“The Advanced Taser” A Medical Review”, Bleetman & Steyn from the
www.taser.com has some interesting test results and some significant errors.
The Taser device they tested was a 26 W unit (M-26) that produced peak currents of 18 amps, voltages of 50 kV, and used 11 microsecond pulses.
There is little other data presented about the actual wave shapes used, but
“Cardiac Safety of Neuromuscular Incapacitating Defensive Devices”, McDaniel et al. shows representative waveforms for a model X26 Taser provides additional details, including some actual wave shapes.
The basic shape shown is a series of 3 short pulses (the ~11 microsecond pulses referred to by Bleetman & Steyn) followed by a larger ‘pedestal’ duration pulse of approximately 50% of the peak amplitude but with a duration of ~50 microseconds. The 11 microsecond pulses have a predominate frequency of ~45 kHz, while the longer pedestal provides a frequency component of ~10 kHz.
While these may appear to be higher frequencies compared to typical biological signals, they are actually not considered high at all from an electrical engineering view. A reference is made to “skin effect” limiting the tissue penetration depth in Bleetman & Steyn.
The human body is not a ‘good’ conductor like a metal. Metals do not allow the presence of an electric field. Nor is it an insulator. Skin effects occur in conductors and limit the penetration of the electric field based on the conductivity of the material. Skin effects also occur in dielectrics based on the dielectric constant of the material.
http://niremf.ifac.cnr.it/tissprop/htmlclie/htmlclie.htm#atsftag provides a database of body tissue electrical properties as a function of frequency. This site quickly shows that there is no significant skin effect present at either of the frequencies predominant in the Taser waveform. At 45 kHz, no tissue type has a penetration of less than ~1.68 meters for CSF. Muscle shows a depth of 4.16 m. At 10 kHz the CSF again shows the shortest penetration, but it has now increased to 3.56 meters.
The current from the Taser is going to flow in an impedance defined distribution between the probes with no appreciable skin effect from frequency. The Taser frequencies are simply not high enough. Since the probes are in the skin and underlying tissue the current will tend to take the shortest path, but current does not flow in a straight narrow cylinder. The site above also has conductivity data for various tissue types, and this will have an effect on the actual current path taken. The higher the peak current the more spreading is going to occur. The ability of tissue to carry current is limited by the ion density present within the tissue. As more and more ions are involved in moving current, the dynamic impedance of the tissue appears to rise. This forces the current to expand in to a larger cross section within the body. Since the current cannot move outwards from the skin, the only path is to penetrate deeper into the body. The fact that the TASER is purported to affect nervous tissue more than muscle is noted by Bleetman & Steyn. The actual conductivity of heart muscle is ~2.9 times better than the nerve tissue at 45 kHz, and ~3.6 times better at 10 kHz.
Despite the Taser’s manufacturers data of producing p to 50 kV, the testing by McDaniel et al. was conducted at a fixed voltage of 6 kV and relied on matching delivered charge to anesthetized pigs. This assumption of charge delivered being the critical matching parameter is not justified in the paper. While charge is directly related to current (charge per time) it may not be the correct matching parameter, and the se of a lower voltage will have serious effects on the current distribution within the body.
Arguments are also made regarding persons with pacemakers being less likely to ever be involved in a Tazer incident. While this indicates a low probability of such an individual being on the receiving end of a Tazer, it does nothing to determine what the results might be if they were ‘Tazed’. Assumptions about safety could easily lead to more frequent use and this increased exposure could tend to negate the low probability assumption.
I do not have access to the schematics of a Tazer to see exactly how the device is creating the high voltage and delivering it. A typical method would be to use a transformer to step the voltage up from the initial low battery voltage and then charge a capacitor. The discharge characteristics would then be extremely dependent on the impedance of the load circuit (the human body). Remember the probes have pierced the high resistance of the skin and are applying the voltage directly to the much lower impedance internal tissue. This is very consistent with peak currents of 18 A. While the energy delivered (joules) is a fraction of a typical external defibrillator, the defibrillator is designed to force complete depolarization of the cardiac nerves and restore a normal sinus rhythm. Values below this will have differing effects. McDanieal et al. has a ‘safety index’ defined as the ratio of the discharge to induce ventricular fibrialtion (VF) to the discharge of an NMI device. The attempt at a linear fit between this safety index and the pig’s weight is at best poor, and indicates other confounding factors are likely present in the experimental data.
AI is in typical fashion over reacting, but the safety of the Tazer across a wide population does not appear to be clearly established. It is obviously safer than a firearm, but it is not ‘completely safe’ and this should be taken into account when establishing protocols for the use of the Tazer.
And just as a note, wavelength is not the factor that would be of any importance. I believe you are actually speaking of the period of the delivered energy. This is the type of confusion created by the sloppy use of terms.
