| In applications where ozone is generated and used | | | | by weight (%wt/wt). This can be a very useful unit |
| for air or water treatment application, it is important | | | | of measurement. If the mass flow of feed gas for |
| to be able to know and communicate the | | | | an ozone generator is known, say by using a thermal |
| concentration of ozone. There are techniques for | | | | mass flow meter, than knowing the %wt/wt will |
| measuring the ozone present in air or water, for | | | | allow one to know the mass of ozone by simple |
| example, the use of UV radiation at 254 nm, but | | | | multiplication. In order to derive the %wt/wt from |
| there still need to be units of measure to translate | | | | the g/Nm3 value, one must know the density of |
| these results into meaningful numbers for technicians | | | | ozone and the density of the feed gas. To calculate |
| to use. | | | | the %wt/wt one divides the quantity (100 X the |
| A unit of measurement in the gas phase grams of | | | | measured g/Nm3) by the following value:g/Nm3 - (the |
| ozone per normal cubic meter usually written as g | | | | ratio of feed gas density to ozone density) X g/Nm3 |
| Nm3. the term normal in this expression is critical | | | | + feed gas density |
| since a gases density is influenced by pressure. | | | | So, if the measured concentration of ozone were |
| Normal in this case refers to the the conditions at | | | | 100 g/Nm3 and the feed gas was oxygen (1,428.96 g |
| which the measurement was taken, or the condition | | | | Nm3) then knowing that ozone has a density of |
| to which it has been adjusted. Normal here refers to | | | | 2,143.93 will result in a % wt/wt value of 6.84. If air |
| standard pressure and temperature as agreed to | | | | is the feed gas the result would be different, i.e. |
| internationally: P (Standard) = 1 atm or 760 mmHg or | | | | about 10% lower than the value 6.84 shown above. |
| 1.01325 bar and T (standard) = 0 degrees C or 32 | | | | In water treatment applications we normally use the |
| degrees F or 273.15 degrees K. Since actual | | | | terms ppm, which is really part per million by weight |
| measurements will be done at conditions most likely | | | | (ppmw). We could also use the terms g/m3 since 1 g |
| different than the international standards, the | | | | m3 = 1 ppmw. The density of water changes very |
| measurement devices are fitted with the ability to | | | | little with changing temperature. We normally are |
| compensate for these differences and present the | | | | referring to dissolved ozone when we talk about the |
| results in g/Nm3. Thus, the measured g/m3 value is | | | | concentration of ozone in water. Since ozone is |
| multiplied by (P (Standard)/T (Standard)) X (T | | | | produced as a gas, care must be taken to insure that |
| (measured)/ P(Measured)). | | | | the gas bubbles have been separated from the |
| Another unit of measure that is often used is ppmv, | | | | water prior to making a measurement. Ozone can be |
| which is part per million on a volume by volume basis. | | | | measured using the UV method mentioned above or |
| The measurement ppmv is proportional to g/Nm3: | | | | by an electrochemical method. Both methods can |
| 466.43 ppmv = 1 g/Nm3. | | | | result in accurate reading in ppmw when carried out |
| Another important unit of measure is percent weight | | | | properly. |