THE MG8-11 CO2 SENSOR MODULE APPLIED TO THE DrDAQ

SUMMARY

Introduction

Sensor theory & Excel calculator

Sensor signal conditioning circuit (buffer) & other hardware

DrDAQ application

CO2 Sensor operation

Additional technical information

Temperature behaviour and aging

INTRODUCTION

Measurement of CO2 concentration in air is useful in many processes. CO2 meters/sensors generally available are expensive and in many cases have a digital output which requires additional microprocessors for data decoding. The MG-811 is a solid electroloyte, cell type sensor with analog output. Its operating principle is similar to that of a pH electrode cell and its output voltage decreases with increasing CO2 concentration.

This sensor comes from China, is very inexpensive and is sold here:

http://sandboxelectronics.com/?p=147

The sensor assembly is complete with a small PC board holding the sensor’s socket, a signal conditioning circuit, a threshold detector circuit and a switching regulator to feed a stable voltage to the sensor’s internal heating element. The above mentioned page also provides some very sketchy technical information, which does not really help to achieve acceptable performance.

This post provides the missing technical information necessary for reliable and accurate measurement and recording of CO2 concentration in air using Picolog and the DrDAQ USB logger, as well as an Excel calculation tool giving complete insight into sensor’s operation.

SENSOR THEORY AND THE EXCEL CALCULATOR (1)

The MG-811 sensor cell outputs a voltage Vs inversely and linearly proportional to the base 10 logarithm of concentration of CO2 in the air present inside the cell. We define the following quantities:

Vo............This is the value Vs assumes at a CO2 concentration of 400 PPM and is the REFERENCE voltage.

Vc............This is the value Vs assumes at a concentration of 1,000 PPM (0.1% in a 99.9% air volume).

ΔVs = Vo - Vc............This is the REACTION VOLTAGE and defines the slope of the PPM Vs. Voltage Output curve.

Since we have an inverse relationship, Vo is by definition the maximum value possible, so that ΔVs is always positive.

C............This is the CONCENTRATION of CO2 expressed in PPM (parts per million).

The MG-811 is capable of measuring C in the range 400 – 10,000 PPM (0.04% to 1%). Unfortunately the output voltage reference Vo varies from sensor to sensor in a range of 200 to 600 mV and accurate calibration is necessary. To define the slope of the Concentration Vs. Output voltage curve, the sensor’s output Vc at 1000 PPM must also be calibrated. When these two parameters are known, the operational range of the sensor is fully calibrated and defined by the following equation:

Vs = 400 + (ΔVs / ((LOG (400) – LOG 1000) * (LOG (C) – LOG (400))) [1]

Vs = 400 - ( ΔVs / (0.398 * (LOG (C) -2.60206) [2]

Where Vs and ΔVs are in mV, C is in PPM and LOG is the natural logarithm (Base 10).

Any MG-811 sensor can fall into or near a Vo reference, approximating all other values as shown below:

MINIMUM:……….Vo=200 mV………. ΔVs=30 mV………Vc=170 mV……….Vs @ 10,000 PPM = 94.6 mV

MEDIUM:…………Vo=400 mV………. ΔVs=60 mV………Vc=340 mV……….Vs @ 10,000 PPM = 189.2 mV

MAXIMUM:………V0 =600 mV……... ΔVs=90 mV………Vc=510 mV……….Vs @ 10,000 PPM = 283.8 mV

In a future post I will upload the Excel tool "MG-811 calculator" which automatically computes all PPM values from Vs, for Vo minimum, medium and maximum. The enclosed PC screen describes the tool operation.