The Tooke Paint Inspection Gage is a precision tool for inspection and thickness measurement (in accordance with ASTM D 4138) of single or multiple coats on any substrate, and for micro-scopic observation and measurement of substrate and film defects.
The Tooke Gage uses an illuminated 50-power microscope equipped with a “universal scope” with a measuring reticle that accommodates measurement in mils, microns, and millimeters. The gage mounts three tungsten carbide cutting tips for precise incision of the work surface. Standard cutting tips are 1X, 2X, and 10X. (An optional 5X tip is available.) Also, a gauge can be ordered with any three tips desired, or with a single precision-ground tip and two blanks.
Pic.1. Replacement tungsten-carbide cutting tips. Available in 1X (45°), 2X (26.6°) and 10X (5.7°) configurations
Direct measurement of total coating thickness and thickness of individual coats of paint is a unique capability of the Tooke Paint Inspection Gage. Thus, in addition to routine use, it often serves as a “referee” instrument to calibrate indirect or non-destructive thickness measuring instruments.
(Note: Each universal microscope is validated before sale against a certified gage block traceable to the National Institute of Standards & Technology (NIST). An OEM calibration certificate is available for an additional fee.)
Finish Coat Thickness
Primer Coat Finish
Pic.2. Geometry of Thickness Measurement
Pic.3. Grooves of 1X, 2X, and 10X Cutting Tips
Used Tooke gauges to see microscopic cracking, a tendency for brittlenes, blistering, or other microscopic anomelies in coatings. use the surface microscop of the gauge to inspect the substrate under the coating for surface contamination, mill scale, and the quality of the abrasive blast. tooke gauges are used frequently in failure analysis.
There are many different manufacturers and models of tooke gauges available. the basic operting principles are very similar and are detailed in this section
The Tooke Gage has been used to assess sandblast cleaning work, to measure plating and paint thickness on ceramics, metal, wood, and concrete, and even to measure protective backing thickness on mirrors. It is virtually the only tool for measuring paint on plastics. The quality of the incision in the film discloses much about the characteristics of the brittleness and adhesion of the material.
Tooke gauge offers a quick, versatile method to examine coatings and take destructive measurements of coating thickness, (some models even have a cross hatch adhesion test process added to their unit)making it a portable, easy-to-use gauge.
make sure batteries, cutters, and cross hatch cutter are properly fitted into the instrument following manufacturer’s instructions.
Remove the batteries from the gauge if it remains unused for a long period of time. This prevents damage to the gauge in the event of malfunction.
- Mark the surface to be tested with a stroke of the black marker pen provided with the gauge. Ensure there is a distinct contrast between the color of the pen ink and the coating. Different ink colors may be required.
- Cut the coating at right-angles to the pen mark as follows:
-Place the gauge on the specimen with both legs or wheels in contact with the surface of the specimen (this ensures that the knife blade produces an exact vertical cut with no tilting to one side).
-Pull the gauge toward you; as you pull, apply a little pressure.
-Slight pressure is normally sufficient to penetrate through to the substrate. Heavier pressure may be required for very thick coatings and very hard surfaces.
- Position the gauge so the microscope lens is over the cut.
- Illuminate the cut.
- Look through the microscope lens and adjust the focus until the cut is clearly visible.
“Line A” is on the edge between the substrate and the beginning of the primer-coating: begin your measurement there. “Line B” marks the top of the primer coating/beginning of the top coating (light blue). “Line C” is the incision into the top coating, made easier to see by using the black marker provided with the Tooke Gage. This count divided by the tip designation (2) is the film thickness. If the result should be less than 2 or more than 20 mils, the inspector may wish to utilize 10X or 1X cutting tips respectively.
- Align the reticle scale so that the scale divisions are parallel to the cut. Note that one side of the cut has a straight edge and the other side is likely to be ragged.
- Measure the width of the cut coating (or coatings) by counting the number of reticle divisions.
To convert the width of the cut coating into coating thickness, multiply the number of reticle divisions by the resolution of the reticle given in Table 1.
Table 1. Paint Inspection Gauge Measurement Range(s)
|Paint Inspection Gauge
(Tooke Gauge Universal Scope)
|Paint Inspection Gauge (Tooke Gauge Non-Universal Scope)|
Manufacturer’s Practical Maximum Thickness Range
Always refer to the model-specific manufacturer’s instructions for detailed testing procedures.
Use the paint inspection gauge in accordance with the following national and international standards;
-ASTM D 4138
Gauges do not contain any user-serviceable components. Original factory calibration is done by setting the guide studs in precise alignment with the cutting tip. Make checks with precision-applied coating film standards.
For very high-precision work, maintain painted panels of known thickness and check the instrument against these panels periodically.
In the field, verify the cutting tips are in good condition. If the coating tears or is difficult to cut through, the cutting tips may be worn. Replace immediately before further use.
The Elcometer 121-4 gauge measures coating thickness with DFT maximum of 1600 -m(63 mils). It is fitted with a 50X microscope.
The accuracy and repeatability of instruments are highly dependent on the individual performing the test and how they interpret readings.
Question readings when they are outside of known values. Be sure to use the proper conversion factor for the cutting blade used.
Common errors when performing the test may include:
-Not enough pressure applied to cut through the coating to the substrate.
-Operator pushed gauge away rather than pulling the gauge toward self.
– Read results on the wrong side of the cut line.
-Used the wrong cutting blade for applied coating thickness on the test subject.
- Micro-Metrics Company, Tooke Paint Inspection Gage, date of access: 10 January 2017, http://www.micro-metrics.com/og202.htm
- Paul N. Gardner Company, Tooke Paint Inspection Gauges, date of access: 10 January 2017, https://www.gardco.com/pages/filmthickness/df/tooke_gauge.cfm
- Elcometer, Paint Inspection Gauge Operating Instructions, date of access: 10 January 2017, http://www.elcometer.com/images/stories/PDFs/InstructionBooks/121_4.pdf