WEF Discussion Forums
Laboratory Management and Technical Issues
LDO probe calibration verification?
We are having some BOD blank failures over 0.20 and want to make sure it is not from a poorly calibrated DO probe. We have a HACH HQ3d LDO probe that we calibrate according to the method - water saturated air in a BOD bottle. The calibration is checked by aerating a water sample for 30 minutes and then allowing it to settle for another 30 minutes. This sample is read with the LDO and that number is compared to the theoretical calculation of oxygen using the Benson/Krause method noted in standard methods (4-132). Is this the correct way to verify the LDO probe calibration and what is an acceptable number? 0.20? How often should the sensor cap on the LDO be changed? Should we check the zero with sulfite solution on a regular basis?
We verified our old membrane DO probe with the winkler titration, should we run the winkler and compare it to the LDO probe numbers? or just stick with the theoretical calculated numbers. should they all argee within 0.2 mg/L?
any thoughts on LDO calibration problems?
The correct way for checking calibration of the LDO meter/probe is in Hach's method, Peter. There are two such methods floating around, the old one which says all you have to do is check calibration (which was a big selling point when Hach first started marketing the LDO), and the new one that as I interpret it requires calibration and other QC testing that takes routine use of the LDO system from "easy" to, in my opinion, a nightmare.
But that wasn't your initial query. You are having blank problems and are wondering if meter/probe calibration could be the problem. If such IS the problem, you would expect to have as many negative blanks (i.e., where DO seems to increase) as you do positive blanks (i.e., where DO appears to decrease, the more common blank problem). If that is not your situation, I suspect your blank problem originates somewhere else.
I haven't seen these additional calibration and QC testing requirements. Could you elaborate on them?
Go to http://www.perrybrake.com/BODsolutions.html and click on the third bullet from the bottom.
Our lab has been using the LDO probe for a while now and absolutely love it. We calibrate using water saturated air, although we don't need to calibrate very often because it is usually right on the money.
I do remember reading the the sensor tip needs to be changed once per year.
We have used the HACH LDO for a few years and love it. We keep the LDO that is used for BOD's IN the BOD incubator so it is always at 20 degrees...and...we calibrate the meter IN the incubator. The ambient temp of our lab is higher than 20degrees. Works like a charm. The meter also indicates barometric pressure...it is right on with the barometer, too.
We were having trouble with blanks, too. Too much DO depletion. Switched to making the dilution water with store bought distilled water. GREAT blanks, now.
Get Perry Brake's book on BOD...a great resource, easily understood, and has great suggestons.
While doing a little research on LDO and calibrating LDO meters, I noticed that the following terms were all being used to describe calibration using a BOD bottle filled with 1/4 inch of water and equilibrated (shaken):
saturated air calibration
air saturated WITH water calibration
and the one that appears to be currently in vogue but to me is the most confusing:
water saturated air calibration
The main problem is that the two words "water saturated" are acting collectively as an adjective when used with the word air, whereas if you simply invert them, giving you "saturated water" - that refers to something totally different, with the word "saturated" now being the adjective acting on the word "water" (and the "air" is implied).
I think that a lot of people read "water saturated air" and mentally visualize it with the word "WITH" added, which then gives it the oppose meaning it was intended to have! "Water saturated air" and "water saturated WITH air" are opposites, and therein lies my issue with the term "water saturated air"!