TIMBER PRODUCTS October 2020 HT Newsletter Volume: 10/14/2020

     Surrogate Block Protocol

When surrogate blocks are used in a heat chamber, make certain the following processes are being followed: 

Surrogate blocks must be as big or bigger than the size of the biggest pieces of wood in the chamber, including connected boards.  For example, pallets with a 2x4 stringer where an additional two 2x4’s were fastened to it for the repair, would need 4x6 surrogate blocks in the chamber to ensure the pallets with this sort of repair were properly heat treated.  Blocks that are smaller than the largest piece of wood or combined wood in the heat chamber is a nonconformance.

Surrogate blocks should be at least six inches in length with a probe hole centered on the block end or side, so that it can fit into the pallet. Blocks where probes are not centered on the block end or side are nonconforming.

Surrogate block probe holes should be drilled to represent the center of the thickest wood in the chamber and should not be too deep or too shallow.  Drilled holes should not be so large in surrogate blocks that the probe temperatures do not accurately reflect the core readings for the thickest wood it represents.  For example, a facility drills a 5/8” hole in a 2x4 surrogate block for a probe when a ¼” hole was all that was needed.  A small amount of countersinking is allowed for backing placement.  Improperly drilled blocks are nonconforming.

Surrogate blocks must not be cracked to where heat can conduct into the block and potentially increase probe temperatures.  Blocks with cracks large enough to allow heat to conduct into it are nonconforming.

Prior to use, surrogate blocks must be no hotter than the ambient outdoor temperature for the time of year they are used.  This is confirmed by reviewing the heat chamber reports.  As an example, if the ambient outside temperature is 50°F but the starting probe temperatures in the blocks are 100°F on the heat chamber report, then the blocks were not allowed to properly cool down before being re-used and this is nonconforming.  It is best to have two or more sets of blocks, so this issue can be avoided. Blocks can be painted different colors to assist the facility in making sure blocks are properly rotated on back-to-back charges.  Heat chamber reports revealing hot blocks were used is a nonconformance.

Surrogate blocks should be placed inside the pallets being treated so airflow and heating reflect the same conditions as the pallets.  Charges where Surrogate blocks are not placed inside pallets are considered nonconforming.

Any situations outside of the parameters described above is considered nonconforming.