Xylem WQ pH SENSORS User Manual download pdf (Page 4)

Global Water

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III. Sensor Installation

General

a. Water quality sensors have many applications and therefore many

installation options. All the sensors are fully submersible and may be

suspended by their waterproof cables in the water to be monitored.

b. Do not install the water quality sensor in applications that contain

solvents. Over time, many solvents can deteriorate the cable and the

sensing element.

c. Install your water quality sensor so that it is easily accessible for

calibration purposes. You may need to remove and reinstall it in the

future, so plan ahead!

d. The sensors will not function correctly if mud, silt, leaves, or garbage

buries them. The pH, Dissolved Oxygen, and ORP/Redox sensors will

fail if they are left out of the water for extended periods of time. Install

the sensor in a way that will avoid these conditions.

e. All Global Water water quality sensors produce a 4-19 mA output

signal. 4-19 mA is an industrial standard signal for process control

monitoring. Most PLCs (Programmable Logic Controller), RTUs

(Remote Telemetry Unit), and data acquisition systems accept this

signal directly. If the system only accepts voltage signals, the sensor

output must be converted to a voltage signal by reading the voltage

across a precision resistor in series with the signal wire. Since Ohms

Law states that V = IR, if the 4-19 mA signal is dropped across a 250

ohm resistor, the output will be 1 to 4.75 volts DC. If the 4-19 mA

signal is dropped across a 125 ohm resistor, the output will be halved

to 0.5 to 2.375 VDC. The 4-19 Ma signal wire is connected to the

datalogger voltage input terminal. The resistor is placed between this

input and the ground terminal of the datalogger’s battery. The power

(or voltage to the sensor) must be connected to positive battery

terminal of the datalogger.

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Xylem WQ pH SENSORS User Manual Page 4