Below are some random notes on shooting continuity devices, not a comprehensive design guide. Use or reliance on the information and materials contained herein is entirely at your own risk.
Blasters check the continuity of their cap circuits with a blasting galvanometer, now sometimes called a blasting ohmmeter or blaster's multimeter. The Institute of Makers of Explosives (IME) defines these meters as follows: "Blasting Galvanometers and Blasting Ohmmeters are electrical resistance measuring devices designed specifically for the testing of electric detonators and blasting circuits. They may be powered by either a silver chloride cell or a dry cell with special current limiting circuitry so that the test current is below specified limits. Blaster's Multimeters are versatile, multipurpose test instruments designed to measure resistance and voltage in electric blasting operations. They can be employed for measuring stray current."
Most logging trucks are equipped with some sort of shooting line continuity meter. SIE's old design used a pair of mercury batteries, while the old GO / GOI / MLS design used a silver chloride cell. The GO / GOI / MLS APM 401 Auxiliary Patch Module contains a silver chloride cell based continuity checker circuit. If you need a replacement silver chloride battery, Blasters Tool & Supply Company (800-634-6250) has them at $18.48 each (their part number 14824). Silver chloride cell designs are supposed to be inherently safe, but there are designs floating around that should be modified to insure proper current limiting. At the very least, the current limiting should be in the form of two resistors, one on each side of the battery (even silver chloride cells). Here are schematics of the GO / GOI / MLS and SIE updated designs. Do not replace silver chloride cells with regular carbon zinc or alkaline batteries without modifying the current limiting circuitry. Do not attempt such modifications unless you know what you are doing.
On March 1, 1994, the American Petroleum Institute (API) published "Recommended Practices for Oilfield Explosives Safety," API Recommended Practice 67 (RP 67). The section on surface equipment states that the test current from the meter used to perform resistance checks shall not exceed 25 milliamperes or 10% of the no-fire rating of the detonator in the circuit, whichever is less.
The approximate no-fire current for the old non-resistorized Dupont E-2B was 0.2 amps. The Ensign-Bickford EB-140 (a resistorized E-2B), EB-85, EB-105, EB-109, EB-153, and EB-161 all carry a published no-fire current rating of 0.2 amps and an all-fire current rating of 0.8 amps, but a review of the current sales literature reveals some manufacturers with caps having no-fire current ratings as low as 0.15 amps, though these are not commonly used in oilfield applications . Accordingly, the current should be limited to around 20 milliamps for oilfield blasting galvanometer / ohmmeter / continuity meter applications. It is also strongly recommended that only resistorized detonators / blasting caps be used since they are inherently safer than the old non-resistorized caps previously commonly available.
If you have any doubt about the safety of your blasting galvanometer / ohmmeter / continuity meter, take it out of service immediately, and have it checked by a competent service facility.
See also Shooting Panels and Shooting CCL Surface Amplifiers. If you need more information please feel free to contact us