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.
Results
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 |
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%.
A high-frequency magnetic field is generated by the probe. When a conductive target enters this field, eddy currents change the probe impedance. The proximitor converts this to a voltage proportional to gap distance.
200 mV/mil = 7.87 mV/μm for API 670 systems. European: 8 mV/μm. The relationship is linear within the specified range. Calibrated for 4140 steel.
API 670 recommends −10V DC (≈50 mil = 1.27 mm) for standard 8mm/200 mV/mil probes. This centers measurement in the linear range for maximum dynamic range.
8mm probes: 0.25–2.3 mm. 5mm: 0.25–1.3 mm. 12mm: 0.5–4.0 mm. Output is linear within ±5% in this range. Outside: errors increase rapidly.
Calibrated for 4140 steel. Correction factors: stainless 304 ≈ 0.85×, titanium ≈ 0.60×, brass ≈ 0.50×, aluminum ≈ 0.40×. Apply factor to sensitivity.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional vibration measurement and shaft monitoring solutions. Proximity probe compatible instruments. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
