Experimental Transmitters

Through the years Vemco, working closely with marine biology researchers, has developed a number of transmitters incorporating various types of experimental sensors. As originally conceived, these transmitters have often served to instrument laboratory studies aimed a finding correlation between sensor readings and certain aspects of the fish’s behaviour or physiology. A major issue in moving from laboratory instruments to transmitters suitable for use in the field is the necessity to reduce data bandwidth sufficiently for use in a low data rate field transmitter. In many cases, an initial phase using a laboratory transmitter or data logger is key to the development of algorithms necessary for a successful field transmitter.

This page is to outline the various experimental transmitters developed to date and thus give an idea of the kinds of things that might be possible. We emphasize that these are not standard products and our purpose in presenting them is to stimulate discussion with biological researchers who have an interest in working with us to help refine some of these concepts into standard products or to help develop some new sensor type.

A Note on Transmitter Size

Most of the transmitters described are relatively large (16mm diameter typically). Sometimes this is of necessity. It is also a matter of convenience as prototypes can be put together much more quickly and economically than with smaller sized transmitters. However, if necessary, we can discuss the possibility of transmitters suitable for smaller fish.

Mortality Transmitter (3D Accelerometer)

VEMCO's R&D team are currently working on the development of a 3D Accelerometer sensor transmitter. Its initial use will be to determine the mortality of an animal by measuring its acceleration in 3 axes over a period of time. The acceleration data will be processed within the VEMCO tag. Indication of mortality, i.e. Dead or Alive, will be reported back and read by VEMCO receivers. These units are expected to be available during late 2007.

The accelerometer tag can be made available for uses other than measuring mortality. Please contact VEMCO to discuss your specific requirements.

pH Transmitter

The pH transmitter is being used at the University of Hawaii to measure the feeding habits of black tip sharks.

The pH device contains a vial of electrolyte solution. It works by having a continual outflow of electrolyte which prevents contaminating matter (i.e. stomach fluids) from diffusing into the electrode and causing the probe to drift. This is particularly important in diving animals as the increase in pressure can increase contamination of the probe. The device lasts for as long as the reservoir of electrolyte fluid. With the current design, there is enough fluid to last 16 days. It is feasible for a larger animal to carry a larger device with more electrolyte solution and, therefore, a device lasting several months could be developed. Investigations are also underway to look into the viability of extending the life of the current reservoir by reducing flow while still maintaining sufficient accuracy to properly measure feeding.

The prototype sensor comes in a titanium case measuring 25mm x 169mm, which can be opened to refill the electrolyte fluid and replace the batteries.

Heart Rate Transmitter

This is set up as a field transmitter with the following characteristics:

• Size: 16 x 65 mm
• Acoustic Output: 153 dB re 1 uPa @ 1 metre
• Battery Life: 1 month

The basic method of operation is that signals from an electrode pair are analyzed and a pulse is transmitted for each heartbeat. Automatic gain control circuitry ensures correct operation in the event that signal levels change due to movement of electrodes. The transmitter is programmable for the anticipated range of heart rate frequency as well as providing the ability to transmit a minimum repetition rate to facilitate tracking in the event of a very slow heart beat rate.

Differential Pressure

This is set up as a field transmitter and is similar to the V16P except a port is provided for the reference pressure of the sensor. The following two examples of how the transmitter has been configured and used are presented to give an idea of the kind of experiments that can be conducted.

1. Fish Model
   The V16DP-4HR telemeters the pressure difference on each side of an appendage undulating under water. The common use is to correlate differential pressure to swimming speed of fish as a measure of activity. The transmitter measures peak to peak pressure and average pressure every 2-3 seconds. Depth pressure is ignored. Battery life is generally 3-4 weeks depending on mode of use. The transmitter requires replaceable battery packs. Reference - Webber (2001).



2. Cephalopod Model
   This model measures the mantle cavity pressure generated by the contraction of mantle musculature of jetting cephalopods. Pressure is correlated to swimming speed to allow the measurement of swimming speed and energy consumption of cephalopods in the field. Depth pressure is ignored. Battery life is generally 3-4 weeks depending on mode of use. Reference - Webber (1986); O'Dor, et al. (1989).

Data Logger - Accelerometer

This is set up as a laboratory instrument and allows three dimensional acceleration data to be logged. Key characteristics include:

• Size: 16 x 100 mm
• Acceleration range: 8G to less than 0.1 G
• Sampling Rate: 20 per second
• Battery life: 1 week

Data Logger – Other Sensors

The accelerometer data logger could be relatively easily modified to support one or more other types of sensors producing voltage, current or frequency outputs. Up to three channels would be feasible.