Converts between loop current, percent of span, and engineering units for instrument loop checks — linear and square-root (DP flow) characterization, with the five-point checkout table. Runs entirely offline; inputs persist in this browser via localStorage.
Use limits: This does the scaling arithmetic. The transmitter's configured range (read it from the device, don't trust the datasheet), damping, and whether square-root extraction happens in the transmitter or the PLC are configuration facts you verify on the loop — a perfect calculation against the wrong LRV/URV is still the wrong answer. Loop checks follow the approved loop sheets and the commissioning plan.
Five-Point Checkout Table
The standard 0/25/50/75/100% stroke for the loop check — simulate at the transmitter, read at the PLC/SCADA, sign the loop sheet.
%
mA
PV
How the Numbers Are Derived
Linear scaling
% = (mA − 4) / 16 × 100
PV = LRV + (mA − 4)/16 × (URV − LRV) and inverted: mA = 4 + 16 × (PV − LRV)/(URV − LRV)
Works for elevated and suppressed zeros (LRV ≠ 0, including negative LRV for vacuum or bidirectional flow) — the span is just URV − LRV, signed.
Square-root (DP flow) scaling
Differential pressure across an orifice or venturi varies with the square of flow, so when the transmitter sends raw DP as a linear signal, flow is recovered as:
PV = LRV + √((mA − 4)/16) × (URV − LRV)
The consequences on the checkout table are the part that trips people: 50% signal (12 mA) is 70.7% of full flow, and 25% signal (8 mA) is 50% flow. If the SCADA readback during a loop check matches the linear column instead of the square-root column, the extraction is configured in the wrong place (or twice).
Fault bands (NAMUR NE 43)
Readings outside the live 4–20 range are diagnostic, not data:
3.8 – 20.5 mA — valid measuring range including linear over/under-range margin.
≤ 3.6 mA — transmitter signaling failure (downscale), or a wiring/power problem.
≥ 21.0 mA — transmitter signaling failure (upscale).
0 mA — open loop: broken wire, blown fuse, dead power supply, or the meter is in the wrong spot.
The tool flags any converted mA reading in these bands rather than pretending it's a process value.
Field notes
Measure current in series, or read the voltage across the loop's 250 Ω resistor and divide (12.00 mA ↔ 3.000 V) — no need to break a running loop.
Confirm LRV/URV from the transmitter's own display or HART, not from the instrument index — ranges get changed during startup and the paper lags.
Level transmitters mounted below the tank connection have an elevated-zero offset; that's what a negative or nonzero LRV is for.
If SCADA and the field reading disagree by a consistent scale factor, someone scaled 0–100% where engineering units were expected (or vice versa) at the PLC — the five-point table finds this in one pass.
References
ISA-51.1 — Process Instrumentation Terminology (ranges, spans, elevated/suppressed zero). https://www.isa.org/
NAMUR NE 43 — Standardization of the Signal Level for Failure Information of Digital Transmitters.
Project loop sheets and instrument index — the governing configuration documents.