This week’s ESD Q&A question is about Constant Monitors:
Question: What would be the the advantages or disadvantages of using a single wire or dual wire monitor?
Answer:
Single wire constant monitors are ideal for general use applications. They are cost effective and simple to use. Dual wire constant monitors, on the other-hand, should be used for very sensitive applications. They are more expensive, but dual wire constant monitors are the most accurate wrist strap monitoring technology available.
Single Wire Constant Monitors Explained:
Single wire constant monitors work on capacitance technology and use single wire wrist straps. The monitors send a signal down the wire to the wrist strap and measures for capacitance. If there is a mass indicated (approximately that of a person) at the end of the coil cord, the monitor assumes that the person is connected and puts the monitor into an unalarmed state.
Single wire continuous monitor systems are used widely, but they can be fooled. Some people have dry skin and do not make good contact with the wrist strap. The single wire monitors are not able to detect if the person is actually electrically connected. For most applications, this is not a concern. But if you have a very senstive process, you should consider using Dual Wire Continuous monitors.
Dual Wire Constant Monitors Explained:
Dual wire monitors use resistance based technology and dual wire wrist straps. The monitors send a small current through the wrist strap wire and measures resistance of the current as it returns back. Dual wire wrist straps must be used because they are specially designed to rely on the operator’s skin to close that current loop.  When this happens you can be assured that the operator is connected and that voltage is being drained through the wrist strap.  If the wrist strap breaks or the operator’s skin is dry and does not allow for the current to flow from one side of the strap to the other, the alarm will sound.
Dual wire monitoring is the most accurate wrist strap monitoring technology available. Single wire systems are very common and cost effective, but they simply detect that a person is wearing a wrist strap and put the monitor into an unalarmed state. Resistance monitors actually measure the resistance of the wrist strap in combination with the person and alarm if the product exceeds preset levels. This accurate measurement ensures reliability.
Transforming Technologies offers three constant monitor families:
- Ohm Metrics CM400 Series   – Single Wire Constant Monitors
- Spartan CM300 Series    – Single Wire Constant Monitors
- Resistance Ranger CM1600 Series   – Dual Wire Constant Monitors
All of Transforming Technologies’ Constant Monitors can be found on the web here:
https://transforming-technologies.com/product-category/test-equiptment/constant-monitors/