Menu

Sequoia Scientific

LISST-Tau

High-Precision Transmissometer

Optical Transmission and Beam Attenuation

LISST-Tau is a high-precision transmissometer for underwater vehicles, profiling packages, CTDs, and other systems. With high-quality optics and a carefully selected LED source, it transmits a collimated beam through the water, and precisely measures the light transmitted through its 15 cm path. Light modulation with synchronous detection rejects ambient light, while oversampling and averaging yield better than 16-bit resolution. LISST-Tau’s digital controller applies calibration coefficients, corrects for temperature effects, and transmits data from both digital and analog outputs. Included software provides real-time display of data, and functions for checking and updating pure-water calibrations. An optional flow-through chamber simplifies calibrations and experiments. LISST-Tau advances the state of the art for in-situ transmissometers.

NOTE: The LISST-Tau is restricted in its use and can ONLY be sold for civilian and scientific purposes. ALL PO’s for LISST-Tau must be accompanied by a completed end-use statement.

Features

  • Measures optical transmission and beam attenuation
  • Externally powered
  • RS-232 and analog real-time outputs
  • Ambient light rejection
  • Onboard temperature compensation
  • Available with green or red LED source (specify upon ordering)

Specifications

Parameters Measured

  • Optical transmission
  • Beam attenuation

Operating Ranges and Stability

  • Operational temperature range: -3 °C to 40 °C
  • Storage temperature range: -20 °C to 60 °C
  • Beam attenuation range: ~0 m-1 to 30 m-1
  • Linearity (concentration): >99 %
  • Short-term stability (typical standard deviation over 1 minute)
    • Transmission: 0.003 %FS (Green) / 0.005% (Red)
    • Beam attenuation: 0.0002 m-1 (Green) / 0.0004 m-1 (Red)
  • Long-term stability (6 hr test)
    • Transmission: ~0.003 %FS/hr
    • Beam attenuation: ~0.0002 m-1∙hr-1

Technology

  • Optical path length: 15 cm
  • Source wavelength: ~532 nm (Green) or ~650 nm (Red) LED
  • Source spectral bandwidth: <10 nm FWHM
  • Acceptance angle (half angle, in water): 1.0 °
  • Optical transmission @ 16-bit resolution

Mechanical and Electrical

  • Dimensions [Ø x L]: 5.1 cm x 40.6 cm (2.00″ x 16″)
  • Weight [air / seawater]: 1.140 kg / 0.585 kg (2.5 lbs / 1.3 lbs)
  • Depth rating: 2,000 m
  • Sampling rate: 1 Hz
  • External power input: 7 VDC to 25 VDC
  • Current drain @ 12V: 42 mA average during sampling
  • Connector: SubConn MCBH6M

Accessories

LISST-Tau Item # and Description

Item #DescriptionThumbnail
SEQ-FP-LTAU-532NMLISST-Tau 532 nm (green) 15cm Path XMIS (Green LED)
LISST-Tau is a high-precision transmissometer for underwater vehincles, profiling packages, CTD's, and more. Externally powered. RS-232 and analog real-time outputs. Ambient light rejection. Onboard temperature compensation. Uses green LED source. Included software provides real-time display of data, and functions for checking and updating pure-water calibrations.
LISST-Tau

LISST-Tau

SEQ-FP-LTAU-650NMLISST-Tau 650 nm (red) 15 cm Path XMIS (Red LED)
LISST-Tau is a high-precision transmissometer for underwater vehincles, profiling packages, CTD's, and more. Externally powered. RS-232 and analog real-time outputs. Ambient light rejection. Onboard temperature compensation. Uses green LED source. Included software provides real-time display of data, and functions for checking and updating pure-water calibrations.
LISST-Tau

LISST-Tau

SEQ-AC-LTAU-EW1-YEAR EXTENSION OF LIMITED WARRANTY AND LIABILITY
Includes a single (1) year extension to the Standard Sequoia Limited
Warranty, allowing the new instrument to carry a 2-year total limited
warranty (total). Applies only when purchased together with new
instrument. *Note - this is NOT a service contract and only applies to the
terms of the limited warranty.
No picture
SEQ-AC-LTAU-SERVService Plan
One-time service of the instrument within 36 months after delivery.
Includes one-way return shipping from Sequoia, all evaluation, testing
and calibration. Replacement parts and labor costs may apply which are
the responsibility of the owner. Can only be purchased in conjunction
with a new instrument sale, at the same time.
*Note – End-user responsible for return shipment DDP to Sequoia.
No picture

LISST-Tau Accessories *INCLUDED* with LISST-Tau

Item #DescriptionThumbnail
SEQ-AC-LTAU-CBL02LISST-Tau 2 meter integrated communication and external power cable.Picture coming!

LISST-Tau Accessories *NOT INCLUDED* with LISST-Tau

Item #DescriptionThumbnail
SEQ-AC-LTAU-PT6FLISST-Tau 6-pos pigtail 24" only w/ lock sleeve.Picture coming!
SEQ-AC-LTAU-ZINCLISST-Tau Replacement Zin Anode Ass'y (2 Pack)Picture on the way!
SEQ-AC-LTAU-ZINC10LISST-Tau Replacement Zinc Anode Ass'y (10 Pack)Picture on the way!

FAQ

Can the LISST-Tau be ordered with different optical path lengths?

No. The LISST-Tau is offered only with a 15 cm (0.15 m) path length.

Theoretically, a long path length transmissometer (say, 1 m) would enable measurements of transmission values >> 0.999. However, Sequoia has found that the practical implementation of a long path length transmissometer is hampered by mechanical issues, e.g., difficulties in keeping the instrument aligned under pressure. By and large, these mechanical issues offset the theoretical advantage one would expect from the longer path length.

If you want to measure beam attenuation in waters where a path length < 15 cm is required, Sequoia recommends the LISST-200X or LISST-Glider.

Why is the instrument called LISST-Tau?

LISST was originally an acronym for our laser particle sizing sensors. LISST now is just a Sequoia trade mark.

Tau is the name of the Greek letter τ. In marine optics, τ has traditionally been used to denote optical transmission, the fundamental measurement made by the LISST-Tau.

Beam attenuation, c, is related to optical transmission, τ, thus:

c = (1/r) · ln(τ), where r is the optical path length in m (0.15 m for LISST-Tau).

Can the LISST-Tau be used with an external datalogger to supply battery power + store measurements? Do you have a system that you could recommend?

The LISST-Tau has no internal power or memory of any kind. So it can in fact ONLY be used with an external power and datalogging system:

  • Either a PC running the LISST-Tau software (this setup is really only suitable for lab use) OR
  • a dedicated data logger OR
  • integrated into a larger system that provides power and logging (e.g., a CTD, tow body, float, glider etc.).

The LISST-Tau outputs data in either analog or RS-232 format, so any datalogger or system that can handle this will suffice. The LISST-Tau manual (see Software & Downloads tab) describes this.

Sequoia does not recommend a particular logger or system for the LISST-Tau. The best logger solution depends on the exact application – discuss with a datalogger manufacturer / supplier / system integrator, e.g.: