SIGNUX COA05

The SX-COA05 is a digital conductivity probe for continuous water quality monitoring. The measuring electronics are inside the probe, so it connects directly to a PLC, a computer or an IoT gateway over RS485, or to any analogue input through its 4-20 mA output, with no separate controller or transmitter.

The cell constant is chosen when ordering, from k = 0.01 to k = 30, and it sets the measuring range: from 0-20 µS/cm for ultrapure water to 0-300 mS/cm for brines. A built-in NTC provides automatic temperature compensation. The sensing element can be replaced without changing the probe.

Typical applications: thermal power, fertiliser and chemical plants, metallurgy, environmental monitoring, pharmaceutical and biochemical processes, food and drinking water.

• •Continuous, unattended online measurement
• •High accuracy, high stability and strong immunity to interference
• •Automatic temperature compensation
• •Works on its own, without a controller or transmitter
• •RS485 (Modbus RTU) and 4-20 mA outputs
• •All settings over RS485: address, baud rate, calibration, analogue range
• •Replaceable sensing element
• •Settings kept for more than 10 years without power

Specifications

The probe is supplied with a fixed cable with labelled, ferruled ends. The number of wires depends on the version: the 12/24 V version has six wires and also provides the 4-20 mA output; the 5 V version has four wires and RS485 only.

12 V or 24 V version, RS485 and 4-20 mA

5 V version, RS485 only

Every wire carries a printed label with its function. Check the label rather than the colour: the label is what identifies the wire. The 4-20 mA output is a passive current output; its load must stay below 750 Ω.

The order code states the cell constant and the supply and output version. The cell constant cannot be changed afterwards: choose it from the conductivity to be measured.

Optional accessories: mounting bracket and flow cell. Cable lengths other than 5 m on request.

• •In-line: screw the probe into the pipe or tee through either G 3/4" thread. Seal the thread with PTFE tape.
• •Submerged: hang the probe from a rigid pipe screwed onto the upper thread, with the cable running inside the pipe, or use the mounting bracket. Keep the cable entry and the top of the probe above water or fully sealed.
• •Flow cell: for low flows or sampling lines, mount the probe in the optional flow cell.
• •Keep the electrode fully immersed and away from air bubbles; bubbles trapped on the electrode read as low conductivity.
• •Do not exceed 6 bar or 60 °C.

• •When not in use, store the probe in a dark, dry and ventilated place.
• •The sensing element is a precision part and cannot be dismantled. Do not clean it with strong acids or alkalis: they alter the cell constant and the accuracy of the measurement.
• •Before measuring, rinse the electrode with distilled or deionised water and dry it with filter paper, so that no impurities are carried into the sample. Check that the connections are dry; clean dirt with pure alcohol and let it dry before use.
• •After long use, or when readings drift, recalibrate the probe with standard solutions (see Calibration).
• •If calibration is no longer possible after cleaning, the sensing element has reached the end of its life: replace it. The measuring cable is dedicated to the probe and cannot be replaced by another one.

The probe uses a three-point correction: zero, slope and compensation. Calibration is done over RS485 with the function-call registers R12 to R14 (0x000C to 0x000E), either from the SIGNUX tool or from any Modbus master.

A function call takes three writes: parameter 1 into R13, parameter 2 into R14 and, last, the function number into R12. Writing the three registers at once with function code 16 starting at R12 has the same effect. When the call succeeds, R12, R13 and R14 return to 0; if it fails or a parameter is wrong, R14 reads -1.

Procedure: rinse the electrode, immerse it in the standard solution, stir gently and wait for the reading to settle. Then send the call with the value of the standard multiplied by 10. Use a low standard (for example 73 µS/cm) for the zero point and a high one (for example 1413 µS/cm) for the slope. Examples 2 and 3 in the Modbus chapter show the exact frames.

Read registers (03): request

Read registers (03): response

Write one register (06): request and response are identical

Write several registers (16): request

Write several registers (16): response

All values are 16-bit, high byte first. Decimals are implied: a register marked «1 decimal» holds 10 times the value, and one marked «2 decimals» holds 100 times the value.

• UINT16: 16-bit unsigned integer
• INT16: 16-bit signed integer
• Settings are read from R2 to R11 and changed only through function calls on R12 to R14.

Master request: 01 03 0000 0002 C4 0B |||

Slave response: 01 03 04 1ACC 09C4 3A D7 |||

0x1ACC is 6860 in decimal, so the conductivity is 6860 / 10 = 686.0 µS/cm. 0x09C4 is 2500, so the temperature is 2500 / 100 = 25.00 °C.

73.0 × 10 = 730 = 0x02DA. Function number 1, parameter 1 = 0x02DA, parameter 2 = 1 (zero point). The three registers are written at once from R12 with function code 16.

Master request: 01 10 000C 0003 06 0001 02DA 0001 FA FC |||

Slave response: 01 10 000C 0003 40 0B |||

1413.0 × 10 = 14130 = 0x3732. Function number 1, parameter 1 = 0x3732, parameter 2 = 2 (slope point).

Master request: 01 10 000C 0003 06 0001 3732 0002 35 05 |||

Slave response: 01 10 000C 0003 40 0B |||

After a successful call, R12, R13 and R14 read 0. If R14 reads -1 (0xFFFF), the call was rejected: check the function number and the parameters.