What is a Dissolved Oxygen Meter?

Antoine Lavoisier coined the term 'oxygen' in 1777. He discovered the word using the Greek words 'oxys', meaning 'acid' and 'genes', meaning 'producer', as he thought that oxygen was a part of all acids. While, today we know that this is not true, what we do know is that all living organisms need oxygen to survive.

Oxygen is the third most common and abundant chemical by mass in the universe, the first two being hydrogen and helium. It is produced in the Earth's atmosphere by plants during photosynthesis.

The atomic weight of oxygen is 8, and thus, its valency is 2. The most commonly found compound of oxygen is water. In addition to this, oxygen also forms an integral part of silica.

While some gases like nitrogen (N) and ammonia (NH₃) chemically react with water, other gases like oxygen (O) do not react with water at all. Dissolved oxygen can be defined as the physical distribution of oxygen in water.

The main sources of this element in water are atmosphere and the process of photosynthesis by plants. Waves and flowing water mix the air with water, while aquatic plants produce oxygen as a byproduct during photosynthesis.

The amount of oxygen that can be present mainly depends on factors such as temperature, salinity, and atmospheric pressure of water. Additionally, this amount can increase with a decrease in temperature of the water, and can increase with an increase in atmospheric pressure.

This meter is used to measure the amount of oxygen present in a unit volume of water.

Now, the next question that arises is, why do we need to measure the quantity of this element in water? This is because it indicates if the water is useful for a specific application like water treatment for plants, sewage treatment works, river monitoring, and fish farming.

The various types of meters available to measure the amount of oxygen are:

• Polarographic (Clark Cell Method) Sensor Oxygen Meter: This meter uses an external voltage by keeping the potential difference between the anode and cathode to less than 0.5 volts. The difference in the voltages of cathode and anode provides the amount of oxygen.

• Galvanic Sensor Oxygen Meter: This meter contains cathode and anode dipped in electrolyte which induces charges on them. The difference in the charges yields the amount of oxygen. This does not use an external voltage, and the difference between the anode and cathode is greater than 0.5 volts. These are more stable and accurate when compared to the Polarographic oxygen meters.

• Optical Fluorescence Oxygen Meter: This meter contains sensors that glow in contact with oxygen. The time period for which they glow indicates the amount of oxygen. These meters are extremely suitable for long-term measurements in groundwater, as it is not sensitive to contaminants or for that matter, aging.

While purchasing an oxygen meter, range and accuracy offered are the key specifications to look out for. Portable hand-held equipments are very handy to be carried to different location points, whereas permanent setups are more useful in laboratories.

The typical features that should be available with the meter that you are purchasing are:

• Self-calibration
• Event Triggers
• Battery Packs
• Filters

The technical data that a portable oxygen meter should be able to capture is:

• Oxygen range: 0.01 mg/l - 2000 mg/l
• Resolution: 0.01 mg/l
• Temperature range: 0 - 50ºC
• Automatic air pressure compensation
• Automatic temperature compensation
• Correction for salinity

The appliance works based on the principle of an oxygen sensor attached to an acoustic transmitter. When the equipment is turned on, it determines the level of dissolved oxygen in water and transmits it to a hydrophone connected to the receiver.

Multiple transmitters that receive signals at different frequencies can be used to receive multiple data. The distance over which the signal can travel depends on various factors including the occurrences of waves, bubbles, and ship traffic.

The meter uses the galvanic type of oxygen sensor to measure the level of oxygen in the water. The dissolved oxygen transmitters are attached to aquatic animals. When externally attached to them, these transmitters provide information, as to how the variation in the oxygen level measurements impacts the behavior of the individuals temporally and spatially.

Some of the industrial uses of a dissolved oxygen meter are given below; however, this is a concise list that is expected to grow with time.

• Analysis of boiler feedwater for industries
• Wastewater treatment plants
• Pollution control in rivers and lakes
• Measurement of ionic concentration for pharmaceutical companies
• Analysis of drinking water

There was a time when philosophers were poisoned because they thought freely and unconventionally, and today scientific expansions know no limits. In a nutshell, advancements never stop and as long as we exist, we will keep on moving towards a newer and better tomorrow. All we need to do is contribute our bit towards this development in any way we can.