Analog Input Scaling in BAS: 4–20 mA and 0–10 V Done Right

If a pressure sensor reads 2× high or a temperature jumps around, the problem is frequently scaling, range selection, sensor type mismatch, or a wiring issue—not the controller. Use this checklist to validate signal type, confirm raw values with a meter, and apply correct linear scaling.

Problem

Analog inputs read incorrectly (offset, inverted, or clipped at min/max), or different devices read the same sensor differently.

Root Cause

Typical causes:

• Configured as 0–20 mA while the sensor outputs 4–20 mA, or vice versa
• Wrong input mode (voltage vs current)
• Wrong min/max engineering range
• Wiring error (loop power, wrong terminals)
• Thermistor curve mismatch (e.g., 10k Type II vs 10k Type III)

Solution

Step A: Confirm the signal electrically before touching software

1. Identify the sensor output type from documentation or label: 4–20 mA, 0–10 V, thermistor, or RTD.
2. Measure the signal with a meter at the controller input or test points:
   • For 4–20 mA: confirm the live current is plausible for operating conditions.
   • For 0–10 V: confirm the voltage moves with the process variable.

Step B: Validate controller input mode

Ensure the analog input channel is configured for the correct electrical mode (mA vs V), and confirm any shunt resistors or jumpers the hardware requires.

Step C: Apply correct linear scaling (4–20 mA)

The standard convention is 4 mA = 0% span and 20 mA = 100% span.

Engineering units formula:

Percent = (mA − 4) / 16
EngineeringValue = Percent × (EUmax − EUmin) + EUmin

Use your BAS scaling block or the BASidekick Sensor & Signal Scaling calculator to implement this consistently.

Step D: Apply correct linear scaling (0–10 V)

Use the same linear approach:

Percent = (V − Vmin) / (Vmax − Vmin)
EngineeringValue = Percent × (EUmax − EUmin) + EUmin

Step E: Detect range mismatch patterns fast

• If the value reads approximately 20% low at the bottom of range, suspect 4–20 mA vs 0–20 mA mismatch.
• If it pegs at max or min despite process changes, suspect wrong input mode or wiring.
• If it drifts or is extremely noisy, suspect grounding or shielding problems.

Step F: Thermistor sanity check

If using thermistors:

• Confirm the BAS is configured for the correct thermistor curve (e.g., specific 10k curve family).
• Validate by measuring resistance and comparing to the correct curve table for that thermistor type.

Example: 4–20 mA Pressure Sensor (0–300 psi)

Measured mA	Calculation	Result
4 mA	(4−4)/16 × 300	0 psi
12 mA	(12−4)/16 × 300	150 psi
20 mA	(20−4)/16 × 300	300 psi

Field Notes / Gotchas

• A common mismatch: a transmitter configured for 4–20 mA is wired to a controller input set for 0–10 V. The controller sees a very small voltage and reports near-zero values even at full process range.
• For loop-powered sensors, confirm the controller supplies the correct loop voltage (typically 24 VDC) before assuming a sensor is faulty.