72-900-06 Rev. F
Page 14 of 38
3.13.1.2 Pyxis ST-500
The Pyxis ST-500 is also an inline fluorometer which outputs both a 4-20mA signal and a
RS-485 interface which responds to PTSA trace chemical. The ST-500 also has a 4-20mA
where 4mA = 0 PPB and 20mA = 200 PPB. Wiring the ST-500 is similar to the Little Dipper
2. The RS-485 bus is not used by the MicroVision EX (The blue and yellow wires). The
only wires necessary to terminate are power to the Pyxis and the 4-20mA loop (Brown or
white (4-20mA+) and Green (4-20mA-)). The clear wire should be connected to the
solution ground.
3.13.2 Calibrating PTSA Probe
4-20mA input probes require a 2 point calibration. In order to calibrate a Pyxis or Turner
inline fluorometer, navigate to the 4-20mA input in question. I.E. Menu -> Settings -> 4-
20mA In 1. For a PTSA probe the units are PPB. Select Type->PPB. Select Back. Scroll
down to Calibrate. For a 2 point calibration you will need a low point and a high point. The
best way to calibrate a low point is to put the probe in deionized water, uncontaminated
with any PTSA. You should see ~4mA as the actual signal. Enter “0000” on the top line.
Before exiting this page the high point will need to be calibrated. This is ideally done in the
actual flow assembly. Place the probe in the flow allow solution to flow across the sensor
to purge any air bubbles out of the sensor. Take a sample with a handheld meter be sure
the mA signal on the controller is stable and enter the high point, the value measured by
the handheld. Alternatively, to calibrate the high point of the sensor, buffer solutions can
be used. Fully submerge the sensor in the known buffer ensure to expel air bubbles which
may have formed on the optics. The known buffer needs to be within the range of the
probes sensing capabilities. Ensure the mA output from the probe has stabilized on the
MicroVision EX under the actual reading. Enter the known value as the high point, upon
hitting enter on the last digit the value will be saved and your probe successfully calibrated.
3.14 Alarm Dry Contact Relay
The alarm dry contact activates for all system alarms and is a volt free signal. Use P4 to
connect the alarm reporting equipment. This relay will close when an alarm condition
exists and will open when no alarm conditions are present. This connection is intended to
be used to signal external process control equipment or indicators (low voltage DC
operated buzzers or lamps) when an alarm condition is present. This relay is not
protected by a fuse and should not be used to voltages above 48VDC.
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