Scale Range and Scale Span are two most confusing terms in measurement. Many a times students get confused with these two terms. Here I have clarified both these terms in the easy language.

In analog instruments, the value of measurand is indicated by pointer on a scale. Scale has some values mentioned on the scale. It has lowest and maximum value indicated on the instrument.

Scale Range

It is defined as the largest reading that an instrument can read. 

Scale Span

It is defined as the difference between the largest and the smallest reading of the instrument.

Let us take an example of an instrument that can take reading to the largest value of Y units. It can take smallest of X units. Now
 
Range of Instrument: Y units ( or X-Y range)
Span of Instrument: (Y – X) units
 
1. Consider a case of pyrometer that is calibrated between 0 to 1000 °C.
 
Range of Instrument: 1000 °C (or 0-1000 °C range)
Span of Instrument: (1000 – 0) °C = 1000 °C
 
Here both Range and Span are same.
 
2. Consider another case of thermometer that is calibrated between 300 to 900 °C.
 
Range of Instrument: 900 °C (or 300-900 °C range)
Span of Instrument: (900 – 300) °C = 600 °C
 
Here Range and Span of thermometer are different.
 
Note: The above definition of Scale Range and Scale Span are applicable for digital instruments also.

Frequency Range

It is defined as frequencies over which measurements can be performed with a specified degree of accuracy. For example, an instrument has 0-300 V range and 0-150 Hz frequency range.

Excercise

Q1. Explain the term, Scale Range?
Q2. Explain the term Scale Span?
Q3. Differentiate the term Scale Range and Scale Span?
Q4. What do you mean by Frequency Range?

Scale Range and Scale Span
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