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Continuous Monitors: What’’s Better – Single-Wire Impedance or Dual-Wire Resistance Monitors?

Dual-Wire Continuous Monitor

Fred Tenzer on Continuous Monitor History and Preference

By Fred Tenzer

First off, both technologies work fine, but one needs to understand the technologies to understand the shortcomings of each. Let me start with the following basic historical information that will make my “preferences” more clear. If you choose to just know the answer, scroll to the bottom, but I believe this information will be very helpful to you in making your decision. All the technologies will also monitor the worksurface ground circuit and while there are some differences, the worksurface part of the monitoring system is NOT discussed below.

History

When people realized that Operator Grounding was the foundation of an ESD Control Program, wristbands and coil cords came into big time use. The first ESD Association Standards meeting was in 1982 and the first standard written produced by the ESD Association was for wrist straps. The weakest part of the system was the coil cord and testing was developed for bending or flex life testing. The minimum flex life was established at 16,000 flex cycles and in the mid-1980’s that was tough to achieve.

Thus, monitoring technology was initially developed to detect initial flex fatigue while it was still in the “intermittent” stage, which is prior to a permanent open being created. Touch testing would almost never detect this “intermittent” failure mode. In addition, if a wrist strap system was touch-tested twice a day and an operator passed at 1:00 PM on Monday and failed at 8:00 AM on Tuesday, all the work that had peen performed at that station after 1:00 PM on Monday would now be suspect and would be a cause for more detailed quality inspections by many companies. Therefore, while discovering an operator grounding problem was good, it was also costly due to increased Quality Control. Thus, monitoring of the “operator ground system” grew in customer desirability and has resulted in technology improvements by some of the manufacturers and inventors of monitoring technology.

Single-Wire Monitoring Technology

Originally, simple “AC capacitance” single-wire monitors were developed. There were many shortcomings of this technology, all stemming from mostly “false negatives” (unit indicating the operator was grounded when he was not) and “false positives” (alarms going off when they shouldn’t). This technology is still around today and is purchased by some because of its low cost, around £25-£35 per operator and a lack of knowledge by the End User. A big plus is being able to use any standard single-wire wrist strap.

Wave Distortion Single Wire Continuous Monitor

The next level in single-wire evolution was “impedance” technology. However, since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat etc. these monitors often have to be adjusted or tuned to a specific installation and operator. Again, there were resulting “false positives” and “false negatives” though this was an improvement over the simple “capacitance” technology. This technology is also still around; the driver is low cost, £30 -£45 per operator.

The top of single-wire monitoring technology is called “Wave Distortion”. What this technology looks at is not the impedance level, but at the waveform generated by the circuit. Current will leak voltage at various points due to the combinations of resistance and capacitive reactance. There is a negligible amount of inductive reactance from the coil cord. By monitoring these “distortions” or phase shifts the Wave Distortion Monitor will determine if the circuit is complete i.e.; the wearer is in the circuit and the total equivalent DC resistance is within specifications given a range of installations. This technology is very reliable, (virtually no “false positives” or “false negatives”) and response time is very fast (<50 ms). In addition, the wrist strap open circuit test voltage is very low at 1.2 volts peak-to-peak @ 1-2 Micro Amps. Thus, a very low voltage is applied to the operator. The cost for this technology is £75 – £90 per operator.

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Introducing Jewel® Workstation Mini Monitor

  • NEW Improved Banana Jack
    Creates a more consistent connection and helps to prevent accidental disconnects with operator’s wrist strap banana plug.
  • Replaces Item 99130
  • Can be used with any brand of single-wire wrist strap and cord
  • Single station Continuous Monitor for operator and ESD worksurface
  • Made in the United States of America
Item Description Price
99135 Jewel® Workstation Mini Monitor, 220VAC $132.09
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