M. C. Miller IR-Free Coupons

IR-Free Coupons can be used to determine the true polarized potential of a buried metallic structure without having to interrupt all sources of current flowing in the soil that have an influence on the structure-to-soil potential. As indicated in the 2004 NACE Standard (Recommended Practice) on “The Use of Coupons for Cathodic Protection Monitoring Applications” [RP0104-2004](can be obtained here), the size of a CP coupon should simulate the largest anticipated coating holiday size on the buried structure occurring in the area of the coupon (the area under investigation). Also, the NACE Standard indicates that the coupon material should be similar to the material of the structure under investigation. Finally, the Standard indicates that coupons can be in ring, cylindrical, circular-plate, and rectangular-plate shapes. M. C. Miller manufactures a variety of “standard” coupon designs for the cathodic protection industry and we also encourage custom orders.

Regarding custom coupons, the following parameters should be specified to M. C. Miller:

• Material requirement (to match the structure) – carbon steel (with desired AISI number), ductile iron etc.
• Area of exposed metal requirement (surface area of coupon metal that will be in contact with the soil)
• Coupon shape requirement – cylindrical, circular plate etc.
• Cable requirement – AWG # of stranded wire (or mm²), type of insulation (THHN, HMWPE, XLPE/PVC etc.) and color of the insulation, number of cables (one or two) and length of cables

Please contact M. C. Miller for custom coupon quotations.

M. C. Miller manufactures a variety of coupon designs as “standard” and the specifications, catalog numbers and list prices for these coupons are detailed below. M. C. Miller also manufactures a 2 inch diameter PVC coupon holder that can be used to secure the coupon. The holder with coupon attached can be conveniently cemented to a customer’s 2 inch diameter PVC tube.

Built to Last

Like all M. C. Miller products, our IR-free coupons are built to last. The wire/coupon interface is environmentally sealed to keep the steel, copper, and solder components corrosion-free, making the coupon virtually indestructible.

Easy Installation

M. C. Miller IR-free coupons can be easy installed using standard installation practices near existing test stations (see NACE Standard RP0104-2004). The M. C. Miller 2 inch diameter PVC coupon holder is a convenient way to couple a coupon with a length of 2 inch diameter PVC tubing which is typically used to shield a reference electrode positioned above the ground (inside the exposed tubing) from stray potential gradients.

Easy Polarized Potential Measurement

After an IR-free coupon has had time to polarize (after being connected to a structure under CP protection), which can take about 1 month, a reference electrode can be inserted into the top of the shielding PVC tube, contacting the compacted soil above the coupon. A high input impedance voltmeter can then be connected between the coupon (via its test station terminal) and the reference electrode. An IR-free potential reading (an accurate polarized potential) is then obtained by breaking the connection between the coupon and the structure, usually through means of a jumper, thus interrupting cathodic protection to the coupon. A coupon-to-soil potential measurement is then taken typically within two seconds of breaking the coupon-to-structure connection.