Instruments Types and Performance Characteristics Report
Instruments Types and Performance Characteristics Report
Instruments Types and Performance Characteristics Report
Performance Characteristics
By Alan S Morris
Reported by :Alon, Hamode
Cordova Jason Clyde
Instrument Types
• Instruments can be classified according to several criteria
which are useful in establishing many attributes like:
• Cost
• Accuracy
• Maintenance
• General Applicability of the instruments
Active and Passive Instruments
Solution
• If these values are plotted on a graph, the straight-line relationship between
resistance change and temperature change is obvious.
• For a change in temperature of 30°C, the change in resistance is 7 Ohms.
Hence the
Measurement sensitivity = 7/30 = 0.233Ohms/°C.
Threshold
• If the input to an instrument is gradually increased from zero, the input
will have to reach a certain minimum level before the change in the
instrument output reading is of a large enough magnitude to be
detectable.
• This minimum level of input is known as the threshold of the
instrument.
• E.g., a car speedometer with typical threshold of about 15 km/h,
means that, if the vehicle starts from rest and accelerates, no output
reading is observed on the speedometer until the speed reaches 15
km/h.
• Is sometimes quoted as an absolute value.
• Is expressed as a percentage of the full scale reading.
Resolution
• There is a lower limit on the magnitude of the change in the input
measured quantity that produces an observable change in the
instrument output.
• Also sometimes specified as an absolute value or as a percentage of
fsd.
• Major factor influencing resolution is how finely its output scale is
divided into subdivisions.
• E.g. a car speedometer with subdivisions of typically 20 km/h.
• This means that when the needle is between the scale markings, we
cannot estimate speed more accurately than to the nearest 5 km/h.
• This figure of 5 km/h thus represents the resolution of the instrument.
Sensitivity To Disturbance
• All calibrations and specifications of
an instrument are only valid under
controlled conditions of temp., pressure
etc. which are defined in the instrument
specification.
• As variations occur in the ambient
temperature etc., certain static
Instrument characteristics change, and
the sensitivity to disturbance is a
measure of the magnitude of this
change.
• Such environmental changes affect
instruments in two main ways: zero drift
and sensitivity drift.
Hysteresis loss
• Figure 2.8 illustrates the output characteristic of an instrument that
exhibits hysteresis.
• If the input measured quantity to the instrument is steadily increased
from a negative value, the output reading varies in the manner shown
in curve (a).
• If the input variable is then steadily decreased, the output varies in the
manner shown in curve (b).
• The non-coincidence between these loading and unloading curves is
known as hysteresis.
• Two quantities maximum input hysteresis and maximum output
hysteresis, as shown in Figure 2.8.
They are normally expressed as a
percentage of the full-scale input or output
reading respectively.
Hysteresis is most commonly found in
instruments that contain springs, like
passive pressure gauge and the Prony
brake (used for measuring torque).
Mechanical flyball for measuring rotational
velocity suffer hysteresis from both of the
above sources because they have friction
in moving parts and also contain a spring
Dead Space
• Therange of different input values over which there is no
change in output value.
• Anyinstrument that exhibits hysteresis also displays dead
space, as marked on Figure 2.8.
• Some instruments that do not suffer from any significant
hysteresis can still exhibit a dead space in their output
characteristics, however.
• Backlash in gears is a
cause of dead space, and
results in the output
characteristic shown in
Figure 2.9.
• Backlash is commonly
experienced in gearsets
used to convert between
translational and
rotational motion.
Necessity for Calibration
• An instrument only conforms to stated static and dynamic
patterns of behaviour after it has been calibrated.
• It can normally be assumed that a new instrument will have
been calibrated when it is obtained from an instrument
manufacturer.
• It will therefore initially behave according to the
characteristics stated in the specifications.
• During use, however, its behaviour will gradually diverge
from the stated specification for a variety of reasons:
• Mechanical wear, the effects of dirt, dust, fumes and
chemicals in the operating environment.
• Therate of divergence from standard specifications varies
according to the type of instrument, the frequency of
usage and the severity of the operating conditions.
• However, there is a time, practically, when the
characteristics of the instrument will have drifted from the
standard specification by an unacceptable amount.
• When this situation is reached, it is necessary to recalibrate,
i.e readjust the instrument to the standard specifications.