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Shims Q & A

What are SHIMS?

SHIMS, sometimes referred to as foils, are small polyester strips of varying thicknesses between 1 mil (25 microns) and 60 mils (1.5 mm). They measure 1 x 3 inches (3 x 8 cm.)

What are my choices?

DeFelsko offers two types of SHIMS as accessories to our coating thickness gages: CERTIFIED and NON-CERTIFIED.

What is the difference between CERTIFIED and NON-CERTIFIED plastic shims?

Although they are made of the same material and cut to the same dimensions, there is a difference in accuracy, packaging and purpose.
NON-CERTIFIED SHIMS are sold as a set of five packaged in one cardboard envelope ($10 per set of five shims). Their labeled thicknesses are accurate to within +0.2 mils (+5 microns). One set is included with every PosiTector 6000, PosiTector 100, PosiTector 200 and PosiTest DFT. They are for use with electronic magnetic, eddy-current and ultrasonic gages. (See Specifications).

CERTIFIED SHIMS are sold individually ($45 - one shim) or as a complete set of eight ($195 - eight shims). Each Certified Shim is individually serialized and labeled with its measured thickness obtained with measurement equipment traceable to NIST. They are packaged in their own plastic protective pouch. One Certificate of Calibration showing traceability to NIST is included with each shim or set of shims. Their labeled thicknesses are accurate to within +0.1 mils (+2 microns). They are for use with only electronic magnetic and eddy-current gages.

How are shims used?

SHIMS are commonly used to simulate a coating. For electronic magnetic and eddy-current gages, they are placed onto an uncoated piece of ferrous or non-ferrous metal, then measured with the gage. Shims may be used individually or stacked to create additional thicknesses.

 

Ultrasonic coating thickness gages can only measure one shim thickness at a time. The shim must be placed over a flat, rigid object such as a glass pane. The substrate material does not have to be the same material as that which is intended to be measured.

Shims provide a quick operational check of the gage by allowing the user to perform practice measurements and to make sure the gage is operating properly. They can also be used to protect a gage when measuring on soft, tacky, rough or hot surfaces (electronic magnetic and eddy current gages only).

 

Some Test Methods and Specifications call for their use when making calibration adjustments in the intended range of use, over the surface of a representative substrate material.

How accurate can SHIM measurements be?

When shims are used, resultant gage measurements are less accurate and must be recalculated by taking into account the tolerances of both the instrument and the shims.

 

When shims are used it is necessary to be aware of the possibility of additional measurement errors. Factors experienced with plastic shims which are not usually present with coated or plated calibration standards include (but are not limited to):

  • permanent creases in the shim due to bending around substrates,
  • gaps between the shim and substrate,
  • variations in the ability of the shim to mold to rough substrate surfaces is dependent on the thickness and rigidity of the shim,
  • distortion due to environmental conditions such as temperature and humidity,
  • an inherent curve in the shim which can act as a leaf spring and cause a magnetic pull-off gage to be “pushed” off the surface prematurely, resulting in an incorrect reading,
  • thickness inconsistencies over the shim surface area,
  • ridges along the shim edges caused by the cutting and trimming processes,
  • permanent “dimples” in the shim due to the pressure of the probe tip,
  • with some materials, shapes and shim thicknesses it is possible that the shim will not lie flat to the surface.

Why can I not use SHIMS with pull-off magnetic gages?

Shims, which are permissible for adjusting Type 2 (electronic) gages, should not be used for adjusting Type 1 (mechanical pull-off) gages including the PosiTest FM and PosiPen. Shims can act as a leaf spring and cause a magnetic pull-off gage to be "pushed" off the surface prematurely, resulting in an erroneous reading.

Can SHIMS be re-certified?

Yes, but the cost effective alternative is to trade them in on the purchase of new ones when they become visibly scratched or worn. Trade-in reduces the price of new CERTIFIED SHIMS by 20%. Additionally, one or more shims can be purchased separately to complete a set.