Meters & Instruments

Q: Is there a simple continuity tester available?

A: Yes. There are different models in the market and their prices vary according to their sophistication. Basically are pen-type devices which can be applied by touching an object, they are connected by a multi stranded cable to a ground point by means of an alligator clip. Some provide multiple usages by just replacing a couple of components, as follow: There is the type which is used to test non powered objects and carry a couple of pen size batteries (AAA), has a small lamp which will light up if the resistance encountered is less or about a specified threshold. Without batteries they can test continuity from live objects in different ranges, depending of the voltage range of the lamp used eg </= 100V AC/DC or </= 600V AC/DC, etc… There is a similar pen size tester which uses a 2KHz sound signal to warn of continuity, it uses own battery and also connects through a cable and clip to ground. For a more special continuity tester, use two probes and provides audio warning, a range of resistance up to some 30Ω and using very low voltages (fraction of a volt) but high current, to use in troubleshooting PBAs, these units are more expensive but they will not endanger or bias delicate components.

Q: What is involved in a continuous monitoring meter?

A: These devices are mains powered and usually form part of the gear employed in a workstation as to satisfy ESD standards and their intention is to continuously monitor the ground paths for the workstation and/or individuals, should the operator’s ground cord fail whilst connected to the monitor and/or the surface where installed onto, it will raise audible and visual LED alarm indications, the idea is not having to implement scheduled/periodic managed tests, the unit usually just snaps into a couple of 10mm sockets attached to the work surface, the monitor uses the AC ground as test reference. There are a few technologies available to apply eg double wire provisioning and wave distortion monitoring. The pulse testing monitoring method is sometimes applied onto double wiring but is considered not to be fully compliant with international standards as a current pulse is sent around the loop covering only part of the time and there may be ESD discharges in between pulses, standards specify constant monitoring. The wave distortion is a technique based in the phase shift of the voltage/current waves.

Q: How a surface resistance and resistivity tester works?
A: This is a megohmmeter, normally a digital instrument with a LCD display or a display of LEDs at different surface resistance and resistivity ranges, the latter covering de conductive eg 10E2 to10E4, the antistatic dissipative eg 10E5 to 10E11, and insulating ranges over 10E11.Typically the meter is used to measure resistance in between a test point on the surface to ground (RTG), between two test points on the surface (RTT) [these measurements must comply with EOS/ESD-S4.1]. The measuring of resistance from a surface point to ground is done by using a clip probe, or something similar, as to connect to ground and a special surface probe which facilitates the connection of the second electrode (this is a 5 pounds cylindrical weight treated with conductive material like carbon). The measurement of resistance in between two surface to surface points is done with two of the above mentioned special probes.

Resistivity on flat homogeneous surfaces (also on multilayered surfaces noting RH and temp at the time of measuring) can be measured quickly by use of a couple of linear electrodes fitted under the tester’s box, parallel to each other in two of the sides of a square (dimension does not matter). Current is made to flow in between the electrodes’ surface giving a resistivity value in Ω/sq and the test complies with ASTM D-257.

There are usually two ranges that can be used, this depends of the surface resistivity to measure, if a more or less conductive surface then the 10V battery feature is to apply and the range may be from 10E3Ω/sq to 10x10E6Ω/sq, if dissipative, then the 100V feature is to apply from about 1MΩ/sq to 1000GΩ/sq. The test goes for no longer than 20s so a 100V amplified voltage can be generated from low battery and sustained for some time. The meter may use battery or a power AC adapter.

Q: What is a Electrostatic field meter?

A: This instrument could be analogue or digital and is a portable device which is utilised to measure charges field on materials, people or machinery, it also assesses the polarity, this without touching the object, a common range in a field meter is +/-20KV to some +/-30KV, at a known distance from de object, say 1in, at a manufacturer’s specified tolerance, typically the box is made of a conductive material. Usually provides buttons for the hold and zeroing functions. As the distance from the measured object is difficult to assess a mechanism may be provided to aid. Grounding of the meter is achieved by the user using a wrist strap connected to ground with the same hand which is holding the device and operating the buttons as to do the measurements. They use batteries to operate.

Q: How a touch tester works?

A: A touch tester is typically a box which is powered from the mains through a low voltage power pack. The local earth (green/green-yellow) is taken from the mains outlet and passed onto a wrist strap banana socket. The wrist strap grounding cord to be tested is plugged into the banana socket whilst is attached to the wrist of the wearer. When the wearer touches a plate provided on top of the meter with the wrist strap on, a measurement of resistance in the loop is made by the meter, if the resistance found is in the range of 800KΩ to 8MΩ then the test of the wrist strap passes, if out of this range then an alarm is produced.

Q: What is in a wrist strap and footwear station?

A: A battery powered test station is installed on a plate that goes on a wall somewhere convenient on a floor allowing for staff themselves to check their wrist straps and ESD footwear. There is a tester that can do tests in a couple of ranges, there is a banana jack to insert the grounding cord of the wrist strap once the wearer touches a small metal plate on the instrument current passes through the cord, wrist strap and hand loop to the plate, and alarm (LED and beep) is raised should the resistance be found to be over/under a threshold. The ESD footwear, say heel and toe straps are measured similarly through a metal plate supplied which is located over the floor. This test station is quite convenient as to comply with current standards advising of daily testing.