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Calibrating DO Meter
LabLady
Posted: Monday, December 28, 2009 6:16 PM
Joined: 12/28/2009
Posts: 1


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

 

Thanks!


Brent Dickey
Posted: Tuesday, December 29, 2009 8:06 AM
Joined: 9/28/2009
Posts: 18


Auto cal on saturated air with YSI 5100 meter. What's a Winkler happy


Perry Brake
Posted: Wednesday, December 30, 2009 5:58 PM
Joined: 12/16/2009
Posts: 69


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.

 

 


James Royer
Posted: Thursday, December 31, 2009 1:01 PM
Joined: 9/21/2009
Posts: 98


I would agree with Perry that you need to calibrate the DO meter the same all the time. I made a post on waterandwastewater as to calibration. If the concern is that the LDO values are different than other meter readings then a winkler titration would determine which is the correct DO readings. A 0.8 to 1.0 mg/L difference makes a big difference for large dilutions. the LDO has not been approved everywhere.


adri
Posted: Monday, January 4, 2010 4:06 PM
Joined: 1/4/2010
Posts: 6


I have just started at a new facility and here we are wetting the probe with dH2O, and letting it hang in the air until it has stabilized then pushing cal, personally I don't like this method at all.  Has any one else come across this? 


Perry Brake
Posted: Monday, January 4, 2010 4:39 PM
Joined: 12/16/2009
Posts: 69


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.

 

 


Old Lag
Posted: Monday, January 4, 2010 5:00 PM
Joined: 12/15/2009
Posts: 3


I cannot see how this method can work (just wetting the membrane).  The electrode will quickly consume the DO in the small amount of water wetting the tip and you will be calibrating at zero.    Won't work.


adri
Posted: Monday, January 4, 2010 7:25 PM
Joined: 1/4/2010
Posts: 6


Thanks people, unfortunately the probe is not getting used for BOD, but is getting used for measuring the DO in the receiving environment.  I think that I will have another try to find the manual and press this issue (unfortunately one of many).   


Perry Brake
Posted: Monday, January 4, 2010 8:53 PM
Joined: 12/16/2009
Posts: 69


What meter are you using?


adri
Posted: Tuesday, January 5, 2010 1:13 PM
Joined: 1/4/2010
Posts: 6


Its a ysi 58, I think it is about 15 years old. 


Anonymous
Posted: Tuesday, January 5, 2010 5:13 PM

You can go online to the YSI website and download the various manuals for their meters.  In it will be the calibration procedure(s), which do not include the one being used in your lab.  Good luck.


Perry Brake
Posted: Tuesday, January 5, 2010 7:53 PM
Joined: 12/16/2009
Posts: 69


  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!  happy



mb
Posted: Thursday, January 7, 2010 10:22 AM
Joined: 1/7/2010
Posts: 1


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.


Brent Dickey
Posted: Thursday, January 7, 2010 10:41 AM
Joined: 9/28/2009
Posts: 18


Looking back over our DO meter calibration logs for the past month we recorded pressure readings between 745 and 769. That's a difference of about 0.3 ppm at 20 degrees. Before we got our YSI 5100 we used a $100 barometer from NCL and it's still working after ten years.


Perry Brake
Posted: Thursday, January 7, 2010 10:48 AM
Joined: 12/16/2009
Posts: 69


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.