The actual type of "Air" being used is always important in gas flow. Some Flow applications such as air sampling devices use ambient air as the supply gas. This is done by connecting a vacuum source to the outlet of the system to pull the ambient/moist air through the test device as well as the molbloc reference. Only sonic molbloc's support this method and do not want the moisture high enough to risk it reaching the point of condensation. The basic setup/flow path for this type of application would generally look as follows:
Room/ambient/humid air --> Air sampling test device --> Sonic molbloc --> Metering valve/flow control --> Vacuum pump/source
*** Note, this is a fairly complex setup and would normally be avoided if the test device can be run with dry gas and with some minimal positive pressure on its inlet. This would allow using a laminar flow element instead in a more traditional flow path setup with it being downstream of the laminar molbloc and outlet open to atmosphere. Sonics are limited in the vacuum range (also based on calibration type) and further complicated by needing to reach a sonic state/back pressure ratio for valid measurements.
In this application we are pulling in ambient air which is thus humid and thus should be compensated for. This compensation is called the Water Ratio. As the name suggests it is essentially determining how much water is in the air. This is NOT simply the humidity value, it is ratio of water mass compared to the air mass.
To calculate the Water Ratio we need to know the current humidity, pressure , and temperature of the gas being pulled into the system. Would typically use something like a lab monitor for the humidity and temperature and a barometer for pressure as typically these are monitoring the lab conditions in which the air that will pulled into the system is at.
*** Disconnecting a molbox pressure tube from the molbloc and venting the line to atmosphere pressure could also be used to get the Pressure value.
The calculation is fully explained in the molbox user manual, but we have created a tool within our Unit Converter to calculate the Water Ratio. This tool is installed as a sperate application if already using our COMPASS for Flow software. You can also download and install a demo copy on any PC and the tools will not expire after the demo period for our website here:
Just enter the current Pressure, Temperature, and Humidity to calculate the Water Ratio. This value can then be entered into the molbox using the Gas button, select "Other" from the menu as Air is a gas mixture, select Air, and you will then be prompted for the "Humidity Ratio" which is the Water Ratio.
If you are using COMPASS for Flow software to run your Tests make sure Tools/Options/molbox tab is set to "Prompt for new value" so that a window will appear to enter the newly calculated value to use on Tests that require this calculation.
*** When completing a Test that uses a Water Ratio (regardless if using COMPASS software or directly inputting this into the molbox menu), make sure the value is reset to 0 when go back to using normal breathing/industrial grade Air as the supply gas. Another way to verify if this correction is being applied or not is when Air is selected on the molbox it will display as "AirW" on the front panel main display screen to indicate a Water Ratio correction is being applied or just "Air" when no correction is being applied.
*** Further calculations might need to be done to similarly correct the DUT/Test Device for the moist air also. Some devices might also already be compensating for a "typical" humidity or similar which might need to be adjusted out for proper comparison to the molbloc Water Ratio. In some cases and especially with low humidity values the Water Ratio correction might be insignificant compared to the device being tested.