Dissolved oxygen meter

The DO8500 is the meter I’d hand to anyone who has to keep records, and I’ve used it. It pairs Apera’s luminescent optical probe with a data logger and an automatic salinity probe.

Apera lists a range of 0 to 20.00 mg/L and 0 to 200% saturation. Accuracy is ±2% of reading or ±0.2 mg/L, whichever is greater, so ±0.2 mg/L at an 8 mg/L reading.

Optical means no membrane and no electrolyte. Apera says the probe doesn’t consume oxygen, so there’s no limit on water flow speed, and readings settle in 2 to 3 minutes with no polarization wait.

It corrects for temperature automatically from 0 to 50°C and for air pressure from 60 to 120 kPa. The DO8500 adds automatic salinity correction through its second probe, where the DO850 needs it entered by hand.

It stores 500 groups of data with time and date and sends them out over USB. It runs on three AA batteries, rated over 200 hours, and has a foldable stand for bench use.

Apera says you don’t need to calibrate before every use. Calibration is two points, zero oxygen and saturated air, and the kit’s calibration cap lets the probe be calibrated and stored in saturated air.

Warranty, per Apera: 3 years on the meter, 2 years on the DO803 optical probe, and 1 year on the DO8032 sensor cap.

In the box: the meter, the 10-foot optical probe, the salinity probe, a calibration cap and a rugged carrying case. Apera gives the probe cap’s diameter as about 1.3 inches (3.3 cm), so check that it fits your sample bottle or sampling port.

My call: download the log after each sampling trip and clear it. Five hundred readings is a season of weekly checks on one pond, not on ten.

The DO850 is the plain version of the DO8500, and I’ve used it. It has the same DO803 optical probe and the same accuracy, without the logger, clock or salinity probe.

Apera’s spec sheet lists 0 to 20.00 mg/L and 0 to 200.0%, at 0.01 mg/L and 0.1% resolution. Accuracy is ±2% of reading or ±0.2 mg/L, whichever is greater, and response is 30 seconds or less to 90% at 25°C.

Temperature correction is automatic from 0 to 50°C and pressure correction is automatic from 60 to 120 kPa. Salinity is entered by hand from 0 to 45 ppt, which is the one check it didn’t max out.

The kit includes the meter, the 10-foot optical probe, a calibration and storage cap and a hard case. Apera also sells it with a 33-foot cable for deeper water.

The meter weighs 313 g, runs on three AA batteries and is rated IP57. It has stable-reading indication and an auto-lock mode that holds the number once it settles.

Apera rates the luminescent cap for over 8,000 hours and says its coating isn’t affected by light or chemicals. The cap is a replaceable part, DO8032.

Warranty is the same as the DO8500: 3 years on the meter, 2 on the probe and 1 on the cap. The Amazon listing says IP67, but Apera’s own spec says IP57, so treat the meter as splash and dust proof, not something to leave underwater.

Apera says the DO850 needs no warm-up time and isn’t flow dependent, which suits still water like a trough, tank or pond edge.

My call: if you test brackish or salt water, look up the salinity first or buy the DO8500. A hand-entered salinity that’s wrong shifts every mg/L reading.

Sper Scientific’s 860067 is an optical meter I’d use where the water smells, and I’ve used it. Sper lists aquaculture, environmental monitoring, wastewater treatment and brewing as its uses.

Sper’s spec table gives 0 to 20.0 mg/L and 0 to 200% saturation. Accuracy is ±0.3 mg/L below 5 mg/L and ±0.4 mg/L from 5 to 20 mg/L, at 0.1 mg/L resolution.

Sper says it stays accurate in the low oxygen range and isn’t affected by acid gases, needs no polarization time, and has no minimum inflow, so it reads in still water.

Temperature correction is automatic from 0 to 40°C. Salinity, 0 to 55 ppt, and barometric pressure, 50 to 115 kPa, are entered by hand. Calibration is two points, 0% and 100%.

The probe is factory calibrated. Sper’s manual says not to calibrate unless it reads wrong or after chemical cleaning, and to soak a dried-out tip in distilled water for 12 hours.

Where it lost points: the looser accuracy above 5 mg/L, manual pressure and salinity entry, no warranty term stated for this model on its page or in its manual, and a sensing tip Sper says isn’t replaceable. Sper rates sensor life at over a year with good care.

Sper’s manual lists an IP67 meter, a 3-meter cable and six AAA batteries, rated for about 10 hours of operation.

Sper’s manual lists what the optical method buys you: lower running costs with no electrolyte changes, longer calibration intervals from low drift, no polarization voltage, no oxygen consumption and no minimum inflow. The meter measures 169 by 78.3 by 43.4 mm and weighs about 400 g.

My call: keep spare AAA batteries in the case. Ten hours goes quickly on a long sampling day.

Extech’s DO210 is the compact membrane meter of the four, and I’ve used it. Extech says it measures dissolved oxygen in water or percent oxygen in air, plus temperature.

Extech’s page lists 0 to 20.0 mg/L at ±0.4 mg/L, 0 to 100.0% saturation at ±0.7%, and 32 to 122°F temperature at ±1.5°F. That ±0.7% is the tightest percent-saturation figure any of the four makers lists.

The probe is polarographic, with the temperature sensor built in for automatic correction. The meter has adjustable altitude and salinity correction, plus min, max and data hold.

The kit comes with the probe, two spare membranes, electrolyte, a 9-volt battery and a soft case. The meter measures 5.3 by 2.4 by 1.3 inches and weighs 15.7 oz with the probe and battery.

Extech lists a 2-year warranty. One quirk: Extech’s own page gives the probe cable as 39 inches (1 m) in one line and 13 feet (4 m) in another, and the listing says 13 feet, so check before you count on the reach.

Where it lost points: a membrane probe scores 6 against optical, ±0.4 mg/L scores 7, and changing membranes and electrolyte scores 6 on upkeep.

Membrane probes work differently from optical ones. Milwaukee’s manual for its own membrane meter says the probe must be polarized first and needs at least 0.3 m per second of water movement for an accurate reading.

Extech lists the probe at 1.1 inches across and 7.5 inches long, small enough for a sample cup. The meter runs on one 9-volt battery and carries a CE mark.

My call: stir the probe gently while you read. If the number climbs when you move it faster, the water around the membrane wasn’t moving enough.