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Eijkelkamp is an advocate of the “air-saturated water” calibration method – that’s different from the “water-saturated
air” calibration commonly used in the past. Here are the steps to air-saturated water calibration:
1
Make sure your instrument’s barometric pressure setting is accurate. (4.18)
2
Put a half-litre of tap water in a litre jar, put on the lid and shake the jar vigorously for one minute. Take the lid off
the jar and let the water stand for about five minutes to let the air bubbles float out.
3
Screw your calibration cup onto the Scuba housing. With the sensors pointed upward, fill the calibration cup until
your aerated water covers the DO cap by a centimetre or so.
4
Wait a few minutes for the temperature to equilibrate.
5
Follow the Scuba control software calibration instructions – remember that you are calibrating % sat, not mg/l, so
select % sat from the list.
What’s the real story on optical DO sensitivity to fouling?
Glad you asked. Several years ago, there was rumour floating around that optical DO sensors were not affected by
fouling. The rumour was only half true.
Suppose you put an optical DO sensor in a river. If you’re just downstream of a rendering plant or oil patch, your
sensor might become coated with grease or oil. Unless that coating is impermeable to oxygen, your sensor will still
give accurate readings (though it may be slow to respond to changes in oxygen). That’s because the coating is not
oxygen-active, i.e. it doesn’t produce or consume oxygen.
On the other hand, if your sensor picks up an oxygen-active coating, for instance of photosynthetic algae. The algae’s
respiration can cause the sensor to report exaggerated swings in diurnal oxygen pressure because the algae have
their own micro-environment of oxygen pressure – and the optical DO sensor thinks that the oxygen pressure
immediately adjacent its membrane is representative of the rest of the world.
The manufacturers of optical-DO sensors recommend that you not calibrate the zero-DO point. However, we support
zero-DO calibration in the Scuba software, and think it’s a good idea to check your sensor’s zero from time to time in
either of three ways:
1
Dissolve a few grams of sodium sulfite and a pinch of cobalt chloride in a half-litre of tap water. You can buy this
solution ready-to-use but be careful not to aerate the solution by pouring it numerous times.
2
If you’re like me and think the sodium-sulfite method is yesterday’s news, you can prepare zero-oxygen water by
bubbling nitrogen through water. Use bottled gas and an aquarium-type airstone (If you’re using a high-pressure gas
bottle, please use a two-stage regulator to prevent unnecessary excitement). After bubbling the gas through, say, a
litre 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 your Scuba 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 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 surface (the black
material; about a centimetre diameter) with a cloth and soapy water.
Optical dissolved-oxygen sensors
usually
have very low drift rates
(compared to the old Clark sensors), so practice will show you how
often to calibrate your optical sensor. You might also find that one or
other of the calibration points does not require calibration every time
you set the other point.
The tip of the optical dissolved oxygen sensor (HDO) must be replaced
periodically, typically once every 4 years. If your SRF reports less than
100% or if you notice that the sensor’s readings are getting noisy (i.e.
jumpy), then it’s probably time to change the tip by unscrewing the
old tip and replacing it with a new tip. Recalibrate and you’re ready to
go.
