L.2 - Transmitters Flashcards
What is a transducer?
> the device that performs the measurement function in a control system
> it is a device that receives information in the form of one quantity and converts it to information in the form of the same or another quantity
What is a signal conditioner?
it it used to convert the transducer output into a more useable standard form
What does a transmitter consist of (w/ details)?
1) Transducer (performs measurement)
2) Signal Conditioner (scales, filters, linearizes, amplifies, compensates)
List some typical transducers:
1) Thermocouple (temp to mV)
2) Accelerometer (acceleration to mV)
3) Bourdon Tube (pressure to deflection/motion)
4) Photo Diode (light intensity to current)
What is a transmitter?
it is a transducer that responds to a measured variable by means of a sensing element and coverts it to a standardized transmission signal which is a function only of the measured variable
Why do we care to use standard range signals?
to ensure compatibility and interoperability among instruments from various manufacturers
What are two most common signals used?
1) Pneumatic
2) Electronic
What are pneumatic signals?
the pressure of a gas used to carry signal information
How do pneumatic instruments work, what is the standard pneumatic signal range?
> they are powered by a compressed gas and use tubing to route signals between instruments
> standard range: 20-100kPag, 3-15Psig
What are electronic signals? what is the standard electronic signal range?
> electrical voltage or current carries signal information
> voltage: 1-5V, Current: 4-20mA
Define the range of an instrument:
> it is the region between the limits within witch a quantity is received, measured or transmitted
> expressed by stating the lower and upper range values
What is meant by a static transfer function?
it describes the steady-state input-output relationship of the transmitter Eqt’n
What is static error?
it is the difference between the measured value and the actual value
What is accuracy?
it is the degree of conformity of an indicated value to a recognized/ideal value
What is span accuracy?
[ (actual value - ideal value) / ideal span ]*100%
What is point accuracy?
[ (actual value - ideal value) / ideal value ]*100%
List some error types in a transducer system:
1) Bias error
2) Span error
3) Repeatability
4) Resolution
5) Linearity error
What is a bias error?
it is the actual output - ideal output at the min. range value/LRV
What is span error?
[ (actual input span - ideal input span) / (ideal input span) ]*100%
What is linearity error?
it is the deviation from ideal after the bias and span errors are removed
What is repeatability?
the ability of an instrument to produce the same measured value of a fixed given input on a number of independent trails (assuming all other variables remain constant)
What is resolution/precision?
the smallest measurable change of the input or output variable
What is the term called for a transmitter starting at 4mA instead of zero?
Live zero
Why do we star the transmitter at 4mA instead of zero?
In case of a broken wire or any breakage, so that we have a value other than zero to record
