General purpose simulation system (GPSS)

General purpose simulation system is a process-oriented language originally designed by Geoffrey Gordon in 1961 which is used for discrete event modeling. GPSS, which was developed by IBM Corporation, is a very dynamic language. Modifications were being done on this language many times and hence exist many versions of this language now. Now GPSS become a multi-vendor language. GPSS is a very popular simulation language and it started with the advent of mainframe computers. GPSS is a superior language with in-built mechanism for the collection of data, analysis, production of tabulated output data and execution. In 1977, Henriksen developed a new language which is known as GPSS/H. This language is reported to be five times faster in execution time. GPSS/H offers some unique features like a real value clock for recording the simulation time, ability to read and write external files, enhanced control statements, increased number of mathematical functions, capability of generating unlimited number of independent random number series and capability of generating random variants from different probability distributions. A block-by-block construction model is used in GPSS. Block time is an integer which gives an idea about the number of units required to execute the action represented by the block.

Luminescence

Luminescence is the phenomenon in which a crystal absorbs energy and emits the energy back in the visible range. There are two varieties here-florescence and phosphorescence. Fluorescence is an instantaneous re-emission. Phosphorescence which is also known as afterglow takes a longer time to glow. Luminescence is greatly influenced by the presence of impurities and imperfections in the material. These impurities are called activators and introduce additional energy states in the bandgap of the material. These energy states are called recombination centers and traps. If the discrete energy states are located near the center of the bandgap, they are referred as recombination centers. All the other energy states in the bandgap are called traps. Thus a trap is an imperfection which is capable of capturing an electron or a hole. The existence of recombination centers and traps introduces a number of newer possible transitions in the semiconducting material. Luminescence finds applications in electroluminescent lamps, fluorescent tubes, cathode ray oscilloscopes and television displays. The phenomenon of luminescence may be classified into photoluminescence, cathodoluminescence, electroluminescence, chemiluminescence, thermoluminescence, triboluminescence and injection luminescence. Photoluminescence is produced by photons. Cathodoluminescence by high energy electrons, electroluminescence due to ac electric fields, chemiluminescence due to some chemical reactions, thermoluminescence by simulation at low temperatures, triboluminescence is due to mechanical energy and injection luminescence is due to forward biased p-n junction.

Manometers

Manometers are used for the measurement of pressure. In a single-tube manometer, a piezometer is connected to a pipe at the point where the static pressure of the fluid is to be measured. Piezometers are suitable for fluids under measurement. In a twin-tube manometer, two tubes of equal cross-section are connected together to form a U-tube. Both gas pressure and liquid pressure can be measured by using twin-tube manometers but care should be taken for not to mix or react chemically the liquid/gas under measurement with the manometric liquid. For low gauge pressures, water is taken as manometric liquid and in all other cases mercury is advisable. We can also employ a sealing liquid as a buffer to avoid reaction. The cistern type manometer requires only one reading while measurement. For the measurement of low differential pressures arising in gas-flow metering, ring-balance manometers are used. An electric displacement transducer is used in Bell-type manometers. Some other commonly used manometric liquids are transformer oil, dibutylphthalate, carbon tetrachloride and tetraboromethane.