HyQual Multi-probes User Manual v3, 08.07.2022
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ing surface according to the amount (partial pressure) of oxygen in the sample. The oxygen-active com-
pound fluoresces by absorbing energy in the form of blue light and then emitting energy as red light.
Oxygen “quenches” that fluorescence, so the more oxygen, the less fluorescence.
In each measurement cycle, the blue light is first turned on, and then turned off. The red-light receiver
measures the time it takes, after the blue light is turned off, for the fluorescence to die off. This value is
proportional to dissolved oxygen.
DO readings are corrected for the water sample’s temperature and salinity (if you have a C
onductivity
sensor).
HyQuest Solutions
recommends the “air
-
saturated water” DO calibration method, as opposed to the
“water
-
saturated air” calibration commonly used in the past. Here are the steps to air
-saturated water
calibration:
1.
Make sure your instru
ment’s Barometric Pressure setting is accurate. (C.14)
2.
Open the Calibration menu and select “ADO %sat” for
HyQual
3.
Put a half-liter of tap water in a liter jar, secure the lid, and shake the jar vigorously for one mi-
nute. Take the lid off the jar and let the water stand for about five minutes so air bubbles float
out.
4.
Screw your calibration cup onto the HyQual probe housing and remove the cup lid. With the
sensors pointed upward, fill the calibration cup until your aerated water covers the DO sensor
by a centimeter or so.
5.
Wait a few minutes for the temperature to equilibrate and the sensor to arrive at a steady read-
ing.
6.
Now just follow the calibration instructions on the screen.
The manufacturers of optical-DO sensors usually recommend that you not calibrate the zero-DO point
because the zero-DO point drift is very low. However, The HyQual probe supports zero-DO calibration,
should you wish to check your sensor’s zero from time to time, using any of three methods:
1.
Dissolve a few grams of sodium sulfite and a pinch of cobalt chloride in a half-liter of tap water.
You can buy this solution ready-to-use but be careful not to aerate the solution by pouring it
back and forth numerous times.
2.
If you do not wish to use the sodium-sulfite method , you can prepare zero-oxygen water by
bubbling nitrogen through water. Use bottled gas and an aquarium-
type air stone. (If you’re
using a high-pressure gas bottle, please use a two-stage regulator to prevent unnecessary ex-
citement.) After bubbling the gas through, say, a liter of water for, say, 10 minutes, you should
have a good zero.
3.
The simplest way to check zero response is with nitrogen gas. Wrap the sensor-end of The
HyQual probe
with a plastic bag, and feed nitrogen gas into the bag. Make sure there’s another
hole at the opposite end of the bag for the air to escape, otherwise you won’t get a good zero
and the exploding bag will cause unnecessary excitement. (If you’re using a high
-pressure gas
bottle, please use a two-stage regulator.)
Optical dissolved-oxygen sensor maintenance is little more than occasionally cleaning the sensing sur-
face (the dark material, about a centimeter diameter, at the tip of the sensor) with a cloth and soapy
water.
Optical dissolved-oxygen sensors usually have very low drift rates . Practice will show you how often to
calibrate DO, and you might find that one or the other of the calibration points (zero or 100% satura-
tion) does not require calibration every time you set the other point.
