Decagon Devices MAS-1 Operator's Manual - Page 8
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MAS-1
3 INTEGRATING THE MAS-1
3
Integrating the MAS-1
A 4-20 mA system generally consists of a sensor, transmitter, power
supply, and device to read the current being transmitted through
the current loop. The MAS-1 is an integrated sensor and 4-20 mA
transmitter. When the MAS-1 is powered by the Power Supply, it
transmits a current though the loop that is proportional to the soil
dielectric permitting and therefore the soil volumetric water content.
In figure 1, the current loop is shown by the dotted line labeled I=3
to 20 mA the arrows indicate the direction of the current.
Figure 1: 3 to 20mA current loop diagram
The MAS-1 uses a microcontroller to regulate the interval at which it
takes measurements. It takes one second from the time it is powered
up to take its first measurement and transmit current though the
loop. The trasnmitted current will reach a stable value within four
seconds of power up. After the initial four second startup, measure-
ments are taken every one second, while the current in the loop is
continuously maintained. Since the measurement intervals are con-
trolled by the MAS-1 itself, there is no need to pulse the excitation
voltage. A constant supply voltage should be applied in order for
the MAS-1 to function as it is designed
3.1
Wiring
Conventional (PLC)
A programmable Logic Controller (PCC) is typically used to read
5