Dissolved oxygen is one of the most important electrochemical measurements of aqueous solutions.
Oxygen in water is produced by a number of means. Either as a required quantity due to the use of aerators, as a byproduct of photosynthesis from algae and other photosynthetic organisms, and from aeration as water travelling course picks up oxygen (this can include movement through piped installations as well as water travelling through a waterfall).
The amount of oxygen dissolved into water is called the dissolved oxygen.
Dissolved oxygen measurements take two forms - either the percentage saturation of water, or the concentration of dissolved oxygen, normally as a ratio of milligrams per litre (mg/L) which translates as parts per million. (PPM).
For many process control applications - dissolved oxygen is of paramount importance.
Dissolved oxygen in boiler feed water or boiler make up water - will enable a boiler to become corroded. The resulting corrosion significantly weakens the boiler, reduces the efficiency of the boiler and will eventually result in the boiler having to be replaced.
Alternatively - with aquaculture and marine farming applications - dissolved oxygen levels are paramount to ensuring that livestock is able to survive, with significant variation in the dissolved oxygen levels required depending on the origin of the marine life. For instance - crabs, lobsters and oysters require minimal levels of dissolved oxygen - somewhere in the region of 2 mg/L. Tropical fish like clown fish, and fresh water fish such as salmon require much higher levels of dissolved oxygen - in excess of 6 mg/L in most circumstances.
In addition to process control - dissolved oxygen is an online measurement that can be used in conjunction with a requested biological oxygen demand measurement (BOD). When effluent is discharged from a process into water courses - if the BOD is too high it can have significant environmental impact. The environmental monitoring of a water course in conjunction with the effluent monitoring allows us to ensure that a water treatment or effluent treatment process is working properly and not impacting upon the environment.
Factors Impacting Dissolved Oxygen Measurements
Dissolved oxygen is impacted by a number of different factors. As the oxygen is dissolved into water - a change in atmospheric pressure has significant affects on the amount of dissolved oxygen a body of water can hold. Mechanical vacuum removal systems are also very effective at removing dissolved oxygen from boiler feed water.
The Chart below shows dissolved oxygen concentration levels at different altitudes.
An increase in temperature also has significant affects on the amount of dissolved oxygen that a body of water can hold. With an increase in temperature the concentration of dissolved oxygen that a body of water can hold decreases significantly.
Dissolved Oxygen - Salinity and Relative Humidity.
In addition to the temperature and atmospheric pressure, the salinity of the solution has a bearing on the amount of dissolved oxygen that can be held. The dissolved oxygen concentration is inversely proportional to the salinity of the measured solution. It's worth noting that the salinity of the solution only affects the dissolved oxygen concentration and not the percentage saturation of dissolved oxygen.
The relative humidity of the atmosphere within which the measured solution is located also has a bearing on the concentration of dissolved oxygen. The humidity effectively adds an additional layer to the atmospheric pressure as the weight of moist air presses down on the body of the solution. The moist air (pH20) is also effected by the temperature - with cold moist air increasing the effective pressure on the solution and warmer moist air decreasing the effective pressure. This is commonly referred to as partial pressure and adjustments to relative humidity can be made. The total affect of the relative humidity on the dissolved oxygen varies by a small percentage .
Our range of dissolved oxygen instruments are designed to offer measurements appropriate for process control applications in environmental, industrial and scientific applications.
We no longer supply the older galvanic dissolved oxygen sensors - our range includes both polarographic and optical (fluorescent) dissolved oxygen sensors. With the polarographic offering accurate and cost effective measurements which require periodic maintenance and calibration and are excellent in applications with a constant flow rate.
Our Fluorescent Dissolved Oxygen sensors offer precision dissolved oxygen measurements. This type of sensor does not consume dissolved oxygen and can be used where no flow is present. In addition - as this sensor uses a microprocessor to provide a 4-20mA signal - these can be fully configured to include automatic compensation for Relative Humidity, Atmospheric Pressure and Salinity increasing the precision of the dissolved oxygen measurement.
Dissolved Oxygen Instruments & Dissolved Oxygen Sensors
A Dissolved oxygen meter is an portable instrument designed to be used to take field readings, laboratory readings, or corroborate the readings of a permanently installed instrument.
Our dissolved oxygen meter is simple to use and simple to calibrate with the option to give readings in either percentage saturation of dissolved oxygen or dissolved oxygen concentration.
The dissolved oxygen sensor includes integrated temperature devices to give automatic temperature compensation readings. In addition a seperate temperature measuring device is included to provide temperature measurements.
Our dissolved oxygen transmitter range uses optical fluorescent dissolved oxygen sensors with integrated electronics to provide an industry standard 4-20mA current output.
The dissolved oxygen transmitters are available as either flow line / pipe mounting sensors or submersible dip mounting dissovled oxygen sensors for use in both environmental and industrial applications.
The dissovled oxygen transmitters are sophisticated devices offering accurate readings even at very low dissolved oxygen concentrations and require very little preventative maintenance or calibration - ideal for environmental installations.
The sophisticated nature of the dissolved oxygen sensors mean that they include automatic temperature compensation as well as compensation settings for atmospheric pressure, salinity and relative humiditiy.
Our Dissolved Oxygen controller range are designed to work with a range of either optical or polarographic dissolved oxygen sensors to provide accurate dissolved oxygen meaurements.
Our dissolved oxygen controller range have control relays for controlling related processes such as aerators or control valves as well as alarm relays and auto-clean relays for fully automating a dissolved oxygen process.
For external monitoring, the dissolved oxygen controllers include industry standard 4-20mA outputs which can be easily integrated into a digital chart recorder, PLC or other similar device.
All of our dissolved oxygen controllers support automatic temperature compensation as well as outputting the percentage saturation of dissolved oxygen and the concentration of dissolved oxygen (mg/L)
Our dissolved oxygen sensor range includes the Clark type sensor, also known as a polarogrpahic membrane type dissolved oxygen sensor as well as our new range of optical fluorescent dissolved oxygen sensors.
Both methodologies of sensor include automatic temperature compensation and can integrate easily into our range of dissolved oxygen controllers, or alternatively will give a 4-20mA current output signal for use with a PLC or chart recorder.
The dissolved oxygen sensors are available for sumbersion or dip mounting, as well as for in-line or in-flow mounting - making them ideal for both environmental monitoring for industrial process monitoring and control.