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V E R I F Y I N G C A L I B R A T I O N
You may from time to time want to verify the calibration of your miniDOT Clear Logger. Do this by
placing the miniDOT Clear Logger in a black 5-gallon bucket containing 4 gallons of fresh water.
(The picture below shows a white bucket so that the miniDOT Clear Loggers are more easily seen.)
The miniDOT Clear Logger’s black end cap is heavy and the miniDOT Clear Logger will tend to flip
so that this end is down. Prevent this somehow. The miniDOT Clear 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 Clear 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 Clear
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.
Use the miniDOT Clear Logger’s miniDOT Plot program to examine the measurements. Saturation
values should be very close to 100%, depending upon the accuracy that you have determined
barometric pressure. If you placed ice in the bucket, then saturation values will still be 100%. You
will see the DO concentration and temperature change greatly as the bucket warms.
The recorded data, when using yeast should show 0% saturation and 0 mg/l dissolved oxygen
concentration. In practice the miniDOT Clear Logger often reports slightly positive values of about
0.1 mg/l, but within the accuracy of the miniDOT Clear Logger.
