Calorie (th) per Minute per Square Centimeter [cal(th)/(min·cm²)]

The unit calorie (th) per minute per square centimeter, written as cal(th)/(min·cm²), measures heat flux density or thermal power per unit area using the thermochemical calorie (1 cal(th) = 4.184 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is often used in laboratory experiments, solar energy studies, and material testing, particularly when small surfaces and longer exposure times are involved. Applications include:

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  Solar radiation measurements on small panels or surfaces

  
  
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  Thermal testing of materials over extended periods

  
  
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  Calorimetry experiments on small samples

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(th)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

Although SI units such as W/m² are standard, cal(th)/(min·cm²) remains useful in historical calorimetry, laboratory experiments, and solar radiation studies, providing an intuitive measure of energy transfer per minute per small surface area.

Watt per Square Inch [W/in²]

The unit watt per square inch (W/in²) measures heat flux density or power per unit area, representing the amount of energy transferred through 1 square inch of surface every second. One watt equals 1 joule per second, so 1 W/in² corresponds to 1 joule of energy passing through 1 in² every second.

This unit is commonly used in high-intensity applications with small areas, such as:

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  Laser beams or focused light sources

  
  
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  Electronic devices to measure power density on chips

  
  
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  Industrial heating processes with concentrated energy

  
  

Heat flux can be expressed as:

where q is heat flux in W/in², Q is energy in joules, A is area in in², and t is time in seconds.

Conversion to SI units:

While W/m² is the SI standard, W/in² is convenient in U.S. engineering, electronics, and laboratory experiments involving small surface areas, where reporting flux per square inch provides an intuitive measure of concentrated energy.