Are there guidelines for measuring low level dissolved oxygen with the IntelliCAL™ LDO101 Luminescent/Optical Dissolved Oxygen (LDO) Sensor?
Guidelines for low level dissolved oxygen (DO) measurements.
The following Guidelines are recommended for measuring dissolved oxygen (DO) below 1 mg/L O2:
- Perform a 100% and a zero point calibration. The zero point calibration should be done last to “condition” the LDO probe surface for low DO concentrations (see When is it recommended to perform a zero calibration on the IntelliCAL™ LDO101 Standard Luminescent/Optical Dissolved Oxygen probes? ).
- It is necessary to check the salinity of the sample with an IntelliCAL™ CDC401 Conductivity Probe, in order to adjust the salinity correction factor on the HQd meter. The salinity correction factor can be entered manually or automatically (see Can the IntelliCAL™ Dissolved Oxygen probes automatically correct for salinity? )
- Always work with a closed glass beaker (with lid or stopper), where the LDO probe is “sealed” to atmosphere.
- The LDO probe should dip into the sample by a minimum of 2 cm above the metal disk of the temperature sensor. It is recommended to use a glass bottle with a funnel, that is filled with sample (this would be similar to the setup used for Biochemical Oxygen Demand (BOD) measurements).
- Larger sample volumes are recommended (300-500 mL would be ideal) for more reproducible results.
- The solution to be measured should always be stirred very slowly. A recommended stirring speed would be one full turn of the magnetic stir bar per 3-5 seconds.
- Make sure the stirring is the same for ALL measurements when testing low level DO.
- After inserting the LDO probe into the sample, let it rest there for 5-10 minutes to equilibrate for temperature (this is usually the main time consuming step). In some cases, if the sample and LDO probe temperatures differ by more than 5°C, it can take up to 15 minutes or more to get a stable temperature and DO reading.
- Observe the development of the DO and temperature measurements. It may require up to five measurements (when using the "Press to Read" mode) to have a final reading. Please note, that below 1 mg/L DO, the goal should not be to have a quick measurement. Since the conditions are really hard for the sensor, it will take patience to obtain an accurate result. Usually after four to five measurements, the signal / DO reading should be stable.
- For statistics, repeat the procedure thoroughly in order to get reproducible results.
- When changing the sample type (chemical, concentration, salinity etc.) start from beginning.
- Cleaning is also important, especially when using samples containing organics, microbials, or food/beverage with alcohol, sugar etc. (see What is the correct way to clean the sensor on a dissolved oxygen LDO® probe? )