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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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