
D – ALTIMETER
DPN 402197 © TSS (International) Ltd Page 5 of 18
Figure D–2: Switch S1 layout
When the switch has been switched to the correct position the electronics board can
be inserted into the pressure housing first ensuring the interconnecting cable is free
alongside the printed circuit board. Ensure the board is slid down so the end of the
board sits to one side of the internal cable at the bulkhead connector taking care not
to damage the internal anodised finish of the O ring seal area as the PCB is slid
down, (see figure D–3).
Figure D–3: Reassembly of the unit.
Ensure the rubber “O” rings are free from contamination and if necessary remove and
clean the O rings and grooves before re-greasing with silicone or compatible O ring
grease.
The area around the O rings must be meticulously clean to ensure a good pressure
seal when the unit enters the water. Inspect with a magnifying glass to make sure the
surface of the O ring and housing are clean.
D.3 THEORY OF OPERATION
This section describes the operation of the ALT250. The theory describes the general
principles of acoustics and the technical description covers the basic operation of the
altimeter. Do not attempt to repair the altimeter unless you are an experienced elec-
tronics technician used to working with surface mount components.
D.3.1 Operating Principles
The altimeter determines the round trip time of the Sonar pulse travelling from the
transducer through the water column then reflected off the seabed and received back
at the transducer. The time the Sonar pulse takes to make this journey equates to the
two way distance. The distance can be determined by comparing the measured time
to the known speed of sound in water then dividing by 2 to get the one way distance.
The speed of sound in water varies according to conditions, (due mainly to salinity
and temperature). The altimeter defaults to 1480 metres/second which is a default
average for speed of sound in sea water.
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RS232/TTL
INTERNAL/EXTERNAL
BAUD 2400/9600
SHUT DOWN
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