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Saturday, December 3, 2011

Temperature : Basics





Temperature:

A short introduction to temperature:

Temperature (sometimes called thermodynamic temperature) is a measure of the average kinetic energy of a systems particles. Temperature is the degree of "hotness" ( or "coldness"), a measure of the heat intensity.

When two objects of different temperatures are in contact, the warmer object becomes colder while the colder object becomes warmer. It means that heat flows from the warmer object to the colder one.

Degree Celsius (oC) and Degree Fahrenheit (oF) :
A thermometer can help us determine how cold or how hot a substance is. Temperatures in science (and in most of the world) are measured and reported in degrees Celsius (oC). In the U.S., it is common to report temperature in degrees Fahrenheit (oF). On both the Celsius and Fahrenheit scales the temperature at which ice melts (water freezes) and the temperature at which water boils, are used as reference points.

           • On the Celsius scale, the freezing point of water is defined as 0 oC, and the boiling point of water is defined as 100 oC.
           • On the Fahrenheit scale, the water freezes at 32 oF and the water boils at 212 oF.
 
On the Celsius scale there are 100 degrees between freezing point and boiling point of water, compared to 180 degrees on the Fahrenheit scale. This means that 1 oC = 1.8 oF.

Thus the following formulas can be used to convert temperature between the two scales:
tF = 1.8 tC + 32 = 9/5 tC + 32
tC = 0.56 (tF - 32) = 5/9 (tF - 32)
where
tC = temperature (oC)
tF = temperature (oF)

Kelvin - K :
Another scale (common in science) is Kelvin, or the Absolute Temperature Scale. On the Kelvin scale the coldest temperature possible, -273 oC, has a value of 0 Kelvin (0 K) and is called the absolute zero. Units on the Kelvin scale are called Kelvins (K) and no degree symbol is used.

Because there are no lower temperatures than 0 K - the Kelvin scale does not have negative numbers.
A Kelvin equal in size to a Celsius unit:
1 K = 1 oC
To calculate a Kelvin temperature, add 273 to the Celsius temperature:
tK = tC + 273.16 
 
Degree Rankine - R :
In the English system the absolute temperature is in degrees Rankine (R), not in Fahrenheit:
tR = tF + 459.67 

Temperature Sensors - Comparing Types:

Comparing advantages and disadvantages of thermocouples, RTDs and thermistors temperature sensors :

Attribute
Thermocouple
RTD
Thermistor
Cost
Low
High
Low
Temperature Range
Very wide
-350oF
+3200oF
Wide
-400oF
+1200oF
Short to medium
-100oF
+500oF
Interchange ability
Good
Excellent
Poor to fair
Long-term Stability
Poor to fair
Good
Poor
Accuracy
Medium
High
Medium
Repeatability
Poor to fair
Excellent
Fair to good
Sensitivity (output)
Low
Medium
Very high
Response
Medium to fast
Medium
Medium to fast
Linearity
Fair
Good
Poor
Self Heating
No
Very low to low
High
Point (end) Sensitive
Excellent
Fair
Good
Lead Effect
High
Medium
Low
Size/Packaging
Small to large
Medium to small
Small to medium

Article Source : www.engineeringtoolbox.com


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Basics of Instrumentation & Control


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