Wednesday, August 13, 2014

Need a Very Accurate, Harsh Environment Dissolved Oxygen Sensor?

fluorescence quenching dissolved oxygen sensor
Fluorescence Quenching
Dissolved Oxygen Sensor
(courtesy of ECD)
If you’re looking for exceptionally accurate dissolved oxygen measurement suitable for a wide range of municipal and industrial water treatment applications, you may want to consider a fluorescence quenching (FQ) sensing element. Along with great accuracy, these sensors also provide exceptionally long life and are available with internal electronics that allow the units to self-calibrate, which eliminates costly and time consuming calibration.

These sensors are highly accurate. They provide a maximum error of less than 2 percent, repeatability of ±0.5 percent and resolution of 0.01 ppm or 0.01 percent saturation. They operates over a wide measurement range with three different outputs from 0 to 20 mg/l (0-20 ppm), 0-200 percent saturation or 0-400 hPa (0-6 psi).

Perfect for the most challenging municipal and industrial water treatment environments, these type of dissolved oxygen sensors can withstand ambient temperatures from -20 to 60°C (0-140°F) and measures at temperatures from -5 to 50°C (20-120°F). They can also withstand pressures up to 10 bar (145 psi).

The sensors work on a proven fluorescence quenching method that determines the oxygen concentration of water. The design includes a layer of optically-active, oxygen-sensitive molecules integrated into a cap (which is easy to replace). This layer is highly permeable to oxygen and rapidly attains equilibrium to its surroundings. The cap aligns the optically-active fluorescence layer above the two optical components inside the sensor - the emitter and detector.

The sensor’s emitter flashes a green light at the layer and the layer fluoresces back a red light. The duration and intensity of the fluorescence are directly dependent on the amount of oxygen in the layer. With little to no oxygen in the layer, the response is longer and more intense. The presence of oxygen, however, quenches the fluorescence effect producing a less intense response of shorter duration.

These sensors continuously analyze the oxygen level, the water temperature and air pressure to calculate dissolved oxygen values, and communicate digitally with transmitters or controllers which, in turn, provide a variety of analog or digital outputs (such as 4-20 mA communications with MODBUS RTU or HART®).

FQ (fluorescence quenching) sensors have advantages over amperometric sensors in that the FQ sensors have no membranes to replace or electrolytes to re-fill. There are no anode-cathode components to service or change. This type of FQ dissolved oxygen sensor only requires a change of caps every two years and an occasional wipe-down of the sensor head.