Ozone Measurement Units

In applications where ozone is generated and used for air or water treatment application, it is important to be able to know and communicate the concentration of ozone. There are techniques for measuring the ozone present in air or water, for example, the use of UV radiation at 254 nm, but there still need to be units of measure to translate these results into meaningful numbers for technicians to use.

A unit of measurement in the gas phase grams of ozone per normal cubic meter usually written as g/Nm3. the term normal in this expression is critical since a gases density is influenced by pressure. Normal in this case refers to the the conditions at which the measurement was taken, or the condition to which it has been adjusted. Normal here refers to standard pressure and temperature as agreed to internationally: P (Standard) = 1 atm or 760 mmHg or 1.01325 bar and T (standard) = 0 degrees C or 32 degrees F or 273.15 degrees K. Since actual measurements will be done at conditions most likely different than the international standards, the measurement devices are fitted with the ability to compensate for these differences and present the results in g/Nm3. Thus, the measured g/m3 value is multiplied by (P (Standard)/T (Standard)) X (T (measured)/ P(Measured)).

Another unit of measure that is often used is ppmv, which is part per million on a volume by volume basis. The measurement ppmv is proportional to g/Nm3: 466.43 ppmv = 1 g/Nm3.

Another important unit of measure is percent weight by weight (%wt/wt). This can be a very useful unit of measurement. If the mass flow of feed gas for an ozone generator is known, say by using a thermal mass flow meter, than knowing the %wt/wt will allow one to know the mass of ozone by simple multiplication. In order to derive the %wt/wt from the g/Nm3 value, one must know the density of ozone and the density of the feed gas. To calculate the %wt/wt one divides the quantity (100 X the measured g/Nm3) by the following value:

g/Nm3 - (the ratio of feed gas density to ozone density) X g/Nm3 + feed gas density

So, if the measured concentration of ozone were 100 g/Nm3 and the feed gas was oxygen (1,428.96 g/Nm3) then knowing that ozone has a density of 2,143.93 will result in a % wt/wt value of 6.84. If air is the feed gas the result would be different, i.e. about 10% lower than the value 6.84 shown above.

In water treatment applications we normally use the terms ppm, which is really part per million by weight (ppmw). We could also use the terms g/m3 since 1 g/m3 = 1 ppmw. The density of water changes very little with changing temperature. We normally are referring to dissolved ozone when we talk about the concentration of ozone in water. Since ozone is produced as a gas, care must be taken to insure that the gas bubbles have been separated from the water prior to making a measurement. Ozone can be measured using the UV method mentioned above or by an electrochemical method. Both methods can result in accurate reading in ppmw when carried out properly.