Eureka MANTA+ - Conductivity

Eureka Manta Manual Page 31 of 67
The manufacturers of optical-DO sensors recommend that you not calibrate the zero-DO point. However,
we support zero-DO calibration in the Manta 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-liter 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 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 your Manta
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 d
issolved-oxygen sensor maintenance is little more than occasionally cleaning the sensing surface
(the black material; about a centimeter 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 Eureka 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.
D.8 Conductivity
Eureka uses the four-electrode method to
determine water conductivity. Two pairs of
graphite electrodes are situated in a stable
geometry (you can barely see the electrodes; they
look like two bull’s eyes inside the slot on the
conductivity sensor).