Span
Difference between upper and lower range values eg. Input Span = Input URV-Input LRV
Gain
The gain is the ratio between the output to input span
Repeatability
Describes closeness of output readings when the same input is applied repetitively over a short period of time, with the same measurement conditions, same instrument and observer, same location, and same conditions of use maintained throughout
Accuracy
The degree of conformity of a measured value to an ideal value. Specified as a percentage of full scale or percentage of reading
= S.E/Ideal Value + 100%
= S.E/Ideal Span + 100%
Bias error
The actual output minus the ideal output at the minimum or lower range value
Span error
The slope of the transfer function varies from the ideal. Calculated as (actual input span- ideal input span) and usually expressed as a % of the ideal input span
Linearity error
The maximum deviation from the ideal after bias and span errors have been removed
Sensor
A primary element or sensor is the device which converts the measured value into a form suitable for further conversion into an instrumentation signal
Transducer
A transducer is a device which translates energy from one form to another
Transmitter
A transducer that responds to a measured variable by means of a sensing element and converts it to a standardized transmission signal which ideally is a function only of the measured variables
Typical pneumatic signal range
20-100 kPag
3-15 psig
Typical electronic signals
1-5 V
4-20 mA
In/out of a strain gauge
Strain —> Resistance
Thermocouple
Temperature Difference —> Output Voltage
Bourdon Tube
Applied Pressure —> Displacement
Accelerometer
Acceleration —> voltage
Static Transfer Function
Describes the steady-state input-output relationship of the transmitter (the relationship between input and output variables when the system is stable and does not include the dynamic or transient response characteristics of the instrument).
Output = Gain*(I1-ILRV)+OLRV
What are the advantages of current loops
- relatively insensitive to the resistance of the interconnecting lead wires
-inherent “noise immunity”
-tolerant of large variations in power supply voltages as long as the “loop resistance” is not exceeded
What is “live zero” in industrial current loops?
The 4mA is called “live zero” and verifies conductor continuity and signal polarity.
What is static error?
Difference btwn indicated (measured) value and ideal value
S.E= Measured-Ideal
