Here are a few points highlighting the benefits of each probe. It is up to the user to make an informed decision on what is best for the measurement needs.
Side-looking probes
- Less likely to have the problem of “weak spots” (unless the sampling volume is very close to a side boundary), so they are a little more robust to operate, especially in high flow environments.
- Have higher instrument noise in the vertical than in the horizontal.
- Are well suited for use in wave flumes.
- Have an awful response to flow coming “from behind” the probe.
- Can act as a 2D system in shallow water, if the two beams slanted 65° from the vertical are in air.
- Are more sensitive to the instrument Y velocity (component directed into/away from the center transmitter).
- Because the transmit pulse is typically directed away from the bottom boundary you will be able to measure a bit closer and a bit shallower than you would with the down-looking probe.
Down-looking probes
- The measurement volume can get a few mm closer to the bottom than is possible with the side-looking probes.
- Have a better response in the instrument Z velocity component.
- Provide redundant measurement of the Z component, which can be useful in data screening.
- Have the lowest noise in the vertical.
Check out the Principles of Operation for an overview of the sampling volumes for these two probes.
Updated