Maintenance & Reliability · SOP
Building Automation System BAS Sensor Calibration Verification
This procedure outlines the steps to verify the calibration accuracy of field-mounted BAS sensors (temperature, humidity, differential pressure, CO2) against a NIST-traceable reference. Regular verification ensures energy efficiency, occupant comfort, and reliable equipment control.
Estimated Time
30 min per sensor
Difficulty
Intermediate
Regulation
ASHRAE Std. 111 / CSA Z432
Version
1.0
Purpose
To ensure all Building Automation System (BAS) sensors (temperature, humidity, differential pressure, CO₂) provide readings within manufacturer tolerance. Incorrect sensor data leads to energy waste, system short-cycling, and comfort complaints. Regular verification — at intervals specified by the plant reliability program — catches drift before it affects control logic.
This SOP applies to all field-mounted sensors wired to any BAS controller (Niagara, Distech, Siemens, Johnson Controls, etc.). Verification is performed using NIST-traceable handheld instruments. Results are recorded in the CMMS and a physical calibration tag is affixed.
Scope
This procedure covers:
- All hard-wired and wireless room sensors, duct sensors, outside air sensors, and immersion sensors.
- Typical sensor types: 10K/20K NTC thermistors, 4–20 mA transmitters, 0–10 VDC sensors, and resistive RH elements.
- Verification does not include destructive testing, sensor removal for bench calibration, or repair — those are covered under separate recalibration PMs.
- Performed quarterly on a rotating basis or when a point is flagged by the BAS analytics engine.
Safety Precautions
- If the sensor location requires opening a controller panel or working near live 24 VAC/VDC terminals, complete lockout/tagout per site LOTO procedure. Do not bypass safety interlocks.
- When accessing sensors mounted 2 m or higher, use an approved ladder or lift. Maintain 3-point contact. Do not overreach — move the ladder instead.
- Wear safety glasses and nitrile gloves when handling reference instruments near ductwork or mechanical rooms. If working in an area with moving equipment, also wear high-visibility vest.
- When using CO₂ calibration gas (e.g., 1000 ppm cylinder), secure the bottle to prevent tip‑over. Ventilate the area after calibration. Do not inhale directly.
Required Tools & Materials
- Handheld temperature/humidity reference (e.g., Fluke 971, Testo 605i, or Rotronic HC2A-S). Must have current calibration certificate.
- Differential pressure manometer (e.g., Dwyer 475-1 Mark III) with silicone tubing.
- CO₂ calibration gas kit: certified cylinder at 400 ppm or 1000 ppm with regulator and regulator-to-sensor adapter.
- Laptop or tablet with BAS front‑end software (Niagara Workbench, Distech EC‑gfx, or manufacturer tool).
- Small flat‑head screwdriver (for terminal blocks if sensor needs removal for bench check).
- Calibration tags (non‑metallic, zip‑tie style) and permanent marker.
- ServiceGrid (or other CMMS) for digital logging. (See inline ad below.)
Procedure: Step-by-Step
1
Lock out and verify zero-energy state. If the sensor is wired to a live controller (24 VAC/VDC), de‑energize at the source breaker and apply LOTO. Confirm voltage is absent with a multimeter.
2
Connect to BAS front end. Log into the BAS supervisory software. Navigate to the sensor point (e.g., "AHU-1 Return Temp"). Record the live reading, the object name, and the controller IP/point address. Note the engineer‑in‑charge.
3
Position the reference instrument. Place the reference sensor probe within 15 mm of the building sensor’s sensing element. For duct sensors, insert through the existing test port if available; otherwise, locate as close to the outside air inlet or return duct section as practical. Stabilize both readings for 90 seconds.
4
Compare readings and record offset. Read the stabilized value on the reference instrument. Calculate the difference: BAS reading minus Reference reading. Acceptable limits: Temperature: ±1.5°C (3°F) for indoor, ±2.5°C (4.5°F) for outdoor; Relative Humidity: ±5% RH; Differential Pressure: ±10 Pa (0.04 in H₂O); CO₂: ±50 ppm + 5% of reading. Document both readings and the offset on the verification form.
5
Field adjust or flag for recalibration. If the offset exceeds tolerance, check if the controller supports a field offset (e.g., NiTZOffset in Niagara). Apply a 1:1 offset if available. If no offset parameter exists, create a work order in the CMMS for recalibration or replacement. For critical sensors (e.g., supply air temp leading to freeze protection), escalate immediately to the reliability supervisor.
6
Tag and log. Attach a calibration tag near the sensor with: "Verified [date] by [tech name], offset [value]". In the CMMS (ServiceGrid recommended), enter the sensor ID, readings, offset, and action taken (accept/offset/flag). Set the next verification due date per the PM schedule.