Standard Temperature and Pressure Conditions

Purpose and applications of standard temperature and pressure

Standard Temperature and Pressure (STP) conditions as well as Normal Temperature and Pressure (NTP) conditions are used as reference points in thermodynamics of gases. Normally STP and NTP conditions are used for specifying vapor volume.

This is because, volume of a given number of vapor moles is a function of temperature and pressure conditions. Hence it becomes imperative to specify the corresponding temperature and pressure for volume measurement, whenever gas quantity is specified in terms of gas volume.

Hence universally recognized reference temperature and pressure conditions can be easily used to specify gas volume measured at those conditions. This gas volume can be readily converted to number of moles or gas mass, as temperature and pressure of this standard reference point is easily known.

For example, STP conditions are often used to calculate the standard air density - which is simply air density at standard conditions.

Values of standard temperature and pressure depend on the organization which defines them. Usually the standard pressure is close to atmospheric pressure and standard temperature is close to room temperature value.

Using the standard T & P conditions

• STP and NTP conditions are often used for engineering calculations for a gas flow or gas volume is involved. Volume of a gas varies widely with temperature & pressure. Hence is cannot be reliably be used as a measure of the molar quantity of that gas. Instead when gas volume is considered at a fixed reference temperature & pressure, its 'standard volume' is directly proportional to the molar quantity.
• Gas density measured at STP or NTP conditions is also a good measure of the composition of that gas.
• For example, standard air density (measured at standard T & P) is a good indicator of the humidity or moisture content in air.