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INPUT, OUTPUT CONFIGURATION OF A MEASURING INSTRUMENT
An instrument performs an operation on an input quantity (measurement/designed variable) to provide an output called the measurements. The input is denoted by “i” and the output is denoted by “o”. According to the performance of the instrument can be stated in terms of an operational transfer function(G).The input and output relationship is characterized by the operation ‘G’ such that O=G i The various inputs to a measurement system can be classified into-three categories:

The various inputs to a measurement system can be classified into- three categories. i) Desired input: A quantity that the instrument is specifically intended to measure. The desired input 𝑖 produces an output component according to an input-output relation symbolized by 𝐺𝐷; here 𝐺𝐷 represents the mathematical operation necessary to obtain the output from the input. ii) Interfering input: A quantity to which the instrument is unintentionally sensitive. The interfering input 𝑖𝑙 would produce an output component according to input-output relation symbolized by 𝐺𝑙 iii) Modifying input: A quantity that modifies the input-output relationship for both the desired and interfering inputs. The modifying input 𝑖𝑀 would cause a change in 𝐺𝐷 and/or 𝐺𝑙. The specific manner in which 𝑖M affects GD and G, is represented by the symbols 𝐺𝑀𝐷 and 𝐺𝑀𝑙 respectively. A block diagram of these various aspects has been illustrated in Fig. Example: Consider a deferential manometer which consists of a u-tube filled with mercury and with its ends connected to the two points between which the pressure differentia is to be measured .The pressure differential P1-P2 is worked out from the hydro static (Equilibrium) equation: (P1-P2) = g h (𝜌𝑚 – 𝜌𝑓) 𝜌m and 𝜌𝑓 are the mass densities of mercury and fluid respectively, and h is the scale reading. lf the fluid flowing in the pipeline is a gas, then 𝜌𝑓 << 𝜌𝑚 accordingly the above identity can be re-written as (P1-P2) = g h 𝜌𝑚 Here differential pressure is P1-P2 is the desired input; Scale reading ‘h’ is the output and 𝜌 𝑚 is the parameter which relates the output and the input. A) The manometer is placed on a wheel which is subjected to acceleration and scale indicates a reading even through the pressures P1& P2 at the two ends are equal. The acceleration that constitutes the interference input. The manometer has an angular tilt i.e., is not properly align with the direction of the gravitational force. An output will result even when there is no pressure difference. Here the angular tilt acts as the interfering input. Here scale factor establishes the input - output relation and this gets modified due to i. Temperature variation which change the value of density of mercury. ii. Change in gravitational force due to change in location of a manometer. So, these 2 are modifying quantities. 1) Signal filtering 2) Compensation by opposing inputs. '''3) Output correction.