How does an eddy current proximity probe work?

An eddy current probe generates a high-frequency magnetic field. When a conductive target (shaft) enters this field, eddy currents are induced, which change the probe's impedance. The proximitor driver converts this impedance change to a voltage proportional to the gap distance.

What is the standard sensitivity for proximity probes?

The most common sensitivity is 200 mV/mil (7.87 V/mm) for API 670 compliant systems. European systems often use 8 mV/μm. The relationship is linear within the specified range.

What is the normal gap voltage setting?

API 670 recommends a gap voltage of −10V DC (corresponding to approximately 50 mil or 1.27 mm gap) for standard 200 mV/mil probes. This centers the measurement in the linear range.

What is the linear range of a proximity probe?

For standard 8mm probes: 0.25–2.3 mm (10–90 mil). For 5mm probes: 0.25–1.3 mm (10–50 mil). For 12mm probes: 0.5–4.0 mm (20–160 mil). Output is linear within ±5% in this range.

What target material affects sensitivity?

Probes are calibrated for 4140 steel. Other materials have different sensitivities: stainless steel (AISI 304) ~0.85×, aluminum ~0.40×, brass ~0.50×, titanium ~0.60×. Material correction factors must be applied.

Free Engineering Tool — #031

Proximity Probe Gap Voltage Calculator

Calculate eddy current probe gap voltage, gap distance from voltage, and verify linear operating range. For shaft vibration and position measurement setup per API 670.

API 670Eddy CurrentGap Voltage

Results

Gap Voltage (DC)

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Gap Distance

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Linear Range

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In Linear Range?

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Effective Sensitivity

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Gap Voltage Relationship

Output is negative voltage (larger gap → more negative). Standard sensitivity: 7.87 mV/μm = 200 mV/mil.

Linear Ranges by Probe Size

Probe	Linear Range (mm)	Linear Range (mil)	Voltage Range
5 mm	0.25 – 1.30	10 – 50	−2 to −10 V
8 mm	0.25 – 2.30	10 – 90	−2 to −18 V
12 mm	0.50 – 4.00	20 – 160	−4 to −32 V

Example — Standard API 670 Setup

Given: 8 mm probe, 7.87 mV/μm, 4140 steel target

Target gap = 1.27 mm = 1270 μm → V = −7.87 × 1270 / 1000 = −9.99 V DC ≈ −10 V

Linear range: 0.25–2.30 mm → 1.27 mm is centered ✓

⚠️ Note: Non-steel targets require sensitivity correction. Operating outside the linear range causes measurement errors > 5%.