Monroe Electronics Electrostatic Fieldmeter - Static Monitor - model 177A - Manual (Page 75)

APNE-0016 74
Calculating Web Surface Charge Density
Providing that fieldmeter readings are taken in compliance with the limiting conditions
specified above, web surface charge density (charge per unit area, q/A) can be calculated
from these readings as follows [2,4]:
Surface charge density(σ) = Electric field(E) x Permittivity of free space(ε)
e.g. for a reading of 5,000 V/in:
σ (q/A) = {5,000 V/in} x {8.85 x 10 -12 coulomb(C)/volt(V)•meter(m)} or,
σ (q/A) = 1.74 x 10
-6
C/m
2
, or 1.74 μC/m
2
where: μ = 10
-6
Relating Web Surface Charge Density To The Real World
It is widely reported in literature and texts that the maximum practical charge density a
surface can hold is about 25 μC/m
2
. Discharges to nearby conductors and surrounding air
are responsible for this limitation. It has been stated by Seaver [2] that “Industry has used
an unwritten ‘rule-of-safety’, known as the 5000 Volts Rule, to keep static charge on a web
at a reasonable level. The rule states that if the potential on a free-span of web is kept
below 5000 volts, then the web should remain free of static discharge problems within that
free-span. This rule was established when fieldmeters were usually specified to be
operated at 2.5 cm ( 1 inch) from the web.”
Referring to the charge density calculation in the previous section, where σ = 1.74 μC/m 2 at
E = 5000 V/in, Seaver [2] states “This charge density is over an order of magnitude below
the 25 μC/m
2 required for a guaranteed discharge from the web. Thus, the 5000 Volts Rule
is equivalent to keeping the surface charge density below one tenth the discharge value.”
and “The 5000 Volts Rule is a good safety rule for non-conductive webs, but it must be
remembered that the measurement needs to be made with the fieldmeter held 2.5 cm from
the surface of a true free-span of web. The 5000 Volts Rule ensures the web has a surface
charge density that is less than 2
μC/m 2 .”
Lower charge levels, while safe from discharges, may still attract contaminants and cause
contamination problems in many processes.
Continuous Monitoring Of Electric Fields In High-Risk Machine/Product Performance
Areas
Antistatic measuring and control systems are needed in practice because the conditions for
static elimination always change [10].
Permanent fieldmeters are especially necessary in hazardous areas to verify nonhazardous
web surface charge density levels. The probes are installed immediately downstream of
passive or active ionizers to make sure the ionizers are working correctly. This scenario
assumes all rollers but the gravure and impression to be outside the classified area. All
electrically powered equipment installed in these classified locations must be approved for
the solvent atmosphere in which they are located, so both the probes and ionizers must be
approved for the classified areas in which they are installed.
For details on continuous monitoring, see the next document in this series, APNE-0016,
Static Control in Web Converting.