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DISSOLVED OXYGEN PROBE MAINTENANCE
Q - How often should I calibrate my dissolved oxygen meter?
A - As a rule, dissolved oxygen probe maintenance calibration should be performed or verified daily, before sampling starts.  But in general, dissolved oxygen probe maintenance calibration frequency is determined by the user and the importance of the data.  The more critical the data, i.e. when used for compliance purposes, the more attention that should be paid to timely dissolved oxygen probe maintenance calibrations.

The calibration of newer optical based dissolved oxygen probes is very stable but we still recommend that they be verified on a regular basis to ensure accurate data.

Your DO data is as good as your dissolved oxygen probe maintenance calibration.

Q - How do I verify my dissolved oxygen probe maintenance calibration?
A - Place the dissolved oxygen probe in its calibration environment and check to see that the meter is reading the calibration value for the current barometric pressure.  For example, if your 'true' barometric pressure is 750, divide this number by 760 and then multiple by 100% to calculate what your dissolved oxygen probe and meter should be reading in water-saturated air or air-saturated water.

750/760 x 100 = 98%

A post-check of your dissolved oxygen probe can also be performed with this value in mind.  If, for instance, you calibrate your dissolved oxygen probe to 98% and then conduct your DO testing, you can place the probe back into the same calibration environment and it should read +/- 2% (+/- 1% on optical) of 98% once stable.

Q - What is 'true' barometric pressure?
A - Dissolved oxygen probes need to be calibrated to "True" barometric pressure, i.e. a barometric pressure value that has not been corrected to sea level.  Laboratory barometer readings are usually "true" (uncorrected) values of air pressure and can be used "as is" for dissolved oxygen probe calibration.  Weather service readings are usually not "true", i.e., they are corrected to sea level, and therefore cannot be used for dissolved oxygen probe maintenance calibration until they are "uncorrected".  An approximate formula for this "uncorrection" is:

True BP = [Corrected BP] - [2.5 * (Local Altitude in ft. above sea level/100)]

Q - If a calibrated dissolved oxygen meter is turned off, then back on, does it need to be recalibrated?
A - There is no need to perform a maintenance calibration of your dissolved oxygen probes when the meter is turned back on.  Dissolved oxygen probes and meters are designed to record and save the maintenance calibration values so no recalibration is required at power-up unless it requires normal calibration as mentioned earlier.

You can "verify" that the dissolved oxygen meter is holding its maintenance calibration by knowing the original calibration value (98% as an example) and the probe should read within +/-2% (+/- 1% on optical) of 98% upon power on once it's stable.

Q - After turning the dissolved oxygen probe and meter on, how long should I wait before calibrating or taking a measurement?
A - There really is no set time period to wait for a dissolved oxygen probe or meter to be ready for measurement.  Regardless of the dissolved oxygen probe type that is used, wait for the temperature and dissolved oxygen values to become stable.  For dissolved oxygen probes and meters that use a polarographic probe, achieving stability takes about 5-15 minutes.  For galvanic and optical dissolved oxygen probes, the readings will reach stability almost immediately after turning on unless the ambient temperature has changed.  For each dissolved oxygen probe type, it is recommended to make sure there are no water droplets on the sensing element (membrane) to help ensure quicker stability and ensure calibration accuracy.

For the best maintenance calibration results, it is very important to have the dissolved oxygen probe and meter in an environment where the temperature is stable and does not change prior to, or during the calibration procedure.

Q - Is it necessary to perform a maintenance recalibration if there is a change in altitude or barometric pressure after the initial dissolved oxygen probe calibration?
A - No.  Dissolved oxygen probes are calibrated to and measure the partial pressure of oxygen.  Therefore, after performing an accurate maintenance calibration, the dissolved oxygen probe and meter will automatically compensate for changes in pressure.  For dissolved oxygen probe and meters measuring % DO, the maintenance calibration value will be 100% regardless of the altitude or barometric pressure and the internal barometer of the meter will be used to keep the saturated value at 100%.

Q - For consumptive electrochemical dissolved oxygen probes, how do I know when I have enough stirring?
A - When increasing the amount of stirring does not result in an increase in the dissolved oxygen probe readings, enough stirring is being supplied.

Q - How often should the membrane be changed on an electrochemical dissolved oxygen sensor?
A - As a general rule, we recommend for dissolved oxygen probe maintenance that the membrane be changed every 2-8 weeks.  This is dependent on the dissolved oxygen probe sampling application.  For laboratory or portable DO meters, keeping the dissolved oxygen probe membrane clean and in a moist environment between uses will lengthen the membrane life.

When measuring samples with high levels of hydrogen sulfide, a weekly dissolved oxygen probe membrane change will reduce sensor cleaning and maintenance and thus provide better performance.  When measuring in clean water, dissolved oxygen probe membrane changes can easily go beyond 4 weeks without negative effects.

Q - How often do electrochemical dissolved oxygen probes need maintenance?
A -Maintaining electrochemical dissolved oxygen probes is relatively easy, but is only necessary when the sensor no longer performs to factory established specifications.

When calibrating, many instruments perform a self maintenance dissolved oxygen probe performance check.  If the dissolved oxygen probe is outside of its normal working parameters, the instrument will give an out of range indication.  Before performing maintenance cleaning of dissolved oxygen probes, always try fresh probe electrolyte solution and a new membrane.

It is normal for a polarographic dissolved oxygen probe's silver anode to darken over time due to the build up of silver chloride.  This typical darkening will not affect the sensors performance so do not clean the dissolved oxygen probes just because they 'look dirty'.  Long term exposure to hydrogen sulfide can cause darkening of the anode that will influence a dissolved oxygen probes performance.  Typical symptoms will be jumpy readings, inability to calibrate and/or low dissolved oxygen probe current, both of which may respond well to maintenance cleaning.  Typically, the dissolved oxygen probes will require cleaning or servicing about once per year.

Go to the troubleshooting and maintenance section of the dissolved oxygen probe and meters instruction manual or call our technical support department for the best advice regarding resolving these issues.

Q - How often should I replace an optical dissolved oxygen probe sensing element?
A - The optical dissolved oxygen probe sensing elements are warranted for 1 year but may last longer.  Be sure to keep the dissolved oxygen probe sensing element clean and stored in a moist environment between uses to obtain the longest usable life possible.

Information provided by YSI Incorporated.

For more information on dissolved oxygen probe maintenance calibrations please click here.
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