Analog Flashcards
(10 cards)
What is Thermocouple
When heat is applied to the junction of two dissimilar metals, an emf is induced at the open end.
Emf proportional to heat
The mV output from the thermocouple is too small for wiring back to most control devices , what to do?
It is usual to have a conditioning circuit incorporated into a transmitter.
The conditioning circuit produces one of the common process control signals like 4-20 mA and needs to be matched with the thermocouple.
Explain RTD :
What are they made of?
What are the replacing
What do they need and why?
Resistance temperature detectors are temperature sensors that exploit the predictable change in electrical resistance of some materials with change in temperature.
Mostly platinum so called platinum resistance thermometers (PRT).
They are slowly replacing the use of thermocouples in many industrial applications below 600° C.
They require a small current to be passed through in order to determine the resistance, change in voltage drop as resistance change with temperature is measured and used to indicate temperature.
The simplest RTD :
Uses:
Used in:
Why?
Uses two wires
Only used if accuracy is not required
Cuz the resistance of the connecting wires is always included which leads to errors in the signal
How to minimize errors in RTD
Using three wire configuration, the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the brigade
so the lead resistance cancelled out
Explain four wires in RTD ?
How
Why is it not a perfect solution
The better alternative provides?
Increases the accuracy and reliability of the measured resistance.
A standard two terminal RTD is used with no other pair of wires to form an additional loop that cancels out the lead resistance
Wheatstone bridge method uses a little more copper wire and not a perfect solution
It provides full cancellation of spurious effects and cable resistance of up to 15 ohm can behandled
Advantages of platinum resistance thermometer: (4)
1 - accuracy
2. Low drift
3- wide operating range
4- suitability for proposition operation
Limitation of platinum resistance thermometer
Parley used in applications above 660°C, above it it’s more difficult to prevent the platinum from being contaminated with impurities from the metal sheath.
That’s wing in laboratories they replace the metal sheath with glass construction
Compare between thermistors and platinum RTD
Platinum Rtd:
Less sensitive to small temperature change
Have slower response time
Thermistors:
Operate on a smaller range of temperature
What is linear variable differential transformer?
Electrical transformer used to measure the linear displacement.
The transformer has three solenoidal coils placed end to end around a tube. A cylindrical core is attached to the object who’s position ment to be measured.
