Oxygen measurements now possible for everyone!

One Cue Systems offers you a low cost oxygen meter, that allows you to measure oxygen levels without toxic chemicals. This meter consists of a robust oxygen electrode, designed and built by One Cue Systems, and a general purpose multimeter.
As for accuracy, this meter is not inferior to expensive laboratory instruments, because the accuracy of any oxygen meter depends on the care with which the meter is calibrated. So, read the manual (making measurements) thoroughly!

Oxygen is indispensable for plants and animals

They need it for their respiration. In water only a small amount of oxygen can be dissolved: about 10 milligrams per liter. Fish pump water along their gills the whole day to get sufficient oxygen. You can well work out that they get anxious as the oxygen content goes down. This happens especially at night as all plants, animals and bacteria only breathe. By day this is not the case, because in daylight the water plants produce oxygen.

There are numerous situations in which oxygen plays an important role:

Is the oxygen content in my aquarium or pond not too low?
What is the influence of plants and animals on the oxygen content in water?

What is the influence of cyanide-poison on the respiration?
What happens to the oxygen content during algal blooms by day and by night?
How large is the self-purification capacity surface water?
What is the influence of the quantity of light on photosynthesis speed?
What is the influence of temperature on respiration and photosynthesis speed?
How large is the Biochemical Oxygen Demand (BOD) of waste water?

Maybe you can come up with other ideas, but: how do you measure the oxygen in all these situations? The most oxygen meters are specially developed, scientific instruments, and thus expensive. One Cue Systems on the other hand brings a low cost oxygen meter on the market, that is made of simple components and that allows you to make you own oxygen measurements.

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How does oxygen arise?

Oxygen is a "waste product" that arises when (green) plants, with the help of sunlight, transform simple chemical compounds into organic matter (photosynthesis). In this way water plants add oxygen to the water. Although plants can produce oxygen, they also need oxygen for respiration.

Oxygen consumption

Next to the uptake of sunlight plants, like most other living organisms, also provide in their energy need through the combustion of organic material. For this process oxygen is necessary. This combustion is not dependent on the presence of sunlight and continues thus 24 hours per day. While the oxygen production stops at night by lack of sunlight, the oxygen consumption by water plants and other organisms proceeds as normal. Consequently, the oxygen content in surface water becomes lower by night than by day. Especially at the end of the night, around sunrise, oxygen contents arise that are considerably lower than at the end of the day.
This phenomenon especially takes place in algae-rich waters. In such waters, the bottom is covered mostly by a thick layer of dead organic material (died off algae). This is a sizable food source for bacteria and moulds (decomposers), that live here in large numbers. The demolition process that takes place here on a large scale, demands considerable quantities of oxygen. By day, this furnishes little problems, through the large oxygen production of the yet living algae. At night, through the absence of oxygen production, the oxygen content in algae-rich water drops however steeply (sometimes even till zero), with possible fish mortality as a result. This phenomenon especially occurs at the end of the summer. Then many algae are present (large oxygen consumption), while less oxygen is produced by shortening of the light period.

Click on the image for processes involved in oxygen production and consumption (Dutch)


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Oxygen levels in water

The oxygen levels in water are not solely dependant on oxygen production and - consumption by plants and animals. It is, as it happens, also possible that oxygen enters the water from the air above the water. Air contains a constant percentage of approximately 21% oxygen. Where air comes into contact with water, oxygen will dissolve in water. The quantity oxygen that dissolves depends on a number of factors:

Time. Required to mix the oxygen into the water until balance with the open air is reached. The speed with which oxygen dissolves in water is high when the water surface is in motion, for example in quick moving streams or by strong winds (waves).
Air pressure. At low air pressure less oxygen dissolves than at high pressure. This hardly plays a role.
Salinity. The saltier the water, the less oxygen dissolves. The solubility of oxygen in marine water is therefore approximately 20% lower than in fresh water.
Temperature. The higher the water temperature, the less oxygen can dissolve, whereas fish consume more oxygen at higher temperatures. The maximum quantity of air oxygen that you can dissolve in one liter of water (saturation value) depends on the temperature.

See table below with the temperature in C and the saturation value in mg/L.

temp

O2 temp O2 temp O2 temp O2 temp O2
0 14.60                
1 14.19 11 11.01 16 8.90 31 7.41 41 6.31
2 13.81 12 10.76 22 8.72 32 7.16 42 6.22
3 13.44 13 10.52 23 8.56 33 7.16 43 6.13
4 13.09 14 10.29 24 8.40 34 7.05 44 6.04
5 12.75 15 10.07 25 8.24 35 6.93 45 5.95
6 12.43 16  9.85 26 8.09 36 6.82 46 5.86
7 12.12 17  9.65 27 7.95 37 6.71 47 5.78
8 11.83 18  9.45 16 7.81 38 6.61 48 5.70
9 11.55 19  9.26 29 7.67 39 6.51 49 5.62
10 11.27 20  9.07 30 7.54 40 6.41 50 5.54


Absolute and relative oxygen concentrations

The oxygen content of water is expressed in milligrams per liter (mg/L).
The (absolute) oxygen content in milligrams per liter is a good measure to determine whether fish have enough oxygen to their disposal. The absolute oxygen concentration is however less useful to compare the oxygen household of waters with different water temperatures. A different water temperature means also another saturation value (maximum quantity of oxygen to dissolve).

In order to compare the oxygen levels of waters with different temperatures the relative oxygen content or oxygen saturation value is used. With this unit, the measured oxygen content is given in per cents of the saturation value at the concerning water temperature. When, for instance, one measures an oxygen content of 5.5 mg/L at a temperature of 11 C, this is precisely half the amount of oxygen that this water could contain (saturation value at 11 C is 11.0 mg/L). The water is then saturated only for 50% with oxygen. A saturation value of 50% is called undersaturation (under 100%).
Sometimes, however, oversaturation (above 100%) occurs in situations where more oxygen is dissolved in the water when compared to air-saturated water. This is usually caused by a very high oxygen production of green plants and algae. So much oxygen is produced that a part of this escapes to the air in the form of tiny oxygen bubbles. These bubbles also strip other dissolved gasses (nitrogen, carbon dioxide) from the water. The extra space that arises because of this, is taken in by oxygen and so the oversaturation arises.

In water with a stable oxygen household, the oxygen saturation percentage remains in general between 80 and 120%. An oxygen saturation far under 80% can indicate strong biological decomposition, values above 120% may indicate algal bloom.

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