Are there guidelines for measuring low level dissolved oxygen with the IntelliCAL™ LDO101 Luminescent/Optical Dissolved Oxygen (LDO) Probe?

Document ID

Document ID TE5813

Published Date

Published Date 06/03/2018
Question
Are there guidelines for measuring low level dissolved oxygen with the IntelliCAL™ LDO101 Luminescent/Optical Dissolved Oxygen (LDO) Probe?
Summary
Guidelines for low level DO reading.
Answer
Guidelines for measuring dissolved oxygen (DO) below 1 mg/L O 2:
  • Perform a 100% and a zero point calibration. The zero point calibration should be done last, to “condition” the LDO surface for low O2
  • It is necessary to FIRST check the salinity of the sample with an IntelliCAL™ CDC401 Conductivity Probe, in order to adjust the correction on this factor in the HQd meter. Afterwards corrections on Salinity have to be done manually, because there is no HQd software function to do this afterwards.
  • Always work with a closed glass beaker (with lid or stopper), where the LDO probe is “sealed” to outer air.
  • The LDO probe should dip into the sample by minimum of 2 cm above the metal disk of the temperature sensor. It is recommended to use a glass bottle with funnel, filled with sample like for BOD measurements.
  • The more sample volume the better (300-500 mL recommended) for more reproducible results.
  • The solution to be measured should always be stirred, very slowly, recommended one turn around of the magnetic stir bar per 3-5 seconds.
  • Make sure that this stirring adjustment is the same for ALL measurements in this lower DO concentration.
  • 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 sample and LDO probe temperatures differ more than 5°C, it take up to 15 minutes and more to get a stable temperature and Do reading.
  • Start reading (press to read mode) and observe the development of the O2 and °C measurement. It needs up to five times pressing read to have a final reading. please note, that below 1 mg/L DO the goal should not be to have a quick reading and done. Since here the conditions are really hard for the sensor, it should be given enough patience to wait for a correct reading. Usually after 4-5 times “READ”, 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, biological things, or food/beverage with alcohol, sugar etc.