Why would I want to measure PAR?
Measuring photosynthetically active radiation (PAR) under the surface of water is useful in a variety of fields, from algal biofuels research to environmental quality. When light passes through a water column, it is attenuated based on the thickness of the column and the turbidity of the water. Measuring PAR underneath the surface of a lake, stream, bay, ocean, pond, or bioreactor can provide an excellent gauge of how much light is available to unicellular and multicellular phototrophic organisms such as algae, aquatic plants, protists, and phytoplankton. This, in turn, can be used as a gauge of the overall productivity of a system, though the specific calculations are often complex and include many other factors.
 
How is PAR measured with the MantaPlus Multiprobe?
Eureka integrates LI-COR PAR sensors with MantaPlus multiprobes to measures PAR at depths up to 200 meters. The LI-193 Spherical Quantum Sensor measures from all directions, while the LI-192 Quantum Sensor is used for upwelling and downwelling applications. Photosynthetically Active Radiation (PAR) is defined as radiation in the 400 to 700 nm waveband. PAR is the general radiation term which covers both photon terms and energy terms.  
 
The LI-193 uses a diffusive sphere to direct light through glass optical filters to a silicon photodiode. The filters create uniform sensitivity to light between 400 and 700 nm, which closely corresponds to light used by most terrestrial and aquatic plants and algae. The angular response of a typical LI-193 is slightly lower in the direction of the connector. When mounted, the low response is usually not significant due to the small proportion of upwelling radiation compared to the total.
 
The LI-192 Underwater Quantum Sensor measures PAR from all angles in one hemisphere. For measurements of downwelling and upwelling, the LI-192 may be adjusted to an upward or downward position on the sonde, depending on the desired measurement.
 
What should I know about PAR measurement in the field?
Eureka has chosen LI-COR sensors for their reliability and robustness.  Eureka’s unique PAR installation configuration eliminates the need for additional hardware to support the sensor.
 
The SI (International System of Units) unit of radiant energy flux is the watt (W). There is no official SI unit of photon flux. A mole of photons is commonly used to designate Avogadro’s number of photons (6.022 × 1023 photons). The einstein (E) has been used in the past in plant science, however, most societies now recommend the use of the mole since the mole is an SI unit. When either of these definitions is used, the quantity of photons in a mole is equal to the quantity of photons in an einstein (1 mole = 1 einstein = 6.022 × 1023 photons).  The MantaPlus reports PAR up to 10,000 µmol/cm2, along with other parameters measured by the multiprobe, such as dissolved oxygen, conductivity and pH.
 
Bio-fouling can be a problem for PAR sensors, particularly those deployed for extended periods.  Any foreign material that accumulates on the active surface of the sensor will reduce the amount of received light.  Therefore, PAR sensors must be cleaned periodically to achieve the best results. 
 
Eureka’s unique PAR configuration.
Most PAR sensors when coupled with multiparameter water quality sondes need additional mounting brackets, or cages, to hold the PAR sensor in place.  This is not necessary for Eureka’s PAR sensors, as they are connected to the body of the sonde. For convenience, Eureka’s PAR sensors may be removed and replaced with a water-proof plug when not in use.
 
MantaPlus Multiprobes with PAR sensors may be configured with battery backs for autonomous self-powered deployment, used with field displays for site-to-site spot checking, or connected to data telemetry stations for real-time remote monitoring.  Eureka sondes equipped with PAR sensors are portable, durable, and cost-effective.