Calibration-record arithmetic · interpolation only
Controlled Proximity-Probe Calibration Interpolator
Estimate gap from voltage, or voltage from gap, only between two approved calibration-record points for one identified probe, extension cable, driver, target and configuration.
Interpolation record
Empirical two-point straight line
b = V₁ − Sx₁
V(x) = Sx + b
x(V) = (V − b) / S
x is physical probe-tip-to-target distance in millimetres, V is the recorded driver output in volts, S is the signed local slope in V/mm, and b is the fitted voltage intercept. The inverse is allowed only when both gaps and both voltages are distinct. The worksheet rejects extrapolation.
For mil input, the exact definition 1 mil = 0.0254 mm is used. Numerically, 1 V/mm = 1 mV/µm and 1 V/mm = 25.4 mV/mil. Negative-output systems can therefore have a negative signed slope; the sign comes from the entered record, not from a hard-coded sensor convention.
| Former claim or logic | Confirmed problem and correction |
|---|---|
| V = −S × gap with no intercept | This forces 0 V at zero gap and discards the system bias/offset. The replacement fits both slope and intercept from two controlled points. |
| “Closer gap produces more negative voltage” | The former wording contradicted the published example for a common negative-output system, where the output changes from approximately −1 V near 0.25 mm to −17 V near 2.3 mm. Direction is now taken from the record. |
| Universal range by 5/8/12 mm probe diameter | Linear range is a complete-system/manufacturer property, not a universal diameter-only table. All diameter presets and green in-range verdicts were removed. |
| Generic factors for 4140, stainless steel, titanium, brass and aluminium | Target material and geometry affect the actual characteristic. Generic multipliers cannot replace a calibration for the selected probe, cable, driver, total length and target. |
| “API 670 standard −10 V target” | A manufacturer may recommend a bias gap for a specific system, but that is not a universal API formula or setpoint. The replacement has no default, preset or compliance result. |
| Partial parsing, automatic result and saved browser state | Blank explicit inputs, strict decimal parsing, submit-only calculation, controlled-record confirmation and safe text output replace implicit/tampered state. |
Manufacturer example is system-specific
The official Bently Nevada 3300 XL 8 mm datasheet, document 141194 Rev. AM describes one system made from its probe, extension cable and Proximitor Sensor. Its stated baseline conditions include an AISI 4140 target, −24 Vdc supply, 10 kΩ load and defined temperatures. For that system it publishes an approximate 0.25–2.3 mm (10–90 mil) linear range corresponding to approximately −1 to −17 Vdc, recommends approximately −9 Vdc at 1.27 mm (50 mil) for radial vibration, and specifies a nominal 7.87 V/mm (200 mV/mil) incremental scale factor with configuration/temperature tolerances.
Using only the two approximate published envelope endpoints as a numerical software test, their midpoint x = 1.275 mm interpolates to −9 V and the inverse calculation returns 1.275 mm. This is a consistency test of the interpolation and the source’s rounded example—not a downloadable preset, a replacement for the actual calibration curve, or an acceptance tolerance.
The same datasheet states that its default calibration is for AISI 4140 steel and that calibration for other target materials is available on request. Its performance statements are conditional on the identified components/configuration; they do not support generic material multipliers.
API 670 and ISO 13373
The official API Standards Plan lists API Standard 670, Machinery Protection Systems, Edition 6, dated 7 July 2025. Detailed requirements are licensed and remain NEEDS_LICENSED_SOURCE here. This page therefore does not invent an API clause, universal bias voltage, linear range, accuracy limit or compliance verdict.
The official ISO 13373-1:2002 page identifies Edition 1 as published and confirmed in 2024. Its scope gives general vibration-condition-monitoring guidance including measurement methods, parameters and transducer selection/location/attachment. It is not cited as a universal proximity-probe gap-voltage transfer table.
Sources and lifecycle checked 13 July 2026. Manufacturer limits apply only to the cited product/configuration; project requirements, the current licensed API edition and the actual controlled calibration record govern.
