Relative humidity is a term frequently used to represent the quantity of moisture or water vapor present in a mixture although it uses partial pressures in so doing. It is expressed as:


Relative humidity is usually considered only in connection with atmospheric air, but since it is unconcerned with the nature of any other components or the total mixture pressure, the term is applicable to vapor content in any problem. The saturated water vapor pressure at a given temperature is always known from steam tables or charts. It is the existing partial vapor pressure, that is desired and therefore calculable when the relative humidity is stated.


Specific humidity, used in calculations on certain types of compressors, is a totally different term. It is the ratio of the weight of water vapor to the weight of dry air and is usually expressed as pounds, or grains, of moisture per pound of dry air. Where Pa is the partial air pressure, specific humidity can be calculated as:



The degree of saturation denotes the actual relationship between the weight of mois­ ture existing in a space and the weight that would exist if the space were saturated:


A great many dynamic compressors handle air. Their performance is sensitive to den­ sity of the air, which varies with moisture content. The practical application of partial pressures in compression problems centers to a large degree on the determination of mixture volumes or weights to be handled at the intake of each stage of compression, the determination of mixture molecular weight, specific gravity, and the proportional or actual weight of components.


Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the U.S. Weather Bureau, give

detailed data about vapor pressure, relative humidity, and dew point at the sea-level barometric pressure of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric tempera­ tures as determined simultaneously by a sling psychrometer. The dry bulb reads ambi­ ent temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer.


All gases deviate from the perfect or ideal gas laws to some degree. In some cases the deviation is rather extreme. It is necessary that these deviations be taken into account in many compressor calculations to prevent compressor and driver sizes being greatly in error.

Compressibility is experimentally derived from data about the actual behavior of a particular gas under pVT changes. The compressibility factor, Z, is a multiplier in the basic formula. It is the ratio of the actual volume at a given pT condition to ideal vol­ ume at the same pT condition. The ideal gas equation is therefore modified to:


In this equation, R is I ,545 and p is pounds per square foot.


Keeping with the subject of pressure, the basic concepts will be treated in the working sequence pressure generation, transmission, storage, and utilization in a pneumatic system.

Pumping quantities of atmospheric air into a tank or other pressure vessel produces pressure. Pressure is increased by progressively increasing the amount of air in a con­ fined space. The effects of pressure exerted by a confined gas result from the average of forces acting on container walls caused by the rapid and repeated bombardment from an enormous number of molecules present in a given quantity of air. This is accomplished in a controlled manner by compression, a decrease in the space between the molecules. Less volume means that each particle has a shorter distance to travel; thus, proportionately more collisions occur in a given span of time, resulting in a higher pressure. Air compressors are designed to generate particular pressures to meet individual application requirements.

Basic concepts discussed here are atmospheric pressure; vacuum; gauge pressure; absolute pressure; Boyle’s law or pressure/volume relationship; Charles’ law or tem­ perature/volume relationship; combined effects of pressure, temperature, and volume; and generation of pressure or compression.

Atmospheric Pressure

In the physical sciences, pressure is usually defined as the perpendicular force per unit area, or the stress at a point within a confined fluid. This force per unit area acting on a surface is usually expressed in pounds per square inch.

The weight of the earth’s atmosphere pushing down on each unit of surface consti­ tutes atmospheric pressure, which is 14.7 psi at sea level. This amount of pressure is called one atmosphere. Because the atmosphere is not evenly distributed about the earth, atmospheric pressure can vary, depending upon geographic location. Also, obviously, atmospheric pressure decreases with higher altitude. A barometer using the height of a column of mercury or other suitable liquid measures atmospheric pressure.


It is helpful to understand the relationship of vacuum to the other pressure measure­ ments. Vacuums can range from atmospheric pressure down to “zero absolute pres­ sure,” representing a “perfect” vacuum (a theoretical condition involving the total removal of all gas molecules from a given volume). The amount of vacuum is mea­ sured with a device called a vacuum gauge.

Vacuum is a type of pressure. A gas is said to be under vacuum when its pressure is below atmospheric pressure, i.e., 14.7 psig at sea level. There are two methods of stat­ ing this pressure, but only one is accurate in itself.

A differential gauge that shows the difference in the system and the atmospheric pressure surrounding the system usually measures vacuum. This measurement is expressed in the following units:


Unless the barometric or atmospheric pressure is also given, these expressions do not give an accurate specification of pressure. Subtracting the vacuum reading from the atmospheric pressure will give an absolute pressure, which is accurate. This may be expressed in the following units:


The word absolute should never be omitted; otherwise one is never sure whether a vacuum is expressed in differential or absolute terms.

Perfect Vacuum

A perfect vacuum is space devoid of matter. It is absolute emptiness. The space is at zero pressure absolute. A perfect vacuum cannot be obtained by any known means, but can be closely approached in certain applications.

Gauge Pressure

Gauge pressure is the most often used method of measuring pneumatic pressure. It is the relative pressure of the compressed air within a system. Gauge pressure can be either positive or negative, depending upon whether its level is above or below the atmospheric pressure reference. Atmospheric pressure serves as the reference level for the most significant types of pressure measurements. For example, if we inflate a tire to 30 psi, an ordinary tire-pressure gauge will express this pressure as the value in excess of atmospheric pressure, or 30 psig (“g” indicates gauge pressure). This read­ ing shows the numerical value of the difference between atmospheric pressure and the air pressure in the tire.

Absolute Pressure

A different reference level, absolute pressure, is used to obtain the total pressure value. Absolute pressure is the total pressure, i.e., gauge and atmospheric, and is expressed as psia or pounds per square inch-absolute. To obtain absolute pressure, simply add the value of atmospheric pressure (14.7 psi at sea level) to the gauge pres­ sure reading.

Absolute pressure (psia) values must be used when computing the pressure changes in a volume and when pressure is given as one of the conditions defining the amount of gas contained within a sample.

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