2-5
2.1.4 Depth/Temperature Measurement (SEN Board)
Depth Measurement
Refer to the simplified depth measurement circuit shown in figure 2.8.
Fig. 2.8 Simplified Depth Measurement Circuit
1) The pressure sensor for depth measurement constitutes a Wheatstone Bridge.
It balances at 0 m and the voltage difference between "+" and " - " becomes zero.
(In fact there is 0.6 mV approximately, so R10 adjusts the off-set voltage to obtain 0 V.)
2) When the sensor (S1 and S2) detects water pressure, the resistance of one sensor
increases and that of the other decreases, developing a voltage proportional to the
depth across "+" and "-" terminals.
3) The voltage detected by the sensor device is once amplified at U1 (1/2) and (2/2), then
led to differential amplifier U2 (1/2). The output of U2 (1/2) detected as DEP-1 at TP3 is 1
to 9 V which corresponds to the depth of 0 - 2000m. U1 (1/2) and (2/2) are an amplifier of
high input impedance and employed to increase measuring accuracy.
4) The succeeding differential amplifier U2 (2/2) works to enhance the measuring
resolution as much as five times. Namely the output at TP4 (DEP-2) varies from 1 to
9 V five times while DEP-1 at TP3 varies from 1 to 9V.
This is controlled by the CPU; the CPU monitors TP3 (DEP-1) voltage and changes
the reference voltage of U2 (2/2) at depths corresponding to every 400m (400m,
80cm...160Cm). Refer to Figs. 2.9 for the operation.
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