A process variables is a process that we can measure and change. There are many process variables in the petroleum industry. However there are only fo
 Pressure
 Flow
 Level
 Temperature
We will concentrate on these four process variables.
PRESSURE
Recall that it is pressure that pushes fluids through pipes and equipment. It can be considered that pressure is the most important process variable. It is therefore important that you have a very good understanding of pressure. Without that understanding you will find it difficult to follow courses that come later in your training. Recall from your science course. PRESSURE (P) is defined as the FORCE (F) applied divided by AREAPressure and LiquidThe diagram (see Figure) shows a force (F) applied to a piston pressing on a liquid in a cylinder. The liquid is considered INCOMPRESSIBLE and the pressure of the liquid on the walls of the cylinder is the same in all directions. This gives the formula:
F
P = 
A
Pressure and Gases The diagram (see Figure shows a force (F) applied to a piston pressing on gas in a cylinder. The gas is COMPRESSIBLE. The volume of the gas will decrease until the pressure of the gas on the walls of the cylinder equals the pressure applied by the piston.F
P = 
A
Pressure UnitsThere is no agreed standard for pressure measurement in the petrochemical industry. Some companies use IMPERIAL UNITS (USA), some use INTERNATIONAL STANDARD METRIC UNITS (ISO) and some use both. The operator must understand both systems and be able to change from one to another.
IMPERIAL  ISO  
FORCE  Pound  Newton 
AREA  Squere Inch  Newton 
PRESSURE  Pound per Squere Inch (Psi)  Newtons per squere metre (Pascal) 
1 psi = 6895 Pa
The Pascal is a very small unit so the KILO PASCAL (KPa) is often used. A bigger unit is the bar. This is the most common ISO unit.
Conversion:100 Kpa = 1 bar
Note: On very old installations the kilogram per centimeter square is still used. For all general purposes.
1 kg/cm = 1 bar
Very small pressures are measured using the height of a column of liquid. The liquids used most are water (H2O) and mercury (Hg).
Absolute, Gauge and Atmospheric PressureThe price of oil or gas depends on the quantity (mass) of the product. The quantity of oil or gas in a given volume depends on the pressure. For this measurement, absolute pressure must be used.
Absolute Pressure  This is the pressure above a total vacuum (there are no particles of matter in a total vacuum). 
Gauge Pressure  This is the pressure measured by a gauge. The pressure above the pressure of the surroundings atmosphere. 
Atmospheric Pressure  The pressure of the air all around you. This is not constant it depends on things like the weather and the altitude of the plant. 
Absolute pressure = Gauge pressure + Atmospheric pressure
Because atmospheric pressure can vary a STANDARD ATMOSPHERIC pressure has to be used. This is normally 1.013 Bar or 14.70 psi. Gauge pressure is written as psig. Absolute pressure is written as psia.
A simple diagram is shown as an example.
Example 1  
Question :  A pressure gauge indicates 11.4 psi. Find the absolute pressure if the atmospheric pressure 14.65 psi. 
Solution :  AP = GP + ATMOSPHERIC PRESSURE AP = 11.4 + 14.65 = 26.05 Absolute pressure = .26.05 psi 
Example 2  
Question :  A pressure gauge reads 3.5 psi vacuum. Find the absolute pressure if the atmospheric pressure is 14.73 psi. 
Solution :  Vacuum pressures are normally given as gauge pressures below atmospheric pressure (zero gauges). That is a negative gauge pressure. AP = ‑ GP + ATMOSPHERIC PRESSURE AP = 3.5 + 14.73 = 11.23 3.5 psi vacuum pressure = 11.23 psi absolute pressure 
FLOW
The simplified diagram (see Figure 1 5) shows a tanker being loaded from a storage tank. The amount of oil loaded must be accurately measured to know how much it costs. The total flow (quantity) of oil into the tanker can be measured in two ways:
 by volume, in barrels or cubic meters.
 by mass, in metric or imperial tons
For control purposes the rate of flow (how fast the ship is loaded) is also measured. Rate of flow units can also be given in either volumetric or mass units.
Rate of flow by volume (volumetric)  Barrels / Hour Cubic Feet / Min. Cubic Meters / Sec 
Rate of flow by mass  Tons / Hour Kilograms / Sec. Pounds / Min. 
The petrochemical industry uses many different units and there is no common standard. The following list gives some of the units and their conversion.
VOLUME 

Barrel (Bbl)  = 42 US Gallons = 34.97 Imperial Gallons 
Cubic Foot  = 0.0929 Cubic Meter 
Cubic Meters  = 10.76 Cubic Foot 
Cubic Meters  = 1000 Litres 
1 Litre  = 10000 Cubic Centimetres 
MASS  
Pound (lb)  = 0.454 Kg 
Kilogram (Kg)  = 2.2 lb 
Imperial Ton  = 2240 lb (Long Ton) 
Metric Tonne  = 1000 Kg 
American Ton  = 2000 lb (Short Ton) 
Long Ton  = 1.016 Metric Tonne 
Metric Tonne  = 0.984 Long Ton 
TEMPERATURE
There are different scales for measuring temperatures. The diagram (see Figure 1 6) compares the two common temperature scales; Fahrenheit (Imperial) and Celsius (ISO). The fixed points for both scales are the temperature at which ice melts and water boils at standard pressure.
A temperature in Fahrenheit can easily be changed to Celsius and vice versa. The conversion equations depend on the number of divisions in each scale. Fahrenheit has 180 divisions between the freezing and boiling points of water but Celsius has only 100 divisions. Therefore, the ratio is 180/100 or 9:5. This gives:
ºC = (5(ºF  32)) / 9 or ºF = (9 ºC + 32) / 5