Portable Wrist Strap Tester The Desco Portable Wrist Strap Tester is a battery operated “on demand” wrist strap tester. When the wearer of a wrist strap plugs in their coil cord and touches the metallic surface of the Wrist Strap Tester, a path for current flow is completed from the wearer's skin, through the wrist strap. This action will cause the green “test good” LED to illuminate as long as the resistance of the circuit path is between 750 kilohms and 10 megohms.