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The above diagram illustrates a flat spike using the minimum number of Thermocouples to establish the flat spike. 

• Zones 1 & 2 remain the same temperature at T₁ during the period shown. 

• Zone 3 drops from T₂ to T₁. 

• Zone 4 drops from T₃ to T₂ . 

• Zones 5 &6 remain at T₃.

•  Zone 7 rises from T₂  to T₃. 

• Zone 8 rises from T₁ to T₂ .

• Zone 9 remains at T₁.

The above illustrates the furnace's ability to lose heat axially.  This establishes the maximum slope of the gradient.  To determine how many zones must be utilized to go from T₃ to T₁, use the same number of zones used in the desired flat zone (minimum of three).

The desired flat zone establishes the distance between Z4 and Z6.  The distance between the zone thermocouples is determined by dividing the desired flat zone by 2.  If the slope is used to determine the distance between zones, divide Delta T by the slope, divide by 2, and add 1 to determine the number of zones (minimum of thee and the slope must be within the furnace capability.