Which method (Winkler, Saturated Air, etc.) do you use to calibrate your DO meter and why?

Thanks!

A lot of looks, few replies, LabLady, so I will throw in my 2¢ worth.  We had this discussion on the "old" WEF forum...too bad we have to reinvent the wheel!

I advise against calibrating a DO meter using Winkler, not because the Winkler is inaccurate...if done properly, the titration is very accurate.  Rather, I think it is good practice to calibrate any measuring device as closely as feasible to the way in which it is to be used.  One might conclude from that that I would recommend calibrating with air-saturated water since you are measuring DO in water.  Calibration with air-saturatred water works well, but so does calibration with water-saturated air, and it is the latter that the vast majority of labs use when measuring DO for the BOD test, for example.

You didn't mention BOD, but if your concern is calibration of the meter for the BOD test, the most important thing, in my opinion, is to calibrate and use the meter in exactly the same way on Day-5 as you did on Day-1 since the BOD calculation involves change in DO, not the DO reading itself.

I have never heard of such a procedure and have no faith in it providing the actual dissolved oxygen level.  You have no way of knowing if the air in the room is saturated with DO as you do in an enclosed bottle.  Having said that, however, the calibration procedure might work for the BOD test since you don't use the actual DO level, but rather the difference between two readings.  But an inspector would undoubtedly look unfavorably on it unless you could prove in very simple and convincing terms that the procedure works...and I wouldn't even think about trying to do that.

  LabLady, you can find (and download) the user's manual for the YSI 58 at...

http://www.fondriest.com/pdf/ysi_58_manual.pdf 

The YSI comes with a "calibration bottle" that substitutes for a BOD bottle in creating at 100% DO-saturated environment for air calibration of the meter/probe.  The manual tells you how to use it.  Alternatively, the manual says you can use a BOD bottle when air calibrating.  Also, the manual describes calibrating in water, and by Winkler titration.

The "calibration bottle" might have been lost long ago in your lab and someone mistakenly thought it didn't really do anything other than keeping drops of water off the lab floor, and that you could calibrate without it.  They were wrong!  

We have a YSI 5000 which does not do automatic barimetric pressure calibration.  For years we've left the setting at 760 mm Hg and simply calibrated with autocal.   Now, we are looking at doing manual barametric pressure calibrations by checking the local weather station's pressure reading (19 ft above sea level.) 

Is anyone else here doing manual barametric pressure calibrations?  I'm curious what folks thoughts are about our planned practice as well as our past practice.

I have never run into a lab that uses a constant setting of 760.  By doing that, you are essentially telling the meter the lab is at sea level and the atmospheric pressure never changes.  The first condition (sea level) could be true, but the second undoubtedly isn't.

To calibrate properly using a meter that does not have an integral barometer, you need to determine the pressure from either a barometer (they aren't that expensive!) of from a local airport, weather station, etc.  Be sure they do not give you the pressure corrected to sea level which is what airports are likely to do because it is the corrected pressure that airplanes use. 

Once you have the actual barometric pressure, couple that with the temperature and go to a chart of saturated DO levels at given pressures and temperatures to see what DO value to use in calibrating.  If you are air calibrating in a closed closed container (e.g., BOD bottle) be sure to set the meter to use 100% saturation since that is what the probe is experiencing in the container.  YSI meter manuals mention that, but you have to look hard to find it.

A lot of labs go by what the meter manufacturer tells you to do, and that is use the elevation of your lab to determine the atmospheric pressure.  That would be great if the atmospheric pressure remained constant over a 5-day period...but in most locations, it doesn't even come close all year around.  You must use the actual pressure.