Humidity meters are used to measure the humidity outside. Typically, these would be placed on the outside of your house to accurately measure the humidity.
Hygrometer
There are lots of humidity meters on Amazon.com. Last year, I bough a nice thermometer / humidity meter for less than ten dollars. You can probably still find it on Amazon.
The purpose of the psychrometer is to estimate Relative Humidity of Air. It does it by getting the dry and wet bulb temperatures. Then Relative Humidity is read on a chart.
humidity meter
The dehumidifier removes excess humidity from the air.
Because it doesn't say neither the temperature, the pressure or the humidity of the air. You need to know both the temperature, the pressure and the humidity of the air to say anything about the mass of one cubic meter of air.
The purpose of a gas meter is to monitor how much gas a particular house is using. Customers are then billed according to how much gas they have used from the meter.
The purpose of a ductless air conditioning system is to provide cool and low-humidity air in spaces where there is no method to create an exhaust for heat and the humidity that has been removed from the air inside that space.
Net Meter software monitors network traffic on the computer it is installed on. It's purpose is to monitor and record the internet bandwidth that one uses.
specific humidity
25%
The purpose of a flow meter is to measure the flow of either air or water. They allow one to measure the total volume of air or liquid flow and the rate of the flow.