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3
Screw the reference cap back on to the sleeve. As you screw the sleeve into place, air and excess electrolyte is forced
out of the sleeve through the reference electrode junction (the white, porous circle at the end of the sleeve). This
not only purges bubbles from the electrolyte, but also cleans nasty stuff out of the junction.
Other manufacturers may tell you that their integral, or combination, reference electrode is better. This is not true
First of all, every year or so you have to buy a new combination electrode for about € 275 and install the whole thing
yourself. Second, combination electrodes usually employ “gelled” electrolyte, and are therefore inclined to
calibrating easily in standard pH buffers but measure poorly in low-conductivity waters (like < 200 umhos). With the
Eijkelkamp-style reference electrode you spend a few pennies and a few minutes every month or two refilling the
electrolyte. And its “free-flowing” junction performs well in low-conductivity waters.
4.11 ORP
ORP is measured as the voltage drop across the platinum
membrane of an ORP electrode.
The actual ORP sensor is the 1 mm silver-coloured dot you can see
when looking down at the pH sensor – if your Scuba has ORP. A
reference electrode is used to complete the voltage-measuring
circuit. Because platinum does not react with ions in the water,
it won’t give or take any electrons from those ions unless they
are very persuasive. The potential (voltage) created by this refusal is what you’re actually measuring as ORP.
ORP electrode maintenance is nothing more than occasionally cleaning the platinum surface with a soft cloth and
soapy water. If the platinum is discoloured, you can polish the ORP electrode with very light abrasive, like 900-grit
wet-and-dry sandpaper (please be careful not to polish the pH glass bulb). The important part of ORP maintenance is
refilling the reference electrode (4.9).
ORP uses a one-point calibration:
1
Rinse your sensors several times with the ORP standard you’ll use for calibration.
2
Fill the calibration cup with enough ORP standard to cover both the ORP and reference electrodes.
3
Follow the Scuba control software calibration instructions after selecting ORP_mV to calibrate.
4.12 Depth and vented depth (stage)
Depth is measured by a strain-gauge transducer as hydrostatic water pressure. The deeper you go in the water, the higher
the pressure.
Eijkelkamp’s depth sensors are usually inside the instrument, with a small pressure port that can be seen on the outside
of the Scuba bottom cap. They require no regular maintenance, but you might check occasionally to make sure the
pressure port is not clogged. If it is, use something soft, like a toothpick, to clear the port of obstruction.
