Precision Measurement Engineering, Inc.
(760) 727-0300
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12
Verifying Calibration –
You may from time to time want to verify the calibration of your
miniDOT Logger. Do this by placing the miniDOT Logger in a black 5-gallon bucket
containing 4 gallons of fresh water. (The picture below shows a white bucket so that the
miniDOT Loggers are more easily seen.) The miniDOT Logger’s black end cap is heavy and
the miniDOT Logger will tend to flip so that this end is down. Prevent this somehow. The
miniDOT Logger must be placed in the bucket with the black end cap upwards. Otherwise
bubbles will accumulate in the black end cap area and miniDOT Logger will not sense the
DO in the water correctly. Use an aquarium pump and air stone in the water to provide a
bubble stream. Cover the bucket with a black lid. The idea is to prevent light from enabling
algal growth.
Record measurements for several hours or a day, but in any event long enough for the
miniDOT Logger’s temperature to come to equilibrium with the water. During the experiment,
find the local air pressure, either from measurements or from a local weather station. Watch
out... weather stations often report barometric pressure referenced to sea level. You must
determine the absolute barometric pressure at your elevation.
A more comprehensive experiment is to additionally place ice in the bucket and mix until the
water temperature is close to zero degrees. Next, remove the ice. Place the bucket on a
towel or piece of cardboard and cover the top of the bucket with a towel. Record for 24 hours
as the bucket temperature gradually returns to room temperature.
After recording the bubbled water, you may also remove the air stone and gently mix a
packet of baker’s yeast into the bucket together with a tablespoon of sugar. The water must
be only slightly warm to the touch but not more than 30 deg C. These organisms will deplete
all the dissolved oxygen in the water. Cut a disc of thin plastic film just large enough to lay
on top of the water. Place this on top of the water. Do not stir or bubble after placing the
film. Record measurements for at least an hour or more.
