Instrument Types: Active vs. Passive, Analogue vs. Digital, and Smart vs. Non-smart, Slides of Engineering

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“Instrument

types”

Prepared by:

zainab ather

Instrument types:

 Active and passive instruments

 Null-type and deflection-type instruments

 Analogue and digital instruments

 Indicating instruments and instruments with a signal output

 Smart and non-smart instruments

Active and passive instruments: Resolution  (^) With the simple pressure gauge shown, the amount of movement made by the pointer for a particular pressure change is closely defined by the nature of the instrument.  (^) In an active instrument, however, adjustmentof the magnitude of the external energy input allows much greater control over measurement resolution. Whilst the scope for improving measurement resolution is much greater

Active and passive instruments: Cost

 Passive instruments are normally of a

more simple construction than active

ones and are therefore cheaper to

manufacture.

 Therefore, choice between active and

passive instruments for a particular

application involves carefully balancing

the measurement resolution

requirements against cost.

Null-type and deflection-type instruments: Accuracy  (^) The accuracy of the deflection type pressure measurement instrument depends depends on the linearity and calibration of the spring, whilst for the second it relies on the calibration of the weights.  (^) As calibration of weights is much easier than careful choice and calibration of a linear-characteristic spring, this means that the second type of instrument will normally be the more accurate. This is in accordance with the general rule that null-type instruments are more accurate than deflection types.

Null-type and deflection-type instruments: Usage  (^) In terms of usage, the deflection type instrument is clearly more convenient. It is far simpler to read the position of a pointer against a scale than to add and subtract weights until a null point is reached.  (^) A deflection-type instrument is therefore the one that would normally be used in the workplace. However, for calibration duties, the null-type instrument is preferable because of its superior accuracy. The extra effort required to use such an instrument is perfectly acceptable in this case because of the infrequent nature of calibration operations.

Analogue and digital instruments  (^) A digital instrument has an output that varies in discrete steps and so can only have a finite number of values. The rev counter is an example of a digital instrument. A cam is attached to the revolving body whose motion is being measured, and on each revolution the cam opens and closes a switch. The switching operations are counted by an electronic counter. This system can only count whole revolutions and cannot discriminate any motion that is less than a full revolution

Analogue and digital instruments: Computer Interface  (^) An instrument whose output is in digital form is when there is a need to be interfaced to a control computer. Analogue instruments must be interfaced by an analogue-to-digital (A/D) converter.  (^) A/D converter adds a significant cost to the system. Additionally, a finite time is involved in the process of converting an analogue signal to a digital quantity, and this time can be critical in the control of fast processes where the accuracy of control depends on the speed of the controlling computer. Degrading the speed of operation of the control computer by imposing a requirement for A/D conversion thus impairs the accuracy by which the process is controlled.

Smart and non-smart instruments  (^) Self calibration capability  (^) Self-diagnosis of faults  (^) Compensation for random errors  (^) Adjustment for measurement non-linearities