Soiling ratio reading above 1.00

By definition, the soiling ratio should always be less than or equal to 1.00.

However, sometimes the soiling ratio measured with an RDE300 series system will be greater than 1.00. Usually this means that the RDE300 unit was not properly calibrated with the clean and dirty reference PV devices as outlined in the RDE300 Series User Guide.

Sometimes though the calibration may be properly performed and yet the measured soiling ratio drifts slowly to values above 1.00 over time.

When troubleshooting this condition, here are several factors to consider.

If using an RDE300 system with an automatically washed reference cell:

• Check the alignment between the reference cell and designated soiled module. They need to be co-planar within the specs shown in the user guide. If not, the baseline reading may shift over time as the peak sun elevation changes throughout the year.
• Check whether you are using distilled water in the washing unit reservoir, as directed in the user guide. If you are using regular tap water, mineral deposits may build up over time on the reference cell surface, creating white clouds which block light. These are not removed by the water spray. To resolve this issue, clean the cell manually, remove the water from the reservoir, and refill the reservoir with distilled water only.
• Check whether the reference cell is fouled by some substance (such as bird droppings) that is sticky and is not being removed by the water spray. This is unusual but does happen occasionally. In this case, clean the cell manually and resume normal operation.

If using an RDE300 system with two modules:

• Check the alignment between the designated clean and soiled modules. They need to be co-planar within the specs shown in the user guide. If not, the baseline reading may shift over time as the peak sun elevation changes throughout the year.
• Check whether the designated "clean" module ("PV1") has been cleaned properly. Sometimes site crews may not completely clean the module as intended.
• If the clean module is indeed clean, but the soiling ratio is still reading above 1.00 when both modules are clean, re-do the calibration described in the user guide. Sometimes this is necessary if the designated clean module has degraded.