Dissolved Oxygen Sensor: How Does a DO Probe Measure Oxygen in Water?

After a hot, still summer night, the fish in a pond hang at the surface gulping air, even though the water looks clear. That is the trouble with dissolved oxygen: you cannot see it, and it can swing widely within a few hours. A dissolved oxygen sensor makes that invisible number measurable. In short, an optical probe shines light on a dye whose glow is dimmed by oxygen, while an electrochemical probe measures a tiny current produced when oxygen is reduced at an electrode. Both convert their signal into milligrams per liter or percent saturation.

To turn that reading into a decision, you also need to know which values are normal, why warm water holds less oxygen, where the probe should hang, how to calibrate it and at what point fish are in danger.

What is dissolved oxygen and why measure it?

Dissolved oxygen, usually shortened to DO, is molecular oxygen held between water molecules. It enters the water from the air at the surface, helped along by waves, waterfalls and aerators, and during the day it is produced by algae and aquatic plants through photosynthesis. It is consumed by the respiration of fish, plants and microorganisms and by the breakdown of organic matter on the bottom.

People measure it wherever life or a process depends on it. In fish farming, aquaculture oxygen monitoring determines how densely you can stock and how much you can feed. In wastewater treatment plants, the DO reading in the aeration basin drives the blowers; typical setpoints sit around 1.5 to 2 mg/L. In lakes and streams, a falling oxygen level is an early sign that runoff, fertilizer or an algae bloom is tipping the balance. And in a backyard pond, a DO sensor is often the only way to catch an overnight oxygen crash before it costs fish.

mg/L vs % saturation: how is dissolved oxygen reported?

Two units are common, and they answer different questions. Milligrams per liter (mg/L) is the absolute concentration: how much oxygen is actually in one liter of water. Percent saturation (% sat) describes how much oxygen the water holds relative to what it would hold in equilibrium with air at its current temperature, salinity and air pressure.

A quick example shows why you should never mix the two. At 10 °C, 8 mg/L is roughly 71% saturation, because fresh water at that temperature can dissolve about 11.3 mg/L. At 25 °C, the same 8 mg/L is almost 97% saturation, because the saturation concentration has dropped to about 8.3 mg/L. The percentage tells you whether the water is currently gaining or losing oxygen to the air; the mg/L value tells you whether there is enough for your animals.

Readings above 100% are not a measurement error. On sunny days, algae can supersaturate the water, and values of 120 to 150% are common in nutrient-rich ponds. Those ponds often crash hardest at night. The Dragino DR-DO1 reports mg/L together with water temperature. If you also want to see percent saturation, you can calculate it from those two values in your dashboard, as long as every chart is clearly labeled with its unit.

Optical vs galvanic dissolved oxygen sensor: which is better?

An electrochemical sensor, also called a Clark-type electrode, has a cathode and an anode sitting in an electrolyte behind a gas-permeable membrane. Oxygen diffuses through the membrane and is reduced at the cathode, and the resulting current is proportional to the oxygen partial pressure. Galvanic sensors generate their own voltage and measure right away; polarographic sensors need a polarizing voltage and often several minutes of warm-up. Both consume oxygen while measuring. If the water in front of the membrane is still, that boundary layer gets depleted and the reading drops too low. Electrochemical probes therefore need a minimum flow across the membrane, plus regular membrane and electrolyte replacement.

An optical sensor, often sold as luminescent DO or LDO, works without any electrolyte. For the Dragino DR-DO1, the manufacturer describes the principle like this: a blue LED illuminates fluorescent material on the inside of the sensor cap, which then emits red light. Oxygen molecules partially quench that fluorescence. The electronics measure the phase difference between the blue and the red light, compare it with an internal calibration and calculate the concentration from it. According to Dragino, the result is automatically compensated for temperature and air pressure. The DR-DO1 is therefore clearly an optical sensor, not a galvanic one.

The table below puts the two methods side by side. The main difference is upkeep, not accuracy.

For a probe that will sit in the water unattended for weeks, the optical method is usually the better choice. For occasional spot checks with a handheld DO meter, an electrochemical unit is often good enough. Still, an optical sensor is not maintenance-free: the cap ages, and biofilm on its surface skews the reading.

What is a normal dissolved oxygen level in water?

There is no single normal value, because the upper limit depends on temperature. Clean, well-mixed surface water usually sits close to saturation, roughly 8 to 12 mg/L, higher in winter and lower in summer. Streams with good water quality generally stay above 7 mg/L. Still water behaves differently: while the upper layers can be saturated, the bottom of deep ponds and lakes often falls to a few milligrams or even zero in summer, because that water barely mixes with the surface.

There is also a daily cycle. Oxygen rises with the sun, peaks in the afternoon and falls through the night, reaching its lowest point just before sunrise. In a heavily planted or densely stocked pond, that curve can swing from above 12 mg/L in the afternoon to below 4 mg/L at dawn.

As a rough guide: values near saturation are good, sustained levels below 5 mg/L put stress on many fish species, and below 2 mg/L the water is considered hypoxic.

How do temperature and salinity affect dissolved oxygen measurement?

Gases dissolve better in cold water. As temperature rises, water molecules move faster and dissolved oxygen escapes more easily. At the same time, fish and bacteria speed up their metabolism in warm water, so they need more oxygen just as the water can hold less of it.

The table below shows the saturation concentration of fresh water at sea level for several temperatures.

Between 0 and 30 °C, solubility almost halves. Salt lowers it further: seawater with 35 g of salt per kilogram holds only about 7.4 mg/L at 20 °C, roughly a fifth less than fresh water. Air pressure matters too; at 1,000 meters of elevation, saturation is about eleven percent lower than at sea level.

For a DO probe, this means two things. First, the sensor has to know the water temperature to convert its raw signal correctly, which is why the DR-DO1 measures temperature itself and compensates for temperature and air pressure automatically. Second, a probe built for ordinary water is not automatically suitable for saltwater. Dragino specifies the DR-DO1 for ordinary water and offers a separate variant for seawater. For brackish water or marine tanks, look for a probe with salinity compensation and seawater-resistant materials.

Where should you place a dissolved oxygen probe?

Placement often matters more than the accuracy of the sensor itself. If you care about the fish, measure where they actually spend time, at a depth that represents the pond as a whole, typically in the middle third of the water column. If you care about the bottom, mount the probe just above it without letting it sink into the sediment.

A common mistake is to place the probe right in the bubble stream of an aerator or in front of a waterfall. There you will see locally high values while the rest of the pond may already be running short. Dragino also notes that bubbles at the measuring point cause inaccurate readings on the DR-DO1. The best spot has normal mixing and some distance from inlets, outlets and aeration.

Fix the probe firmly, for example to a pipe or a dock; the thread on the DR-DO1 body is there for exactly that. A swinging probe turns your trend line into random values. Per the manufacturer, it goes straight into the water without a protective tube, is rated IP68 and handles up to 6 bar of operating pressure. The 5-meter cable does limit how far the transmitter can sit from the measuring point.

How do you calibrate a dissolved oxygen probe?

Optical probes hold their calibration much longer than electrochemical ones, but they still need checking. Dragino’s documentation describes a zero-point calibration for the DR-DO1: the probe is placed in oxygen-free water, and once the reading is stable, the zero point is set with a Modbus command. You can also store the current air pressure as a value so that pressure compensation is correct, for example 101.35 kPa at sea level. Dragino’s documentation mixes command versions, so check the exact registers against the instructions supplied with your probe.

A simple plausibility check needs no chemicals. Put the probe in well-aerated water that has stood open for a while and is at the same temperature as the air. The reading should then be close to the saturation value from the table above. If it is clearly off, either the calibration has drifted or the cap is dirty.

Fouling is the most common cause of bad readings in practice. Algae, biofilm and scale on the sensor cap slow down oxygen exchange, and the probe reads low. Dragino recommends cleaning the probe frequently in complex water conditions and rinsing it with distilled water after measuring. Wipe the cap only with a soft cloth or sponge; brushes and solvents can damage the dye layer. For the DR-DO1, the manufacturer states a lifespan of about one year under normal use. Budget for the replacement from the start.

What dissolved oxygen level is too low for fish?

The threshold depends heavily on the species. Trout and other salmonids from cool streams need a lot of oxygen; they do well above 7 mg/L, start to show stress below about 6 mg/L, and are in a critical range around 5 mg/L. Carp, koi and tench are much tougher and can survive short periods around 3 mg/L, though they barely grow and become more prone to disease. Below 2 mg/L, almost all fish are in danger, and below 1 mg/L losses can follow quickly.

Critical situations typically develop on warm, windless nights, after several overcast days when algae produce little, after a sudden algae die-off, during muggy thunderstorm weather or when an aeration pump fails. In winter, solid ice and snow cover can have the same effect.

That is why aquaculture oxygen monitoring is most useful when it warns you before the fish do. Set a warning threshold with some margin above the critical limit, for example 5 mg/L for a koi pond, and a second, urgent threshold below it. Treat missing or stale readings as a problem in their own right: a probe that has not reported for hours is no reason to relax.

How do you monitor dissolved oxygen remotely with LoRaWAN?

The DR-DO1 itself is a wired probe with no radio. It outputs its readings either as a 4–20 mA signal or over RS485 using the Modbus RTU protocol, and it needs a 5 to 24 V supply at no more than 0.5 W. For remote monitoring, it connects to a wireless transmitter. Dragino offers the WQS-LB for this, a LoRaWAN water quality transmitter that, according to the manufacturer, accepts one to three probes, covering pH, conductivity or ORP alongside dissolved oxygen.

The WQS-LB runs on an 8,500 mAh lithium thionyl chloride battery that can last several years depending on the reporting interval, and it powers the probe only for the duration of each measurement. The default uplink interval is 20 minutes, and you can configure it over Bluetooth or by downlink through the LoRaWAN network. Data travels through a gateway to a network server such as The Things Stack or ChirpStack, and from there to your platform, where you set up trends, thresholds and alerts.

When planning, think about the whole timing chain: the probe needs a moment after power-up for its reading to stabilize, then comes the reporting interval, then processing in the platform. For a pond where oxygen drops over several hours, an interval of 15 to 30 minutes works well. If the reading is supposed to switch aeration automatically, that control loop belongs on a local controller that behaves sensibly even if the radio link drops. Remote monitoring is then a second layer of protection, not the only one.

Where can you buy a dissolved oxygen sensor?

Before you decide, run through the key points once more. For continuous measurement, an optical sensor is usually the better choice, because it needs no water flow, consumes no oxygen and requires no electrolyte. Check that the probe is designed for your type of water, since freshwater and seawater versions are not interchangeable. Look at the temperature range, built-in temperature and air pressure compensation, and the output unit, so you can keep mg/L and percent saturation cleanly apart later. Confirm which interface your transmitter or controller expects, such as RS485 with Modbus or 4–20 mA, and whether the cable length suits your measuring point. Also factor in the effort for cleaning, calibration and replacing the sensor cap. If you want a dissolved oxygen sensor you can monitor remotely, you will find suitable probes and transmitters in our water quality sensors category.