Process Variables

Process is defined as the changing or refining of raw materials that pass through or remain in a liquid, gaseous, or slurry state to create end products. The methods used to control process variables while manufacturing a product is referred as process control. oil and gas, cement, power e.t.c are examples of process industries. Process variable is a condition of process fluid (liquid or gas) that can change the manufacturing process in some way. Common process variables are listed below 

1. Pressure

2. Flow

3. Level

4. Temperature

5. Density

6. PH

Pressure : It is defined as force applied per unit area. P= F/A where, F = force and A = area. Units of pressure are bar, pascal, psi, tor, atm, kg / cm2, lb / in2.

1 Pa = 1N/ m2

1 psi = 1 lb/in2 (English unit) = 6,894 Pascal

1 Torr = 133.322 pascal

1 bar= 100000 pascal

1 atm= 101325 pascal

Flow : it is defined as volume per unit of time at specified temperature and pressure. Units of flow are kg / hr, litter / min, gallon / min, m3 / hr, Nm3 / hr.

Level : The height of a liquid or powder represented in meter ,mm, cm or percentage.

Temperature : It is an objective comparative measure of hot or cold. Units are Degree Centigrade, Degree Fahrenheit, Degree Kelvin, Degree Rankin.




Fahrenheit [°F] = [°C] × 9⁄5 + 32

Kelvin [K] = [°C] + 273.15

Rankine [°R] = ([°C] + 273.15) × 9⁄5

Delisle [°De] = (100 − [°C]) × 3⁄2

Newton [°N] = [°C] × 33⁄100

Reaumur [°Re] = [°C] × 4⁄5

Density : It is mass per unit volume. unit of density is kg/m3

PH : is a numeric scale used to specify the acidity or alkalinity of an aqueous solution. It is the negative of the logarithm to base 10 of the activity of the hydrogen ion. Solutions with a pH less than 7 are acidic and solutions with a pH greater than seven are alkaline or basic.

See Also :

level measurement: direct methods level measurement using hydrostatic pressure ultra sonic level measurement indirect methods for level measurement

Radiation Level Detection

The radiation type level measuring device operates on the principle that radiation can be blocked by any substance to a limited degree. This degree of blockage is measured and translated to indicate level.

The system itself consists of a radiation source, a detector, a conversion circuit, and an indicator. The source and detector are situated above and below the substance to be measured. As the level of the substance increases, the amount of radiation blocked increases. Therefore, the amount of radiation received by the detector decreases. This received radiation is translated into an electrical signal by the converter and sent to the indicator. A typical radiation type level measuring system is shown in Figure

radiation level detector

see also

level measurement using hydrostatic pressure ultra sonic level measurement indirect methods for level measurement level measurement: direct methods difference between sensor and transducer

Ultra Sonic Level Measurement

This type of instrument uses “time of flight” principle. An ultrasonic level measuring device uses reflected sound to determine the height of a substance in a vessel. The time required for a sound wave to be transmitted, reflected, and returned to the device may be translated directly into a level indication. A high frequency sound signal is used for this purpose, normally in the range of 20KHZ to 200KHZ.

The timing circuit of the device is started when the transmitter sends out the sound wave. It is stopped when the receiver detects the return of the reflected signal. The device is calibrated in an empty vessel; therefore, as the level in the vessel increases, the time required for the reflected signal to return decreases.

Although this principle is very accurate and used in many industries, it is not widely used as an indicator in power plants. The device is generally fragile and affected greatly by outside forces. Because of this, it requires frequent calibration to maintain its accuracy. It is, however, used to some extent as a level switch. A typical example is illustrated in Figure .

ultrasonic level measurement

see also

level measurement using hydrostatic pressure indirect methods for level measurement level measurement: direct methods difference between sensor and transducer

Water Level measurement using hydrostatic Pressure

Water Level measurement using static Pressure

The pressure at any point below the surface of a liquid and the level of water in height are related directly. Simply, the pressure at any point below the liquid surface is a measure of liquid level. Level measuring instruments using static pressure utilize this principle.

Static pressure type level instruments are normally only applied to open vessels, because they depend entirely on the static head. Any change in pressure on the surface of the liquid would affect the calibration, because it would cause a change of static head pressure. The pressure and Level relation can be summarized using the equation

h=P/ρg

Indirect Methods for Level Measurement

In case of indirect method or inferential method, levels are measured in terms of other parameters like pressure. Measuring instrument will be calibrated in terms of level. As discussed previously, direct reading level devices are normally for local indication only. When remote indication like in the control room or a control signal is required, an indirect measurement device must be used. There are different types of indirect level-measurement devices like differential pressure, static pressure, ultrasonic, and radiation level measuring instruments. Some are discussed below.

 Differential pressure level measurement

This type of instrument measures the differential pressure resulting from two columns of liquids with unequal height. The height of one column will be always constant and that of the other will be variable. A typical example of this type would be the steam drum water level measuring device, similar to the one shown in Figure.

Level measurement

In industries, Liquid levels are measured either directly or indirectly. In direct method level can be measured by using indicators such as dipsticks, floats, and gauge glasses. Where as in indirect method, it is measured using static or differential pressure instruments. Normally Direct measurements are used only for local indication, meanwhile Indirect measurements are used to provide both remote indication and a control signal.

Direct method

Direct methods are used only for monitoring or getting indications locally. Even though in industries, it is used for various reasons on many types of substances. There are different types of instruments for this purposes. Some of them are discussed below. 

What is the difference between sensor and transducer

The difference between sensors and transducers is one of the most confusing and ever discussed question among instrumentation students. the reason behind rising such confusion is that, both sensors and transducers are highly related and some people use these as synonyms. here i would like to explore some definitions and differences explained by experts.

the below explanation is by john wiley & sons in “Sensors and Signal Conditioning, 2nd. Ed.”:

“A transducer is a device that converts a signal from one physical form to a corresponding signal having a different physical form. Therefore, it is an energy converter. This means that the input signal always has energy or power, i.e. signals consist of two component quantities whose product has energy or power dimension. But in measurement systems, one of the two components of the measured signal is usually so small that it is negligible, and thus only the remaining component is measured.