Argonaut-SW Frequently Asked Questions (FAQ)

What does the "SW" mean?

"SW" is our acronym for "Shallow Water". Although shallow is a relative term, we are referring to water from 0.5 to 16 ft (0.15-5 m) deep.

I’m familiar with some of the other SonTek Argonaut products, what is different about the SW?

The electronics architecture of the Argonaut-SW is very similar to our other Argonaut products, and the data output follows the same format as our Argonaut-SL (Side-Looking) and Argonaut-XR (Extended Range). As such, many of the basic commands and settings are the same, and the SW is fully supported by the ViewArgonaut software program. The product packaging is a bit different. Because the SW is intended for shallow-water applications in channels, the physical design is low-profile (see photo), and the housing is a rugged, black polycarbonate plastic. Another primary difference is that the SW uses a smaller wet-mateable 8-pin connector for power and real-time output communications.

What is the SW's intended application?

The SW is intended to be bottom-mounted, looking up through the water column for most applications. This could be in a canal, natural stream, culvert, or pipe. An important consideration in the design of the SW was that it would be a device capable of calculating and outputting discharge values (flow rates). That is, in addition to measuring water velocity and level, it can also calculate and report the channel flow. This can be accomplished in several ways, including entering the cross-sectional area into the SW, using theoretical equations for flow calculations, or entering an empirically derived "index-velocity" relation.

What is the SW actually measuring?

The SW has three acoustic beams (see drawing at left). When mounted properly on the bottom, one of these beams points straight up, and the other two point up/down stream at a 45-degree angle. The upward-looking beam measures the water level only, while the two slanted beams measure the water velocity via the Doppler method in two dimensions. The level and velocity information is used to compute the flow, mean-velocity, and channel area.

Where in the channel is the SW measuring the water velocity?

When mounted properly along the axis of the channel with one beam pointed upstream and one downstream, the SW will measure the average velocity from an imaginary layer about 0.4 ft (0.12 m) above the transducer head, all the way to the surface. This ability to measure water velocity all the way to the water’s surface is a key technical innovation of the SW. The system first uses the vertical beam to measure the distance to the water surface. The SW will then automatically compute the velocity cell end based on this measured distance. As such, as the water level changes, the velocity measurement will automatically adjust to always measure to the surface no matter how frequently or rapidly the level changes.

Can the SW profile the currents?

Yes. The SW can report up to ten additional water velocity cells. The size and position of these cells are user-selectable. This is in addition to the single dynamic cell mentioned above.

Can the SW measure reversing flow?

Yes. When mounted properly with one beam pointing upstream and the other beam pointed downstream, the SW will be able to detect when the flow changes directions.

What kind of pipes can the SW be used in?

The SW contains flow algorithms for circular or elliptical pipes of 1 ft (0.3 m) diameter and larger; however, for better results we recommend that it only be used in pipes of 18 inches (0.5 m) in diameter or larger. At present, the SW is not suitable for pressurized pipes.

What is the relationship between the clarity of the water and the SW’s ability to measure water velocity?

Doppler sonar instruments such as the SW actually measure the movement of particles in the water, as opposed to measuring the water movement itself. This carries the assumption that the movement of the particles in the water is representative of the movement of the water itself. This actually turns out to be a very safe assumption (on which we have built our business). So if you have a body of water where no particles are present, the physics are such that Doppler current meters will not work. Thankfully, most natural environments almost always have something suspended other than just water molecules (even if it is just tiny air bubbles).

What makes the SonTek Doppler system different from most other manufacturers (who service the process control and irrigation customers) is the sensitivity level. SonTek’s technology is excellent at deciphering reflected acoustic energy from the background noise. As such, even a small amount of particles in the water is usually good enough for most measurements. Please keep in mind that SonTek instruments evolved from high-end laboratory and oceanographic instrumentation, so we are quite experienced in these environments.

Often times the reflective "scatterers" are either microscopic or even transparent organisms. As such, the visual clarity of the water (either observed or measured optically) typically has no bearing on how well a Doppler will work. If you have any questions about whether or not the SW will work in your environment, contact a SonTek representative.

Can I use the SW as a side-looking device?

This is possible, however, you are then limited to using it strictly as a velocity-indexing device. This is because in a side-mounted SW the normally vertical beam becomes useless for detecting the water level. You would then need to use an external stage device to get the level. For side-looking applications, you might want to also consider an Argonaut-SL (Side-Looking) with a vertical beam.

Should I use an Argonaut-SL (Side-Looking) or an Argonaut-SW for my application?

The Argonaut-SL has become increasingly popular in recent years because they are so easy to install and maintain. Shorter cable runs, lower risk of loss/or burial, and lower installation costs are just some of the reasons SLs are preferred in many instances. Because they are fixed mounted on the side of a channel, generally, a side-looking device should be installed such that its location is consistently at 30-70% of the maximum water level for optimal flow performance.

Here are some instances where an SW should be considered in place of an SL.

• Large variations in water level: If the water level falls near or below the level of at which the SL is installed, the device cannot measure properly. Thus, while the SL can work fine in small channels even down to 1.5 ft (0.5 m) or so, it is better suited for sites that do not typically show large variations in water level.
• Stratified Flow: The SL measures in a sampling volume that spans through a horizontal layer. For calculating flow, it is assumed that the water velocity in this layer is representative of the channel velocity. This works exceptionally well for sites where an index-velocity will be performed, and the SL can even detect reversing flow. However, since it measures in only one location along the vertical cross-section, theoretical flow calculations may not hold up if the flow is highly stratified. Within the SW, you can use the optional multi-cell feature to collect incremental velocity data for up to five additional cells.
• Complex velocity profile: In certain complex flow regimes the vertically-integrated velocity measurement of the SW provides more information about the velocity distribution within the channel and can therefore produce a more robust theoretical flow estimate. The SW also works equally well at sites where an index-velocity calibration will be performed. The down side of bottom-mounting is that it can involve a more complex installation, longer cables, and the risk of the unit being covered by debris or buried by silt.

In many cases, both the SL and SW are perfectly viable solutions for the same measurement location. Your decision should then be based on the ease of installation, ongoing maintenance, flow stratification, and how variable the water level is. If you have further questions about this, we recommend that you contact SonTek directly for assistance.

How does the SW differ from the Argonaut-XR?

Basically the Argonaut-XR (Extended Range) comes in a larger housing to accommodate a compass/tilt sensor, pressure sensor, and a third, orthogonal acoustic beam for velocity measurement. The XR does not have a vertical acoustic beam and instead uses a non-vented pressure sensor for water level, so its use as an accurate flow device is limited by its ability to accurately measure water level.

Because the XR measures 3D water velocity (the SW measures 2D velocities) has a lower acoustic frequency, the XR is usually preferred for deeper rivers where an existing level device is already installed, or when the application is more research-oriented and a study of 3D fluid dynamics is of importance.

The XR includes a compass/tilt sensor and is typically installed in environments where either the exact system orientation cannot be controlled, and/or the exact direction of flow is not known or constant. By contrast, the SW is intended for channels where the user can set the system orientation and the exact flow direction is well defined.

Can the SW record data internally and is there an internal battery?

The SW has a 4-MB internal recorder; however, there is no internal battery power. A 7-15 VDC power supply must be provided externally.

Can I run the SW using a topside battery and solar panel?

Yes. The power consumption of the SW is a fraction of a watt and it can easily run off such a setup.

What are the output data formats?

The standard SW supports RS-232 or SDI-12 data output. For a complete description of the output parameters, contact SonTek directly.

Can I output analog data such as a 4-20 mA current loop?

Yes. To do this you need to order a special output module from SonTek that enables the analog output. The limitation is that for every parameter you want to output, an additional module is required. As such, we highly recommend the use of either the RS-232 or SDI-12 outputs if possible.

How long can the power/communications cable be?

If you are using a SonTek-supplied cable, the rule of thumb normally applied is up to 100 m (328 ft) for standard RS-232 or SDI-12.

In addition to water velocity, Level and Flow, what else can the SW measure?

A temperature sensor is also standard part of the package. At present, the SW is not offered with a pressure sensor option, or any external interface option.

Is there an external display available for the SW?

Yes. SonTek offers the Argonaut Flow Display (see photo at right), which enables real-time display of several parameters, including flow rate, velocity, temperature, level, and total volume. More details about the Flow Display can be found here.

Can the SW measure Total Flow (i.e., the total volume of water that flows down the channel over a given time period)?

Yes, by using the external display mentioned above.

How do I mount the SW?

The SW has two through-holes on the housing, which can then be attached (using the supplied machine screws) to a base mounting plate that comes standard with the unit (see photo on right). The base mounting plate has four pre-drilled holes in the corners, or you can modify the plate yourself as need be.

• Argonaut-SW Overview
• Argonaut-SW Applications
• Argonaut-SW Specifications
• Instruments for Hydrology Training Course
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