PosiTector 200 – Using Gates to Identify Density Changes

The PosiTector 200 is typically used to identify total coating or individual layer thickness within a particular measurement range.  The user defines the measurement range by adjusting the two gate settings.  Gate A is the minimum thickness limit with a default setting of 25 microns (1 mil).  Gate B is the maximum thickness limit with a default setting of 1000 microns (40 mils).  By analyzing an application within the thickness range defined by Gates A and B, the PosiTector 200 gage detects the distance from the coating surface down to the most significant density change (see Figure 1).  Such density changes are commonly referred to as “echoes”.

Figure 1 – Density Change Detection

The ability to detect density changes enables the user to make simple gate adjustments to identify or eliminate particular echoes.  For example the user may wish to quantify the coating roughness or measure the thickness of a specific layer in a multi-layer application.  The user may also need to remove undesired substrate echoes from thickness measurements.  Such requirements can be addressed by making a simple gate setting adjustment with the gage menus.

The following example is based on a sample with 8.7 mils total thickness of polymer coating on a medium density fiberboard (MDF) substrate.   The coating thickness includes 2.7 mils of surface roughness.  Figure 2 is a magnified cross-sectional view of the coating.  Figure 3 is a line diagram of the same application.

 

Figure 2 - Magnified View of Polymer Coating and MDF Substrate

 

Figure 3 - Polymer Coating and MDF Substrate

To identify the strongest echo for the sample, the user must first ensure the gate settings include the expected measurement range.  As shown in the PosiTector 200 screen capture (Figure 4), the strongest echo for this sample was the couplant-polymer interface (surface roughness) at 2.7 mils.

Figure 4 – Open Gates

To determine the total coating thickness (i.e. to identify the polymer-substrate interface), the user simply increases Gate A to read higher than the roughness measurement.  As shown in Figure 5, by increasing Gate A up to 2.8 mils, the displayed thickness reading immediately changes to 8.7 mils.  This represents the next strongest echo (total thickness) in the measurement range.  Note that a second physical reading on the sample was not required, as the gage remembers the echoes from the original measurement and displays the results accordingly. 

Note that similar increases to Gate A may be required to instruct the gage to disregard strong surface echoes caused by applications with hard (dense) coatings.

 

Figure 5 – Increasing Gate A

In the event that the strongest echo was the polymer-MDF interface (and not the couplant-polymer interface), the gage would have originally displayed 8.7 mils.  To identify the roughness, the user would simply have decreased gate B until it was less than the polymer-MDF echo.  As an example, with gate B adjusted to 7.5 mils, the displayed result would be as shown in Figure 6.

 

Figure 6 – Decreasing Gate B

A similar adjustment may be necessary to eliminate echoes resulting from the substrate.  Echoes are most likely to occur from very thin hard substrates when the combined coating and substrate thickness is within the measurement range of the gage.  By lowering gate B until it is less than the total sample thickness, the echo representing the bottom surface of the substrate will no longer appear. 

Similarly, non-homogeneous substrates may tend to create multiple echoes as each substrate density change is represented.  Such echoes may be eliminated by lowering gate B to a value closer to the total coating thickness measurement.  With this adjustment, the gage will only display coating related density changes.

To verify that there are no other significant coating related echoes the user can adjust gate A above the roughness and gate B below the total thickness.  If dashed lines are displayed as in Figure 7, it is a strong indication that there are no other echoes (layers).  If a reading appears, as would be expected in a multi-layer application, it is an indication that there is another significant density change such as a primer-polymer interface. 

It is important for the user to understand that the gage will constantly attempt to search for and amplify signals within the measurement range.  This provides the gage the ability to detect gradual density changes such as wet on wet coating applications.  This also increases the possibility of the gage displaying noise from non-homogeneous coatings or air bubbles.  In both instances, readings may appear more random so it is critical to take repeated measurements to verify the existence of a layer.

 

Figure 7 – Increasing Gate A, Decreasing Gate B