OBS501 Smart Turbidity Meter with ClearSensor
®
Technology
21
best possible performance available from today’s state-of-the-art sensor
technology. The sensor reverts to a low-power sleep state between
measurements. A series of measurements is performed, yielding two turbidity
and one temperature value. This measurement cycle takes about 20 seconds.
The measurement cycle is activated by commands via SDI-12, RS-232
terminal commands, or a control line(s) going high (analog measurements).
With SDI-12 and RS-232, the basic values output by the OBS501 are
backscatter turbidity, sidescatter turbidity, and temperature. The OBS501 can
also output a ratiometric measurement that combines the backscatter and
sidescatter measurements. Other diagnostic information is available (see
TABLE
SDI-12 and RS-232 Measurement Commands
(p. 21)
) including the
raw voltage output from the backscatter and the sidescatter sensors, the current
to open and close the shutter, an open and close position count, total open and
close cycles, and a moisture alarm. The OBS501 is shipped from the factory to
output turbidity in TU and temperature in degrees Celsius. The analog output
supports backscatter and/or sidescatter according to the status of a control line.
8.5 Communication Modes
8.5.1 SDI-12
The OBS501 uses an SDI-12-compatible hardware interface and supports a
subset of the SDI-12 commands. The most commonly used command is the
aM!
command, issued by the datalogger. Here,
a
represents the sensor address
(0 to 9). The communication sequence begins with the datalogger waking the
sensor and issuing the
aM!
command. The sensor responds to the datalogger
indicating that two measurements will be ready within two (2) seconds.
Subsequent communications handle data reporting from the sensor to the
datalogger.
The SDI-12 protocol has the ability to support various measurement
commands. The OBS501 supports the commands that are listed in TABLE
TABLE 8-2. SDI-12 and RS-232 Measurement Commands
Commands
Process
Values Returned
aM!
aC!
a = address
Open Wiper
Measure
Close
Send Data
obs (TU)
ss (TU)
temperature (ºC)
wet dry (0=dry 1=wet)
aC1!
Burst Data
Open Wiper
Measure 100 Times
Close
Send Data
bs median
bs mean
bs standard deviation
bs minimum
bs maximum
ss median
ss mean
ss standard deviation
ss minimum
ss maximum
