Eureka Manta Manual
Page 34 of 67
D.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-colored dot you can see when looking down at the pH sensor – if your
Manta 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. (D.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 Manta Control Software calibration instructions after selecting ORP_mV to calibrate.
D.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.
Eureka’s depth sensors are usually inside the instrument, with a small pressure port that can be seen on
the outside of the Manta 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.
Depth calibration is nothing more than “zeroing” the sensor in air, where one assumes the depth to be
zero:
1
Make sure the Manta is not in the water.
2
Follow the Manta Control Software’s calibration instructions.
Notice that the Depth sensor cannot distinguish between water pressure and the air pressure over that
water (i.e. barometric pressure). After you have zeroed the sensor, any change in barometric pressure will
be measured as a change in water pressure. Fortunately, water on Earth is considerably heavier than air,
so the error introduced by barometric pressure changes is small.