So, what is instrument?; what is instrumentation?; why do industry need instrument?
For the beginning, in this session we'll discuss about basic instrument, which talk about basic instrumentation knowledge including principle and definition also.
Note : Our body is instrument system also, but not valid. For example, we can say that some water hot or warm but also we can measure that in the right unit data ( C or F ). Exactly that the reason why we need instrument.
Ok, instrumentation is defined as " the art and science of measurement and control ". Instrumentation can be used to refer to the field and engineers work, or it can refer to the available methods of measurement and control and the instrument with facilitate this. Control system depends on set point and measurement result, and measurement is the first step and the most important action from the control system.
And instruments are the devices which are used to measure attributes of physical system or chemical system. The variable measured can include practically any measurable related to the physical science.
Measurement is the act or the result of quantitative comparation between a predetermined standard and unknown magnitude. The measurement is expressed as a number of the units of the standard ( a real number times unit ), such as distance being indicated by a number of miles or koilmeters.
There are 2 classification of methods of measurement :
1. Mechanic type, or self operated type ( like mercury in the glass thermometer ).
2. Power type, required the source of auxiliary power ( such as compressed air, electrical power, and hydraulic supply.
There are 2 basic methods of measurement :
1. Direct comparasion,commonnly used for measured length.
2. Indirect Comparasion, the comparasion is done with standard through use of calibrated system.In the industry commonly have 5 factor selection of measuring instrument. They are how accurate is the measurement to be made, when are final data required, the cost criterion, in what form data should
be displayed, whether quanttity to be measured has constant value or is it a time variant.
For the measuring system, they should be have primary sensing device, tranducer, intermediate modifying stage, and terminating stage. They all are used for indirect measuring system.
We have some question now, what are the function of instrument?. Before we answer that question we must know the principle of instrument, that is the control system.
Based that words we can say that the function of instrument are :
1. Indication, the value of quantity can be read by movement of needle on a calibrated scale provided in instrument.
2.Signalling, digital or analog ( 4-20 mA ).
3.Transmitting, transmitters are provided in the instrument which provided a signal corresponding to the value of the quantity being measured.
4. Registering, in such instruments instrument merely indiacates, by number or some others symbol of discrete increments, the value of quantitive.
5. To perform various manipulation. Multiplication, differentation, ratio control, etc.
Same as electronic system, control system like instrument has error also.Commonly that error is called source error. In this part we ecspecially talk about 3 source error, they are :
1. Noise, its defined as any signal that doesn't convey useful information.
2.Response time, any measuring has some got response time, i.e., it can't immeditially the input signal applied to it. This is another factor which contributes of the neasurenment.
3. Transmission. In the transmission of information from primary sensing element to indicator the following type error may arise.
After talk about principle of instrument, now we'll talk about variable and process variable. Variable is that suantity or characteristic which is the object of measurement in an instrumentation / control system. Variable generally are classified in two ways : physical characteristic and measurement signal.
And process variable is the current status of the process under control. An example of this would be the temperature of the furnance. The current temperature is called the process variable, while the desired temperature is known as the set point. Specilally, a process variable may be defined as a physical or chemical quantity.
Process variable are measured and controlled for a number reasons, including the following :
Product Quality, Material Saving, Cost Accounting,Production inspecting and testingPlant safety and comfort.