Btu (th) per Second per Square Inch [Btu(th)/(s·in²)]

The unit Btu (th) per second per square inch, written as Btu(th)/(s·in²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 J). It represents the amount of heat energy transferred through 1 square inch of surface every second.

This unit is commonly used in high-intensity thermal applications, engineering experiments, and solar energy studies where energy flux over small surfaces is significant. Typical applications include:

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  Heat flux from industrial furnaces or radiative heaters

  
  
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  Solar radiation on concentrated panels or small surfaces

  
  
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  Thermal testing of materials under high-intensity conditions

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in Btu(th)/(s·in²), Q is energy in Btu(th), A is area in in², and t is time in seconds.

Conversion to SI units:

While SI units like W/m² are standard, Btu(th)/(s·in²) is useful in U.S. engineering, laboratory experiments, and historical references, providing an intuitive measure of high-intensity heat flux per square inch per second.

Calorie (IT) per Hour per Square Centimeter [cal(IT)/(h·cm²)]

The unit calorie (IT) per hour per square centimeter, written as cal(IT)/(h·cm²), measures heat flux density or thermal power per unit area using the International Table calorie (1 cal(IT) = 4.1868 J). It represents the amount of heat energy transferred through 1 cm² of surface in 1 hour.

This unit is commonly used in solar energy studies, building physics, and small-scale thermal experiments where energy transfer over extended periods and small areas is measured. Typical applications include:

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  Solar radiation received on small surfaces

  
  
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  Thermal testing of materials over time

  
  
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  Calorimetry experiments

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(IT)/(h·cm²), Q is energy in calories, A is area in cm², and t is time in hours.

Conversion to SI units:

Although SI units like W/m² are standard, cal(IT)/(h·cm²) remains useful in small-scale experiments, historical solar radiation studies, and building physics references, providing an intuitive way to quantify energy transfer per hour per unit area.