3. Water & Environmental / Dissolved Oxygen

How to Calibrate the Hach LDO Probes

Let us first look at the probe. The Hach dissolved oxygen probes are equipped with a measuring LED that emits a pulse of blue light and a photodiode as a light detector. The probes are also equipped with a sensitive luminescent coated spot that is exposed to the sample.

  • The measuring LED emits a pulse of blue light which irradiates the back of the oxygen-sensitive spot. The coating material reacts with luminescence and emits a pulse of red light where the intensity and time delay are measured.
  • When oxygen is present in the sample stream and contacts the coating, the intensity and time delay of the luminescent light emission are changed.
  • The more oxygen molecules come into contact with the coating, the lower the intensity and the shorter the duration of the red radiation. These changes in the profile curve are used to determine the measurement.

Hach’s LDO probes are available as standard or rugged for their HQ portable meters, these are the probes we’re considering for guide.

Maintenance

Before we cover calibration, let’s take a look at maintenance. This is crucial as no amount of calibration will help if your probe is not correctly maintained.

Based on the model, the cleaning and maintenance frequency, the LDO sensor cap must be replaced after 365 days. After one year of exposure to wastewater, cold and abrasive samples, the normal performance of the LDO sensor cap can no longer be guaranteed.
To ensure optimal performance we recommend a change of the sensor cap for the optical oxygen sensor at least every year. With the new cap, replace also the “i-button”, which contains the calibration data.

Used / damaged LDO caps

For quick response during daily operation, it is recommended to store probes in their storage chamber or tube with some drops of water to maintain a humid environment. In some cases, there is a little sponge inside to hold the water.

Calibration

How often should the LDO electrode be calibrated?

Because the LDO system is less sensitive to changes (no metal electrodes, no electrolyte, etc.), the LDO probe should be checked in 100% O2 standard every month.

Checks Before Calibration

  • The probe must have the correct service-life time stamp. Set the date and time in the meter before the probe is attached.
  • It is not necessary to recalibrate when moving a calibrated probe from one HQ meter to another if the additional meter is configured to use the same calibration options.
  • To view the current calibration, push, select View Probe Data, then select View Current Calibration.
  • If any two probes are connected, push the UP or DOWN arrow to change to the single display mode in order to show the Calibrate option.
  • Prepare the probe for use (see above).
  • If using a rugged probe, remove the shroud from the probe.

Factors to Consider When Calibrating

  • % saturation or mg/L calibration methods are available in the Modify Current Settings menu.
  • The slope value is the comparison between the latest calibration and the factory calibration shown as a percentage.
  • An additional zero point calibration can be added to the calibration routine.
  • The calibration is recorded in the probe and the data log. The calibration is also sent to a PC, printer or flash memory stick if connected.
  • Air bubbles under the sensor tip when submerged can cause a slow response or error in measurement. If bubbles are present, gently shake the probe until bubbles are removed.
  • If a calibration error occurs, refer to the troubleshooting guide.

Calibration Procedure

The standard 100% DO calibration is performed in a BOD bottle* with water-saturated air (bottom is covered with approx. 5 ml tap water).
For 0% DO calibration, a BOD bottle filled with tap water can be purged with N2 gas until no more oxygen is present, or a mixture of ascorbic acid in 5N sodium hydroxide solution can be used. The alkaline ascorbic acid solution needs a few minutes to remove all present O2.
The formerly described 0% standard using sodium sulfite and cobalt salt (catalyst) is hazardous and is no longer recommended.

The following are the steps to follow when calibration the LDO probe, remember should there be an issue with the probe or the connection to the meter you will not get the desired result.

  1. Connect the probe to the meter. Make sure that the cable locking nut is
    securely connected to the meter. Turn on the meter.
  2. Push Calibrate.
  3. Push Methods. Select User Cal – 100%. Push OK.
  4. Rinse the probe cap with deionized water. Blot dry with a lint-free cloth.
  5. Add approximately ¼ inch (6.4 mm) of reagent water to a narrow-neck
    bottle, such as a BOD bottle.
  6. Put a stopper in the bottle and shake the bottle vigorously for approximately 30 seconds to saturate the entrapped air with water. Allow up to 30 minutes for contents to equilibrate to room temperature.
  7. Remove the stopper. Carefully dry the probe cap with a non-abrasive cloth. Put the probe in the bottle.
  8. Push Read. The display shows “Stabilizing” and a progress bar as the probe stabilizes. The display shows the standard value when the reading is stable
  9. Push Done to view the calibration summary.
  10. Push Store to accept the calibration and return to the measurement
    mode. If a rugged probe, install the shroud on the probe.

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