Products > Electrostatic Voltmeters > Non-Contacting

Model 325 High-Sensitivity, DC Stable ESVM

The Trek Model 325 Electrostatic Voltmeter is a versatile instrument used for performing noncontacting electrostatic voltage measurements in applications which include contact potential (surface work function) determination, materials evaluation and electret studies. The Model 325 was specifically designed for high sensitivity applications to allow highly accurate, low noise, noncontacting measurement of electrostatic voltages of 0 to ±40 V over a wide range of probe-to-surface distances.

Special features of the Model 325 allow tuning the performance of the unit to compensate for specific test conditions. A calibrated Null Voltage supply of ±10 volts nulls measured surface contact potential when measuring voltages on associated metallic test surfaces. A response Speed Control adjusts the AC response of the Model 325. A Drift/Spacing Null Adjustment minimizes the variation in monitored voltage values as the probe to test surface spacing changes.

A dual range front panel 3½ digit LED display resolves ±10 mV. The measured voltage can also be monitored through a precision 1:1 voltage monitor output and/or a 10:1 voltage monitor output.

Trek’s patented low impedance probe sensor assures measurement accuracy which is essentially independent of probe-to-test-surface spacing while eliminating the external environmental effects of high humidity and contamination, such as airborne dust, toner, ions and chemicals, on measurement accuracy.

The Model 325 can be operated as a bench top unit or, with optional hardware, in a half 9½-inch or standard 19-inch rack.


Show/Hide Product Technical Information


All specifications are with a Model PD1216P probe with a probe-to-surface separation of 0.4 mm.

Measurement Range
0 to ±40 V DC or peak AC.

1 mV.

Voltage Monitor Output

Better than ±0.05% of full scale.
Voltage Display
Better than or equal to ±2 counts,
referred to the voltage monitor.

Speed of Response (10% to 90%)
At Fastest Speed Setting
Less than 3 ms for a 10 V step.
At Slowest Speed Setting
Less than 5 ms for a 10 V step.

Noise (typical)
Less than 1 mV rms,
referenced to measured voltage.

Drift with Time

Less than 50 ppm/hour, noncumulative.
Drift with Temperature
1:1 monitor output

Less than 50 ppm/°C.
10:1 monitor output
Less than 100 ppm/°C.


During operation, the metallic surfaces of the probe are driven to voltage levels equal to the measured surface voltage value. A probe holding fixture, should be of a insulating material with dissipative properties (107 to 1011 Ω/square) to prevent shorting of the probe to ground. Phenolic or other dissipative type material is recommended to avoid any fixture material charge retention.

Recommended Probe-to-Surface Separation
0.2 mm to 2 mm.

Model PD1216P Sensor Probe
Aperture Orientation

Side viewing.
Aperture Size
4.6 mm (0.18").
Probe Body Type
Round body.
10 mm Diameter x 56 mm L
(0.4" Diameter x 2" L).

Probe Cable Length
2743 mm ±127 mm (9 ft. ±5").


Null Voltage Source
A calibrated 10-turn dial representing a 10 volt supply, with switch selectable polarity is used to produce zero volts output when the probe is coupled to a known zero volt surface. Also used to null contact potentials on surfaces relative to the probe’s gold sensitive electrode as a reference.
±10 volts
10 mV.

Voltage Display
3½ digit LED display.
Switch selectable for ±10 V or ±40 V full scale.
Resolution 10 V Range
0.01 V.
Resolution 40 V Range
0.1 V.
Zero Offset
±2 counts, referred to the voltage monitor.
Sampling Rate
3 readings per second.

Voltage Monitor Output (1:1)
A buffered 0 to ±40 V BNC output provides a replica of the measured voltage.
1:1 of the measured voltage.
Output Noise
Less than 1 mV rms (measured using the true rms feature of the Hewlett Packard Model 34401A digital multimeter).
Output Current
5 mA.
Output Impedance
100 Ω, nominal.

Voltage Monitor Output (10:1)
A buffered 0 to ±4 V BNC output provides a low voltage
replica of the measured voltage.
10:1 of the measured voltage.
Output Current
5 mA minimum.
Output Impedance
0.1 Ω, nominal.

Response Speed Control
A front panel potentiometer that adjusts the AC response of the Model 325.

Drift/Spacing Null Adjustment
This back panel adjustment minimizes the variation in monitored voltage values as the probe to test surface spacing changes.


108 mm H x 223 mm W x 370 mm D
(4.25" H x 8.75" W x 14.5" L).

3.6 kg (8 lb).

Voltage Monitor Connectors
BNC coaxial connectors.

Ground Receptacle
Banana jack.

AC Line Cord Receptacle
Standard three-prong line cord with integral fuse holder.

Power Switch
A rear panel power ON/OFF rocker switch.

Line Supply
90 to 127 V AC at 48 to 63 Hz
(optionally available at 180 to 250 V AC at 48 to 63 Hz).

Operating Conditions

0 °C to 40 °C.
Relative Humidity
To 85%, noncondensing.


TREK, INC. certifies that each Model 325 is tested and calibrated to specifications using measurement equipment traceable to the National Institute of Standards and Technology or traceable to consensus standards.
Low Voltage Safety Compliance
(EN 61010-1)
Overvoltage Category
CAT II: Local-level mains, appliances, portable equipment.
Pollution Category
Degree 1: Operate in environments where no pollution or only dry nonconductive pollution occurs.

All specifications are subject to change.



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