONAR page while recording and correcting issues quickly when they arise is the best way to ensure high quality BioBase maps.  If you do suspect that your transducer is misaligned while recording, first stop recording, adjust your set up, test, and then begin recording again.  Make note of the filename where you observed the problem and you can later edit your output in BioBase if the file was only partially affected.  If the entire file was erroneous, avoid uploading to BioBase and merging it with other files

Get Expert-level Map Outputs By Only Following a Few Easy Steps
Advances in consumer sonar and gps technology coupled with automated cloud computing has removed a great number of prerequisites for creating high quality aquatic habitat maps.  Yesterday, practitioners were required to possess a great understanding of the fundamentals of hydroacoustics, GIS, and cartography and had to calibrate their hardware by precise "knob-turning."  Today, with Lowrance and BioBase, you can grab a Lowrance HDS off the shelf, take some care installing your transducer, record your sonar while on the water, upload to BioBase, and high quality maps will be produced automatically.  Contact us at info.biobase@navico.com for a free demo!

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