An inline Pyxis Labs probe that measures PTSA fluorescent tracer and turbidity.
¶ What it measures
PTSA in parts per billion, and turbidity in NTU — see Fluorescence (PTSA) and Turbidity. It also reports probe health.
| PTSA range | 0–300 ppb (0–200 ppb on the default 4–20 mA scale) |
| PTSA resolution | 0.01 ppb |
| PTSA accuracy | ±1 ppb (data sheet); the user manual gives ±1% of reading |
| Optics | LED, 365 nm excitation / 410 nm emission |
| Interference compensation | Color and turbidity, up to 150 NTU and 10 ppm iron |
Pyxis publishes no measuring range for the turbidity reading. The ST-500RO (0–40 ppb) and the stainless-steel ST-500SS are variants of the same probe; this page covers the standard CPVC ST-500 (P/N 50661).
¶ Electrical interface
| Outputs | Modbus RTU over isolated RS-485; isolated 4–20 mA |
| 4–20 mA default scaling | Linear, 0–200 PPB |
| 4–20 mA span | Adjustable with the uPyxis app (20 mA point up to 300 ppb) |
| Wireless | Bluetooth, through a Pyxis MA-WB adapter or PowerPACK (not built into the probe) |
| Supply | 22–26 VDC, 1 W; size a separate supply for at least 65 mA |
| Connector and cable | 7-pin IP67 quick connector; 1.5 m (5 ft) bulkhead cable plus a flying-lead adapter cable (MA-1100) |
| Wetted materials | CPVC body and tee, quartz optical channel |
| Process connection | ST-001 tee (supplied), 3/4 in FNPT socket and threaded ports |
| Pressure and temperature limits | Up to 0.7 MPa (100 psi); 4–49 °C (40–120 °F) operating, −20–60 °C (−4–140 °F) storage |
| Ingress rating | IP67 |
Wire colors:
| Wire | Function |
|---|---|
| Red | 24 V + |
| Black | 24 V power ground (Brown in Pyxis’s 2026 UC-50 guide) |
| White | 4–20 mA + |
| Green | 4–20 mA − (internally connected to power ground) |
| Blue | RS-485 A |
| Yellow | RS-485 B |
| Clear | Shield, earth ground |
The 4–20 mA − and the 24 V ground are joined inside the probe. On a controller with a non-isolated 4–20 mA input, you do not need to land the green wire.
Pyxis does not publish the ST-500’s Modbus register map or serial settings; it supplies them on request from service@pyxis-lab.com. The registers the InteliHive uses are listed on the Modbus Register Map. The Modbus address can be changed in the uPyxis desktop app.
¶ Maintenance
The probe stores its own diagnostic and calibration data. The Pyxis MA-WB, an inline Bluetooth device, checks its cleanliness and adjusts its calibration through the uPyxis app.
- Interval — check for fouling, clean and calibrate once a month. Heavily contaminated water may need it more often; clean water may go several months.
- Cleanliness check — connect the probe through an MA-WB (or PowerPACK), open uPyxis, pick the fluid type under Diagnosis Condition, and run Cleanliness Check. The app shows Clean or Dirty.
- Cleaning — soak the lower half of the probe in 100 mL of Pyxis inline sensor cleaning solution for 30 minutes, rinse with distilled water, and check the quartz channel. If it is not clean, soak another 30 minutes and use the soft brush and cotton swabs from the SER-01 cleaning kit. Run the cleanliness check again.
- Calibration — two points after cleaning: Zero Calibration in distilled or deionized water, then Slope Calibration in a Pyxis PTSA-100 (100 ppb) standard. Let the reading settle for a few minutes at each point, and keep direct light off the probe.
- Storage — avoid long-term storage above 60 °C (140 °F). Outdoors, shield the probe from direct sunlight and rain.
- Noisy readings — connect the clear shield wire to a conductor in contact with the sample water, such as a metal pipe next to the tee.
Heavy fouling makes the ST-500 read low, which can cause chemical overfeed. When it controls dosing, limit pump size or run time, or alarm on a high feed rate.
¶ Using it with a controller
Wiring and configuration live with each controller:
- InteliHive — over Modbus RTU, default address 10. In upcoming firmware the controller watches the probe’s cleanliness count and alarms when it reads fouled, and runs the cleanliness check and the zero and slope calibrations for you, without uPyxis. See Modbus Probes — ST-500 and Calibration.
- MicroVision EX — over 4–20 mA. See Configuring a PTSA Probe.
¶ Sources
- Pyxis Lab: ST-500 Series data sheet (PDF)
- Pyxis Lab: ST-500 Series user manual, rev. 3.08 (PDF)
- Pyxis Lab: UC-50 and ST-500 Series startup guide (PDF)
- Pulsafeeder: cleaning and calibrating the ST-500 (PDF)