Velocity Shear Probe

The SPM-38 is the best sensor for measuring microstructure velocity fluctuations in the oceans and lakes. The sensing head is almost exactly the same size and shape as the original sensors developed by Tom Osborn, giving this probe a 30-year proven track record of in-situ turbulence measurement. The probe's operating principle and response characteristics are well established and accepted in the microstructure community.

Operating Principle:

The SPM-38 measures a single spatial component of turbulent velocity fluctuations, in a direction perpendicular to the probes main axis. The sensing element consists of a parabolic, flexible rubber tip.

For small angles of attack, the oncoming flow produces a hydrodynamic lift force proportional to the fluctuating cross-stream velocity component u. The piezo-ceramic beam embedded in the rubber tip translates the lift force into an electric signal that is proportional to u. The length of the sensing tip, here 10 mm, determines the maximum wavenumber resolution of the sensor, approximately 48 cycles per meter (cpm). Using the empirically proven spatial response function, the measurements can be corrected to include wavenumbers up to 150 cpm.

Specifications:

Length	5" (127 mm)
Diameter (max)	3/8" (9.5 mm)
Materials	Stainless/Teflon/Silicone
Pressure Rating	1000 dbar (6000 dBar optional
Range	0 ... 10 s-1
Accuracy	5%
Resolution approx.	10-4 s-1
Temperature Range	-2 ... 35 °C
Sensitivity	0.04 ... 0.09 V/(m/s)2

Example Data:

Spectrum of velocity shear (red) measured with the SPM-38 and the Nasmyth universal spectrum (black) corresponding to a dissipation rate of 8.5 × 10^-8 W/kg. The measaured spectrum has been corrected using the sensor's empirical response correction and is in good agreement with the universal spectrum.