Instrument proportional band

The proportional band of a particular instrument is expressed as a percent of full range. For example, if full range of an instrument is 200°F and it takes a 50°F change in temperature to cause full valve travel, the percent proportional band is 50°F in 200°F, or 25%. Proportional bands may range from less than 1% to well over 200%. However, proportional bands over 100% cannot cause full valve travel even for full range change of the controlled variable. 

Many instrument manufacturers use an alternative term, proportional band (PB), instead of gain. The two are related by... 

These are approximate instrument settings, and may need to be adjusted in process. PB is proportional band. 

Setting all control functions e.g., reset proportional band, etc., of instruments at values expected to be required for operation of the plant. 

Instrument manufacturers sometimes present the scales on proportional action as per cent proportional band which is 100/i p. 

Improper adjustment. Proportioning band and reset should be adjusted to give smooth control. Damping must not be so great that sensitivity is lost. Consult manufacturer s manual for instrument adjustment procedures. 

The resulting proportional controller throttles the steam supply in proportion to the demand. Instruments are designed for a full-scale change of the controlled variable, e.g., 100 F, or from 140 to 240°F in this example. The instrument can be manually set to provide complete on to off valve action over a percentage of the full-scale range of the instrument. This percentage is termed proportional band width. In this example, a 20 per cent proportional band width of 20 F at a set point of 190 F would mean that the control valve would be fully closed at 200 F, fully opened at 180 F, and half open at the set point of 190 F. 

This control action eliminates hunting but creates offset or droop with varying demand changes, such that there is a shifting control point (Fig. 9-21 ). A narrow proportional band setting produces the minimum offset but maximum cycling action and recovery time. Offset can only be eliminated by manually changing the set point of the instrument. 

The use of the flapper-nozzle amplifier increased the sensitivity of the controller but at the expense of its proportional action. The flapper-nozzle was so sensitive that the controller had a proportional range of about 1 per cent of full-scale movement and hence became the equivalent of an on-off controller. Developments during the 1920s concentrated on trying to modify the flapper-nozzle valve action so as to increase its proportional range, and by 1930 several instrument companies offered controllers with a proportional band of 5 to 7 per cent of fiill range. 

The proportional controller allows a continuous adjustment of the power input level (from 0 to 100%) depending on the actual temperature. The range of temperature over which the power is adjusted from 0 to 100% is called the proportional band. The proportional band is usually expressed as a percentage of instrument span and is often centered about the setpoint. Thus, in a controller with a 500°C span, a 5% proportional band would be 25 degrees about the setpoint. Sometimes, the setpoint is located at the upper temperature limit of the proportional band. 

When one pulls a controller out of the instrument panel, several small screws or dials come into view. These are usually labeled proportional band or gain, reset, or rate (or derivative). Turning these screws profoundly affects the performance of the control loop. This is a job best left to the instrument mechanic if he really knows how to tune instruments. Quite often, the operations engineer will have to try his own hand at instrument tuning. This is how to go about it. 

Seebeck used antimony and copper wires and found the current to be affected by the measuring instrument (ammeter). But, he also found that the voltage generated (EMF) was directly proportional to the difference in temperature of the two junctions. Peltier, in 1834, then demonstrated that if a current was induced in the circuit of 7.1.3., it generated heat at the junctions. In other words, the SEEBECK EFFECT was found to be reversible. Further work led to the development of the thermocouple, which today remains the primary method for measurement of temperature. Nowadays, we know that the SEEBECK EFFECT arises because of a difference in the electronic band structure of the two metals at the junction. This is illustrated as follows ... 

XPS of NH3 adsorption was carried out on a SSI (Surface Science Instrument) spectrometer. NH3 was adsorbed at 80 °C on the calcined samples and then outgassed under helium at 350 °C. The proportion of each type of site (Bronsted and Lewis) was evaluated by analyzing the Nls band. 

Band positions in IR spectra are presented here as wavenumbers (T) whose unit is the reciprocal centimeter (cm-1) this unit is proportional to the energy of vibration and modern instruments are linear in reciprocal centimeters. Wavelength (A) was used in the older literature in units of micrometers (/xm = 10-6 m earlier called microns). Wavenumbers are reciprocally related to wavelength. 

It is noteworthy that the width of an absorption line is inversely proportional to the lifetime of the excited state (Heisenberg s uncertainty principle). Hence, for gases, the lifetime is long and the absorption lines are sharp. However, the lifetime is short for compounds in the condensed phase and band broadening occurs. Except for very simple molecules, no instrument allows the observation of individual lines. 

If, as usual, the spectra are not scanned in % transmission but in absorption, it is easy to quantitatively analyze the compound in question (absorbance concentration). If the resolution of the instrument is higher than the half-width of a recorded band, then the height of the band is proportional to the concentration of the species. 

Spectrometer An instrument equipped with a monochromator or a polychromator, a photodetector, and an electronic readout that displays a number proportional to the intensity of an isolated spectral band. 

We should know that the peak height is sensitive to instrument resolution. The peak area measurement under a vibration band shows much less instrumentation dependence. The peak area represents the integrated intensity of the vibration band and is proportional to the square of the change in dipole moment with respect to the normal coordinate as Equation 9.20. 

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