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Section 2: Description of MicroCAT
SBE 37-SMP RS-232
External Power
The MicroCAT can be powered from an external source that supplies
0.5 Amps at 9-24 VDC. The internal lithium pack is diode-OR’d with the
external source, so power is drawn from whichever voltage source is higher.
The MicroCAT can also be operated from the external supply without having
the lithium batteries installed. Electrical isolation of conductivity prevents
ground loop noise contamination in the conductivity measurement.
Cable Length and External Power
Note:
See Real-Time Data Acquisition
in Section 4: Deploying and
Operating MicroCAT for baud rate
limitations on cable length if
transmitting real-time data.
Note:
Common wire resistances:
Gauge
12
14
16
18
19
20
22
24
26
28
Resistance (ohms/foot)
0.0016
0.0025
0.0040
0.0064
0.0081
0.0107
0.0162
0.0257
0.0410
0.0653
There are two issues to consider if powering the MicroCAT externally:
• Limiting the communication IR loss to 1 volt if transmitting real-time
data; higher IR loss will prevent the instrument from transmitting realtime data because of the difference in ground potential.
• Supplying enough power at the power source so that sufficient power is
available at the instrument after considering IR loss.
Each issue is discussed below.
Limiting Communication IR Loss to 1 Volt if Transmitting Real-Time Data
The limit to cable length is typically reached when the maximum
communication current times the power common wire resistance is more than
1 volt, because the difference in ground potential of the MicroCAT and ground
controller prevents the MicroCAT from transmitting real-time data.
V limit = 1 volt = IR limit
Maximum cable length = R limit / wire resistance per foot
where I = communication current required by MicroCAT (4.3 milliAmps;
see Specifications).
Example 1 – For 20 gauge wire, what is maximum distance to transmit power to MicroCAT if transmitting real-time data?
For 4.3 milliAmp communications current, R limit = V limit / I = 1 volt / 0.0043 Amps = 232 ohms
For 20 gauge wire, resistance is 0.0107 ohms/foot.
Maximum cable length = 232 ohms / 0.0107 ohms/foot = 21734 feet = 6626 meters
Example 2 – Same as above, but there are 4 MicroCATs powered from the same power supply.
For 4.3 milliAmp communications current, R limit = V limit / I = 1 volt / (0.0043 Amps * 4 MicroCATs) = 58 ohms
Maximum cable length = 58 ohms / 0.0107 ohms/foot = 5433 feet = 1656 meters (to MicroCAT furthest from power source)
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