CDOM and FDOM water sensors

What CDOM is and How to Monitor its Levels in Water

Coloured, or chromophoric dissolved organic matter (CDOM), is a naturally occurring dissolved matter that absorbs UV light in water. It’s usually made up of tannins that are released from the breakdown of plant material.

A fraction of CDOM fluoresces when it absorbs light of a certain spectrum, and is called fluorescent dissolved organic matter, or FDOM.

Aquaread CDOM sensor

For accurate CDOM/FDOM monitoring, try using our CDOM sensor with the Aquaprobe. This sensor is fixed response fluorometer, which provides excitation at 365nm (UV) and detects any resultant fluorescence between 450nm and 520nm.


Key Features

  • Dense sapphire lenses are used on our CDOM sensor for their high clarity and scratch resistance.
  • Specialised deep UV LED is used to excite the fluorescent dissolved organic matter.
  • Measurement range 0 – 2,000mg/l.
  • Resolution: two auto ranging scales 0.00 – 99.9mg/l, 100.0 – 2000mg/l.
  • Repeatability: +/- 10% of reading or 2ppm (whichever is greater).
  • Simple to install on a wide range of Aquaprobes.
  • Easy 2-point calibration.
  • Site specific calibration with grab sample calibration option.

Our CDOM sensors can be installed into many of our probes – below are some examples. To see all of the probes that can house the CDOM sensor please visit the Products section.



Single parameter Aquaprobe, for use with any of our sensors.



The Aquasonde-5000 has 4 spare ports. They can house any combination of ISE and Optical sensors.



Advanced portable multi-parameter Aquaprobe.

Simple installation

Installing the CDOM sensor into an Aquaprobe is a very simple process. Simply unscrew the blanking plug from an appropriate aux socket, apply some silicon grease to the thread of the sensor (grease provided), and screw in the sensor. Full calibration is required after installation.

Simple calibration

A core aim at Aquaread is to make our products easy to use, which is why we have simplified and automated the calibration process as much as possible. The CDOM sensor is calibrated at 2 points. A zero point and a 100µg/l point using Aquaread’s CDOM-CAL solution.

Grab sample factor

When taking measurements at a particular site there can occasionally be a difference between measured values with the sensor and laboratory data. These differences are caused by a variety of factors.

You can apply a multiplier to the data obtained from our sensor in order to make it more closely correlate with lab data for that particular site. To do this we use a grab sample factor. This factor can be calculated by dividing the average grab sample value (measured in the lab) by the average value measured by our CDOM sensor.

This is a great addition to the software when trying to correlate new data with historic readings from the same site.


Determination of CDOM in the field using fluorescence measurement techniques will never be as accurate as measurements made in a lab using traditional techniques.

Factors adversely affecting accuracy include:

  • Interference from compounds which fluoresce at similar wavelengths.
  • Differences in the fluorescent response caused by temperature.
  • Differences in the fluorescent response caused by ambient light.
  • Interference caused by turbidity.

Fluorescence measurement techniques are ideal for researchers who are interested in detecting the presence or absence of a specific substance and measuring relative fluorescence changes that can be used as an indication of increasing or decreasing concentrations.

Fluorescence measurement techniques are not ideal for quantitative measurement. In order to obtain more accurate results, data obtained with the fluorometer in the field should be post-calibrated with data from standard laboratory analysis of grab samples acquired during the study.

If grab sample data is available, a Grab Sample Factor (GS Factor) should be calculated and input on the calibration screen in order to improve the accuracy of future readings.