Understanding dew point in vaporized hydrogen peroxide applications

VH₂O₂ affects dew point

In bio-decontamination applications that use vaporized hydrogen peroxide (VH₂O₂) the condensation point can be a useful parameter. However, when H₂O₂ vapor is present in the air, deriving the dew point from only the water vapor is insufficient because the H₂O₂ vapor will change the dew point.

In bio-decontamination applications that use vaporized hydrogen peroxide (VH₂O₂) the condensation point can be a useful parameter. However, when H₂O₂ vapor is present in the air, deriving the dew point from only the water vapor is insufficient because the H₂O₂ vapor will change the dew point.

Dew point can never be higher than ambient temperature. If the dew point equals ambient temperature, condensation occurs. This is the point where relative saturation equals 100 %.

In Figure 1, we see that dew point temperatures over 25 °C in the grey area cannot be achieved if the ambient temperature is 25 °C. Where H₂O₂ vapor concentration is 0 ppm (point 1) and 400 ppm (point 2) and the temperature is 25 °C, the water vapor concentration is 18,040 ppm. At point 1, the mixture dew point is 16.1 °C and relative humidity and relative saturation are the same 57.7 %RH. At point 2, where the H₂O₂ concentration is 400 ppm, the mixture dew point is 24.0 °C. Relative humidity remains unchanged at 57.7 %RH and relative saturation rises to 91.5 %RS. We see that by adding 400 ppm of H₂O₂ vapor, the mixture dew point increases by 7.9 °C, relative saturation by 33.8 %RS, and the air mixture is brought closer to condensation.

Mixture dew point at various H₂O₂ concentrations [ppmv]

Water vapor concentration [ppmv]

Figure 1. Both H₂O₂ vapor and water vapor affect the mixture dew point. The lines represent different vaporized H₂O₂ concentrations and the X-axis shows different water vapor concentrations. The higher the H₂O₂ vapor concentration, the higher the mixture dew point is, although the water vapor concentration remains the same.

Temperature and maximum VH₂O₂ concentration

Dew point is highly linked to condensation and can be used to detect when condensation will occur. In Figure 2, we see the maximum achievable vaporized H₂O₂ concentration when the vapor is produced with 35 % and 59 % H₂O₂ liquid. At every single point along the trend lines, relative saturation is 100 %RS, and mixture dew point equals ambient temperature on the X-axis.

Maximum achievable H₂O₂ at various temperatures:

Figure 2. The maximum achievable vaporized H₂O₂ concentration in air when the vapor is produced by vaporizing 35% and 59% H₂O₂ liquid. The maximum achievable H₂O₂ concentration is highly dependent on ambient temperature.

H₂O₂ liquid used to produce H₂O₂ vapor in bio-decontamination applications typically comes as a mixture of water and H₂O₂. For example, 35 % of the liquid’s weight is H₂O₂ and 65 % of its weight is water. When this aqueous solution is vaporized, both H₂O and H₂O₂ vapor concentrations will increase. Both vapors will affect the mixture dew point. Once condensation occurs, neither the concentration of H₂O₂ or H₂O can be increased. Higher concentration vapors can only be achieved by decreasing the liquid’s water content, or by increasing the air temperature. The increase of temperature will increase the gap between mixture dew point and ambient temperature.

Measuring condensation with dew point

Unlike relative humidity or relative saturation, measured mixture dew point is independent of temperature. If temperature is not uniform within the chamber, dew point can be a helpful measurement. 

Relative saturation is a good parameter to detect condensation. However, because RS is temperature dependent, probe placement can be critical. When monitoring condensation using mixture dew point, the measurement probe can be placed more freely. Note that the dew point value is the same at every measurement point in Figure 3.

Figure 3. The chamber has an evenly distributed air and vapor mixture, but there are temperature differences between the three measurement points. Condensation will occur first where the temperature is coldest. Td represents both water and hydrogen peroxide vapor mixture dew point.

Temperature variability can serve as a guide to choosing the parameter to monitor: relative saturation or dew point. Success in measuring H₂O and H₂O₂ vapors starts with understanding the measurement values and the conditions of your application. With this knowledge, you can choose the best parameter to monitor during vaporized hydrogen peroxide biodecontamination processes.