Adjusting Humidity Levels in the CApture™ Portable Fuming System

By Kelly Williams, Director of Laboratory Products

On October 06, 2011 in Articles

The quality and durability of fingerprints developed by the CApture™ Portable Fuming System can be affected by many variables including:

• The initial quality of the latent print and the surface being examined

• The environmental conditions during fuming: air temperature, relative humidity, elevation

• The quantity, quality and formulation of the cyanoacrylate (CA) used

• The temperature of the CA during fuming

• The length of time the CA fumes are released

Because of all of these variables, it is impractical to try to develop a single methodology that will produce optimal results under all conditions. If you are unsure of the operating parameters you should use in a particular circumstance, you should try one or more “trial runs” of control prints before fuming evidence.

Adjusting Humidity Levels in the CApture Portable Fuming System

The humidity level when CA fuming is critical to the quality of print development. Recent scientific tests have determined that the optimum range of relative humidity for the development of fingerprints is 70%-90% with the most high quality marks developing when relative humidity is approximately 80%.¹

It has been determined by calculation and proven by in-house testing that 1 drop of water will raise relative humidity level within the CApture Fuming Chamber by approximately 2%.² When using the CApture, determine the current humidity level and the amount of humidity that needs to be added in order to reach an 80% humidity level.

If humidity levels need to be increased determine the number of water drops to add according to the following chart.

NOTE: Never use the CApture if condensation is present on the clear chamber or on the evidence. The CA fumes will polymerize on the condensate, reducing the clarity of the chamber, and obscuring any fingerprints on materials being fumed. When using the CApture repeatedly, allow adequate ventilation after each run to prevent the buildup of humidity in the chamber.

For example, if the current humidity level is 50%, add 15 drops of water to raise the humidity level an additional 30% to reach the optimal 80% operating humidity level within the chamber.

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¹Paine M, Bandey HL, Bleay SM, Willson H., The effect of relative humidity on the effectiveness of the cyanoacrylate fuming process for finger mark development and on the microstructure of the developed marks. Forensic Science International 2011 Jun 29.
²The humidity testing was done at 75°F and an elevation of 885 ft.