Back to top

What is Fluorescein testing?

Fluorescein is a synthetic organic compound that was the first fluorescent dye to be used for water tracing work. Fluorescein emits a bright green fluorescence and is used for hydraulic model studies and underground water studies. Fluorescein has been almost entirely replaced by rhodamine in recent years, however, there are still applications where the use of fluorescein is appropriate or preferred.


The advantages of using fluorescein for water tracing include its low absorption rate and the fact that it emits a bright green fluorescence, which makes it easier to visualise the progress of your experiment. The disadvantages are that fluorescein is rapidly destroyed by sunlight, fluorescence drops markedly below a pH of 5.5, and many naturally occurring fluorescent materials with similar characteristics can interfere with measurements.

Aquaread’s water fluorescein meter

Aquaread’s fluorescein water tester is just one of the parameters that are designed for use with the Aquaprobe range of multi-parameter water testing probes. The fluorescein probe is an optical electrode that emits light at a 485nm wavelength. This excites the fluorescein, causing it to fluoresce and produce light above a wavelength of 540nm, which is detected by the sensor in the electrode. Our fluorometer is submersible and capable of measuring fluorescein in the range of 0 – 500ug/L at a 0.1 ug/L resolution. This optional optical electrode can be used with each of our Aquaprobe products, which have been designed specifically for use in the field.

When measuring fluorescein levels in water out in the field there are a few things to take into consideration. The measurements can be affected by interference. There are microbiological species and compounds that fluoresce at similar wavelengths to fluorescein, and the fluorescent response can be altered by temperature, ambient light and turbidity. The normal effects of temperature on the fluorescent response of fluorescein is automatically compensated for by the fluorescein water testing electrode.

Why use water fluorescein meters to measure fluorescein levels in water?

Using a portable fluorescein probe, or fluorometer, to measure concentrations of the tracer dye in water means that there is no need to collect water samples and conduct laboratory analysis. It’s also only necessary to use a small amount of dye, because water fluorescein meters can accurately measure concentrations down to the ug/L level.

There are many applications for fluorescein testing in water. It can be used as a tracer or marking instrument in oceanography, in environmental testing simulations to trace and analyse water leakages or contaminants, and for locating rain or groundwater entry points into the sanitary sewer system. These are just a few examples. Fluorescein testing can be used whenever you need to investigate water flow. It is particularly suitable for use underground where it won’t be affected by sunlight.


Our rugged and precise water quality monitoring devices feature the latest technology, protected by durable casing. The AP-7000 offers the user the most options, with 6 additional sensor ports. Suitable for long term deployment, the AP-7000 Aquaprobe features built in self cleaning mechanisms. The Aquaprobe AP-5000 aims to maximise the amount of data that can be collected in one go. Up to 4 additional sensors, such as a fluorescein probe, can be added to the 11 standard parameters. This field instrument packs a lot of punch into just 55mm of diameter.


We also have the AP-2000 and AP-LITE Aquaprobes. Each device is used with our Aquameter which collects all the data, along with the GPS coordinates of each reading. The multi-parameter capability of our devices means that, along with measuring fluorescein levels in water for flow testing, it’s possible to measure a whole range of other parameters that are important for monitoring water quality.

Get into monitoring with Aquaread

If you would like any more information about TDS meters and other total dissolved solids testing equipment, or water quality monitoring in general, please contact us. Click here to see our product range.