SIGNUX PHA01

The SX-PHA01 is an integrated pH transmitter for aqueous solutions. Electrode, reference, temperature sensor and electronics share one body, so the probe connects directly to an RS485 bus with Modbus RTU protocol and reports pH and temperature.

The pH value arrives temperature-compensated: automatically, from the probe own temperature reading, or against a fixed temperature when manual compensation is switched on. The reference uses a double salt bridge, which resists fouling better than a single junction in sewage, aquaculture and process water.

• •pH from 0 to 14.00, resolution 0.01 pH
• •Temperature from 0 to +60 °C, resolution 0.1 °C
• •One-piece design, light and simple to install
• •Double salt bridge reference, with stronger resistance to fouling
• •IP68 protection rating
• •Automatic or manual temperature compensation, switchable over the bus
• •RS485 interface with Modbus RTU protocol
• •Modbus address and baud rate configurable
• •Wide supply range, 7 to 30 V DC

The probe is supplied with a four-core cable with bare ends.

Do not take the colours for granted from memory: not every water probe follows the same convention.

The model code says how the cable ends. It is chosen when ordering and does not change afterwards.

• •One integrated pH transmitter
• •5 m cable (or the length ordered)

• •Submerged installation: run the cable through a rigid waterproof pipe and screw the pipe onto the 3/4 in thread on top of the probe, sealing the thread with PTFE tape. The top of the probe and the cable must stay dry.
• •Pipe installation: screw the probe into the pipe through its 3/4 in thread.

• •Before the first measurement, unscrew the protective bottle, pull the electrode out and rinse it with pure water.
• •Shake the air bubbles off the glass bulb before measuring: a bubble against the bulb changes the reading.
• •Stir the solution, then let the probe settle before reading.

• •The equipment needs no routine maintenance. If it fails, do not open it: contact SIGNUX.
• •The electrode must never be stored dry. The protective bottle holds a soaking solution that keeps the glass bulb and the liquid junction active; keep the tip in it whenever the probe is not in service.
• •Soaking solution: dissolve one sachet of pH 4.00 buffer in 250 ml of pure water and add 56 g of analytical grade potassium chloride, warming and stirring until it dissolves. A 3.3 M potassium chloride solution also works: 25 g of analytical grade potassium chloride in 100 ml of pure water.
• •The glass bulb must not touch anything hard. Any knock or scrape destroys the electrode.
• •Rinse the electrode with deionised water before and after every measurement.
• •An electrode that has been in service a long time goes passive: the slope drops, the response slows down and the reading stops being accurate. Soak the bulb in 0.1 M dilute hydrochloric acid for 24 hours, then in 3.3 M potassium chloride for another 24 hours.
• •The electrode lasts about a year. It is a consumable: replace it when it ages.
• •Calibrate before each use, and at least every three months in continuous service. Dirty or chemically loaded water calls for calibrating more often.

Fouling of the glass bulb or a blocked liquid junction also makes the electrode go passive. The cleaner depends on what has fouled it.

If the electrode is badly passivated and 0.1 M hydrochloric acid does not recover it, the bulb can be dipped in 4 % hydrofluoric acid for three to five seconds, rinsed with pure water and then soaked in 3.3 M potassium chloride for 24 hours. Hydrofluoric acid is dangerous: this is a last resort, for a laboratory, not for the field.

Calibration is done over RS485, with two standard buffers, writing registers 0x0120 and 0x0121 together with function code 16. There are no buttons on the probe.

Work at about 25 °C. Rinse the electrode, immerse it in the buffer with the lower pH and let the reading settle. Then write 0x0001 to register 0x0120 and the buffer value multiplied by 100 to register 0x0121, in a single frame: that fixes the first point.

Rinse the electrode again and blot the water off with a paper towel or a soft cloth, without touching the glass bulb. Immerse it in the buffer with the higher pH, let it settle, and write 0x0002 to 0x0120 and its value multiplied by 100 to 0x0121. The calibration is then complete. Examples 3 and 4 in the Modbus chapter show the exact frames.

All 16-bit values are sent high byte first.

• •UINT16: 16-bit unsigned integer
• •INT16: 16-bit signed integer

The pH register is scaled by 100 because the resolution is 0.01 pH. Other pH probes, such as soil ones, scale by 10: reading this one with that scale gives values ten times too large.

0x0316 is 790 in decimal, so the pH is 790 / 100 = 7.90. 0x0109 is 265, so the temperature is 265 / 10 = 26.5 °C.

The probe reads 7.90 and the reading should be 8.00. The correction is 8.00 − 7.90 = 0.10 pH, which is 10 in register units (100 times the value), that is 0x000A. It is written to register 0x0050 with function code 06.

The response is a copy of the request.

4.01 × 100 = 401 = 0x0191. Registers 0x0120 and 0x0121 are written together with function code 16: 0x0001 to the first and 0x0191 to the second.

9.18 × 100 = 918 = 0x0396. Same two registers, with 0x0002 as the point number.