Prostat CVM-780 - VIII. Capacitance & Accuracy; Capacitance & Accuracy

17
Rev. D / January 2015
CVM-780 Contact Voltmeter
Understanding these variables and the process condions heightens the value of the operator’s measure-
ments.
Metal Tips
Metal ps are far more robust than ceramic materials and less prone to damage. These ps are provided
for general audit and analysis measurements of equipment, chairs, carts, material handling items, tools,
xtures, etc. where many voltage measurements may be required, but close tolerance accuracy, minor
voltage discharge and low level RF generaon are not major factors. Metal ps should be used in these
applicaons to minimize use and potenal damage to ceramic ps.
Note that when using metal ps to measure conductors the slight dierenal voltage between the p
and the conductor at the moment of contact will result in some electron transfer. This, in turn, creates a
very small ESD event, and results in low level RFI signal being generated. These events are not harmful in
all but the most sensive environments, are dicult to see and usually not measureable in most applica-
ons. However, this is why metal ps are recommend for general audit measurements and ceramic ps
for ESD sensive device and assembly measurements.
Ceramic Tips
Ceramic ps provide the most accurate contact voltage measurements, and do so without causing a mea-
sureable ESD event or the resulng generaon of RFI. These materials are compounded and formed to
provide a controlled transfer of electrons upon contact with a conducve object. They are ideal for direct
contact measurement of ESD sensive devices and assemblies, or in areas and situaons where any ESD
or RFI may pose a problem.
Unfortunately, the ceramic ps are fragile, quite brile and can be easily damaged if one is not careful. In
addion, replacements are costly. When making general measurements use the metal ps and preserve
your expensive ceramic ps for crical device and assembly applicaons.
VIII. 
Actual accuracy is dependent on the user’s steady and reproducible technique, and environmental dy-
namics. In parcular, capacitance (C) of the item being measured relave to ground and other nearby
objects. This is based on
Q = CV
And,
Q = V
C
Where,
Q = Charge on the object
C = Capacitance of the item under test related to its size and distance to ground and other objects
V = Resulng voltage on object
Thus, any nearby movement of equipment components, personnel or objects may inuence the item
under test’s capacitance and subsequently its voltage. In a relavely non-dynamic environment measure-
ments are very accurate and stable. As a guideline, dene the measurement environment as the operator
and objects within 1 to 2 feet of the item being measured.
The eect of capacitance can be easily demonstrated. Apply voltage to the oang plate of a Charge Plate
Monitor (CPM). A standard six inch CPM will have a capacitance of approximately 20pF. While observing
the CPM’s plate voltage move your hand slowly back and forth near the surface of the oang plate. If