Water Quality Instruments and Probes

Model 6536E, 6536P-2, 6528C, 6528B and 7422A

  • Overview

    Water quality measurements are performed for water in streams, rivers, lakes and the groundwater system, using boreholes. There are several different measures for water quality, like electro conductivity (EC), pH ( PH), dissolved oxygen (DO). Unidata range of water quality monitoring instruments and probes consists of:

    The 6536 Water Conductivity Instrument with 6536 Probe measures the electro-conductivity and temperature of water in streams, rivers, lakes and the groundwater system, using boreholes. The temperature is measured within the sensing cell to provide a precision correction. Both, temperature corrected and uncorrected, and conductivity measurements are available for recording.

    A Starlog compatible micrologger with 512KB memory is part of the instrument, providing all the standard Unidata data logger features such as SDI-12, intelligent battery supervision and modem interface. Ultra-low power consumption makes the 6536 ideal for remote, unattended operation. This instrument will operate for months from a single model 6910A or 12V battery.

    6536P-2-X probe is designed for use with the 6536E Water Electroconductivity Instrument. It is available in standard cable lengths of 10m and 20m as well as custom lengths up to and beyond 100m.

    Combined with the Starflow 6526J, water flow measurement system, conductivity instrument with probe provides a complete salt load recording package.

    pH double junction probe 6528 have a rugged design and utilises a double-junction reference system to minimise the effects of contamination due to clogged pores or ingress of the sample. The probe is designed as a low power instrument and includes built-in conditioning electronics for direct connection to a datalogger.

    The 6528 pH probe consists of a pH sensor with a voltage output that varies proportionately to the hydrogen ion activity between a reference electrode and an electrode immersed in an alkaline (basic) or acidic solution. These advanced electrodes feature proven flat tip technology which virtually eliminates deposits that can foul the electrode, significantly reducing necessary maintenance.

    Each electrode has a built-in potential matching pin. With this feature, electrode fouling caused by a ground loop current through the reference of the sensor is a thing of the past. These electrodes have been engineered with a replaceable battery to power the amplifier. This feature adds life to the electrode and aids in troubleshooting. With this feature, electrode fouling due to ground loop

    The dissolved oxygen probe 7422 is a galvanic measuring element which produces a millivolt output proportional to the oxygen present in the medium it is placed in. It consists of an upper part with the cathode, anode and cable, and a cap with membrane and electrolyte.

    Oxygen diffuses through the membrane onto the cathode, where it reacts chemically and then combines with the anode. This chemical process develops an electric current, which flows through a built-in resistor. The resistor converts the current (microamps) into millivolts. This millivolt signal is led to the data logger via a two-core cable.

    The probe is virtually maintenance free, requiring only that its membrane be kept reasonably clean. The anode remains fully active through its lifetime of several years. The membrane is very robust and can be wiped clean with a cloth or paper. It is recommended that the membrane be cleaned from time to time to ensure maximum reliability.

    6536E water monitoring conductivity instrument with 6536P-2 probe
    pH probe and Disolved Oxygen probe

  • Specifications

    6536E Water Conductivity Instrument

    Instrument Physical specifications

    Material: Polycarbonate Enclosure
    Size: 230mm x 65mm x 80mm (LxWxH)
    Weight: 1.5kg (including battery)
    Operating temperature: 0°C to 60°C

    Electrical specifications

    Battery: 6 x 1.5V industrial grade D size Alkaline battery pack 6910A (non-rechargeable)
    Battery life: up to 1 year (based on daily schedule)
    External power: External battery power 7-15VDC may be connected
    Communication: 1 x RS232C Baud rates: 300/1200/2400/4800/9600/19200/38400.
    SDI-12: SDI-12V 1.3 recorder (1200 baud smart instrument channel)

    EC Operating Range

    Low: 0-200µS/cm
    Accuracy: ±2%
    Resolution: 0.2µS/cm

    Mid: 200-20,000µS/cm
    Accuracy: ±1%
    Resolution: 2.0µS/cm

    High: 20,000-200,000µS/cm
    Accuracy: ±1%
    Resolution: 20µS/cm


    Range: -20°C to 60°C
    Accuracy: ± 0.1°C
    Resolution: 0.0612°C
    Compensation Range: 0°C to 60°C


    Conductivity: 0 – 200,000 (uncompensated)
    Conductivity: 0 – 200,000 (temperature compensated)
    Conductivity: 0 – 65,535 (uncompensated) Low resolution (for SDI-12)
    Conductivity: 0 – 65,535 (temperature compensated) Low resolution (for SDI-12) Water temperature, battery voltage

    Integrated logger specifications

    Storage memory: Low power CMOS RAM 512k standard
    RAM Back up battery: Lithium Battery 3V, 950mAh
    Time clock: Crystal regulated, +/- 10 seconds/month
    Scan rates: Programmable from 5 seconds to 5 minutes
    Log intervals: Programmable from 5 seconds to 24 hours
    CPU: 80C552 microcontroller, 14.7456 MHz

    6536P Probe

    Material: Derlin Housing, black epoxy
    Size: Dia 15.3mm x 163mm L
    Weight: 500g (including 10m of cable)
    Operating temperature: 0°C to 60°C
    Immersion Depth: 36mm minimum to more than 100m
    Sensor Cable: 10 metres, 20m & 50m with SQL connector, custom lengths available
    Sensor Type: Four (4) Electrode, cocentric graphite
    Pressure Resistance: 10 bar
    Cell Constant: 0.475cm-1 ±2.0%
    Thermistor Type: Integrated NTC (30kΩ@25°C)

    6528 pH Double Junction Probe

    Material: PVDF Body
    Size: Diameter 25 x Length 141mm
    Weight: 400g including cable
    pH Range: 0-13
    Junction: Double
    Junction Type: Teflon
    Electrolyte: Polymer
    Pressure: 6bar (87 PSI)
    Temperature Range: -5° to 80°C
    Cable: 5m of 2 core

    7422 Dissolved Oxygen Probe

    Material: PVDF Body
    Size: Diameter 58 x Length 59mm
    Weight: 450g including cable
    Range: 0-50mV (2.5 to 5mV per ppm mg/l)
    Output impedance: 1kΩ
    Type: Galvanic cell, self-polarising, self-temperature-compensating
    Temperature Range: 0 to 40°C
    Pressure: Submersible to 100m
    Cable: 7m

  • Ordering Information

    Model Description
    6536E Water Electroconductivity Instrument
    6536P-2-10 Water Conductivity Probe, 10m cable
    6536P-2-20 Water Conductivity Probe, 20m cable
    6536P-2-30 Water Conductivity Probe, 30m cable
    6536P-2-50 Water Conductivity Probe, 50m cable
    7422A Dissolved Oxygen Probe
    6528C Double Junction, Teflon, pH Probe
    6528B Double Junction, Teflon, ORP Probe

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    • Documents