Field Formulas
The relationships behind the calculators, each with a worked example. When you want the math itself, not the tool, it lives here.
Percent of span
Where the value sits on the range. 75 on a 0–100 range is 75%.
Percent from mA
12 mA reads 50%. A live 4 mA is 0%, not a dead loop.
Linear scaling
Any span to any span. 50 °C on 0–100 °C maps to 12 mA on 4–20.
3–15 psi (pneumatic)
The old standard. 50% is 9 psi; 0% = 3 psi, 100% = 15 psi.
Square-root extraction
Flow follows the root of differential pressure. 50% DP is 70.7% flow. Extraction tool →
Flow signal from DP signal
12 mA of linear DP becomes 15.31 mA of flow.
Flow vs differential pressure
Double the flow, quadruple the DP. Why a DP cell needs the root taken before it reads flow.
Error, percent of span
Reads 12.1 mA where ideal is 12 on a 4–20 loop: +0.625% of span. Worksheet →
Turndown (rangeability)
A cell good from 10 to 250 inH2O has a 25:1 turndown.
Zero & span
Zero moves the LRV up or down; span sets how wide the range is. Adjust zero first, then span, then re-check zero.
RTD, Pt100 (IEC 60751)
A = 3.9083×10⁻³, B = −5.775×10⁻⁷, R0 = 100 Ω. At 100 °C, 138.51 Ω. RTD tool →
Thermocouple, cold junction
The meter reads the difference. Type K hot 300 °C, terminals 25 °C ≈ 11.2 mV. TC tool →
Celsius & Fahrenheit
100 °C = 212 °F. A 1 °C step is a 1.8 °F step.
Ohm's law & power
Across a 250 Ω HART resistor, 12 mA drops 3.0 V. Loop powered, two wires, one signal.
Loop burden
At full 20 mA, every 250 Ω needs 5 V of headroom. Add the wiring and the sense resistors before you trust the supply.
Examples are rounded for reading. The calculators carry full precision. Confirm against your facility's procedure and reference standard.
Still not reading right?
If the numbers here do not match what the loop is doing, the problem is usually the installation, the ranging, or the process itself rather than the device. Describe the symptom and a specialist answers by email, at no charge.