Prostat CVM-780 - Page 15

15
Rev. D / January 2015
CVM-780 Contact Voltmeter

When the probe p is not in contact with ground or making a dened measurement, the
measurement system is “oang”. When oang free of a contact measurement any indi-
cated displayed voltage is not meaningful. (See Instrument Maintenance, below.)
Tip Selecon
Tip selecon is a maer of praccality, but will inuence measurement ac-
curacy. Typical measurement error is quite small regardless of p selecon:
≤ ±5%, ±2 Counts from ±25 to ±525 Volts
≤ ±10%, ±2 Counts from 0 to ± 25 Volts
However, performance accuracy will vary within these tolerances depending
on p material and p length. Comparave measurements indicate the fol-
lowing general guidelines for p performance.
Metal Tips Largest deviaon from reference Small ESD/RFI
Ceramic Tips Smallest deviaon from reference No ESD/RFI
Long Tips (0.9 – 1.3”) Largest deviaon from reference
Short Tips (0.4 – 0.6”) Smallest deviaon from reference
Consequently, the least accurate p is a long metal p; while the most accurate is a short ceramic p. In
all cases, accuracy will also vary based on technique, ambient condions, ground reference quality and
other variables discussed below.
Measurement Error
When measuring a constant, precision DC voltage reference, error is typically 1 volt or less, but always
equal to or less than ±2 volts. The circuit is capable of measuring the instrument’s enre voltage range, as
follows:
 
1A 500 V ≤ ±5 V
2A 100 V ≤ ±2 V
3A 50 V ≤ ±2 V
1B 10 V ≤ ±1 V
This procedure conrms the ability of the CVM-780 to measure DC voltage accurately, as would be ex-
pected of any good quality DC voltmeter. Variables within the error range are usually aributed to manual
zeroing of the instrument to ground. Indicaon of praccal measurement performance error in process
applicaons is a dierent maer and most important.
There are many variables in the manufacturing environment that will inuence measurement perfor-
mance of a contact voltmeter. To minimize those variables and esmate instrument measurement er-
ror, contact measurement of a charged precision 20 pF capacitor is made while the capacitor’s voltage is
monitored by a second high impedance contact voltmeter. The procedure creates a stable contact voltage
measurement situaon, as follows: