Accurate CO₂ measurement is critical for applications ranging from HVAC systems to industrial safety monitoring. This guide explains how to calibrate Atomsenses CO₂ sensors using downlink commands, with a focus on the CMD C3: CO₂ Offset protocol.

Why Calibrate CO₂ Sensors?

Field calibration ensures long-term accuracy by compensating for environmental drift. Atomsenses sensors support remote calibration via downlink commands, eliminating the need for physical access in hard-to-reach installations.

Prerequisites

• Atomsenses CO₂ Sensor (Model AS-201 or later)
• Access to device serial port or LoRaWAN® downlink interface
• Atomsenses Configuration Guide (v1.04+)

Step-by-Step: CMD C3 CO₂ Offset Calibration

1. Understanding the Command Structure

The calibration command follows this format:

Header 1	Header 2	CMD	Value	Checksum L	Checksum H
0xFF	0x00	0xC3	XX	CRC Low	CRC High

Value Calculation:
Value = CO₂ Offset (ppm) / 10 in signed char (-128 to +127)

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2. Calculating the Offset Value

Example: Apply -350 ppm offset

1. Divide by 10: -350 / 10 = -35
2. Convert to signed char:
   • Decimal: -35 → Hex: 0xDD

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3. Generating the Full Command

Using our example value 0xDD:

Header 1	Header 2	CMD	Value	Checksum
0xFF	0x00	0xC3	0xDD	0xA0 0x99

Command Line:

FF00C3DDA099

4. Sending the Command

Via LoRaWAN® Downlink:

Use your network server’s payload converter:

json
{
“fPort”: 15,
“data”: “FF00C3DDA099”
}

Verification & Best Practices

1. Confirm Calibration:
   • Send CMD 0xB1: Read Calibration Status
   • Expected response: Offset: -350 ppm
2. Pre-Calibration Checklist:
   • Allow 30-min warmup in stable environment
   • Use reference gas for absolute calibration
3. Error Handling:
   • ERR 0x81: Invalid checksum → Recalculate CRC
   • ERR 0x83: Out-of-range value → Verify signed char conversion

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Advanced Configuration Options

For multi-point calibration and temperature compensation, see:
📥 Download: AtomSenses Configuration Guide v2.4

• Chapter 5: Advanced Downlink Protocols
• Appendix C: CRC Calculation Algorithms
• Troubleshooting Flowcharts

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FAQ

Q: How often should I calibrate?
A: Every 12 months for general use, 6 months in high-dust environments.

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Need Help? Our sensor engineers are ready to assist:
📧 support@atomsenses.com | 📞 +86 188-2286-3607