Environmental Sensor Solutions: Gas Detection

A gas detection sensor’s function is to measure the concentration of gases. These devices have different methods to convert the gas detection element’s output to voltage, meet robust 4-20 mA industrial standards and even feature modules with serial I/O pins for a microcontroller to determine the outcome of the gas measured. The gas detector output signal is typically ratio metrically proportional to the particular input gas level.

This article will discuss the physics and concepts behind various accurate gas sensors. It will also provide the designer key information regarding their proper use and implementation in order to get the maximum benefit from these sensors in a variety of applications, including air quality control, ferment process control, medical monitoring units, combustion and environmental control such as the air-to-fuel ratio in engines.

Oxygen sensors

The oxygen sensors we will discuss In this article employ a well-proven, small zirconium dioxide (ZrO₂) based element at the core.¹ This particular type of design does not require a reference gas that so many other types of sensors need. This allows the capability of the sensor to be operated under high temperature, humidity, and oxygen pressures.

Stabilized ZrO₂ exhibits two mechanisms at high temperatures (>650°C):

1. ZrO₂ partly dissociates producing mobile oxygen ions and thus becomes a solid electrolyte for oxygen. A ZrO₂ disc coated with porous electrodes and connected to a constant dc current source allows ambient temperature oxygen ions to be transported through the material. This will liberate an amount of oxygen at the anode which is proportional to the charge transported (known as electrochemical pumping), which according to Faraday’s First Law of Electrolysis is:
   
   N = it/zF (Equation 1)
   where
   
   N = Number of moles of oxygen transported
   I = Constant current
   T = Time in seconds
   Z = Ionic valence of oxygen
   F = Faraday constant = 96487 C/mol, where mol is mole, a unit of substance, and C is coulombs units of charge
2. ZrO₂ behaves like an electrolyte. If two different oxygen pressures exist on either side of a piece of ZrO₂, a voltage (Nernst voltage) is generated across it.

What is Nernst voltage?

This voltage is proportional to the natural logarithm of the ratio of the two different ion concentrations.
k_B = Boltzmann constant (kB = 1.3x10^-23 J/K)
T = Temperature in degrees Kelvin (K)
e_o = Elementary charge (e_o = 1.602x10^-19 C) where C is coulombs
c_i = Ion concentration in mol/kg where i = 1,2

Either of these properties is used in many variants of oxygen sensors and both in Honeywell sensors such as the KGZ-10 series, the GMS-10 series and the MF010 series (a Honeywell Oxygen Sensors line guide can be found on the DigiKey site here). This removes the need for a sealed reference gas making the sensor more versatile for use in a range of different oxygen pressures and temperatures.

Sensor cell construction consists of the sensing cell. The cell consists of two ZrO₂ squares coated with a thin porous layer of platinum that serve as electrodes. The platinum electrodes provide the catalyst necessary for the measured oxygen ions to be transported in and out of the ZrO₂.

The two ZrO₂ squares are separated by a platinum ring which forms a hermetically sealed sensing chamber. At the outer surfaces there are two further platinum rings which, along with the center platform ring, provide the electrical connections to the cell.

Two outer alumina (Al₂O₃) discs filter and prevent any particulate matter from entering the sensor and also remove any “unburnt” gases. This prevents contamination of the cell which may lead to unstable measurement readings.

The cell assembly is surrounded by a heater coil which produces the necessary 700°C required for operation. The cell and heater are the housed within a porous stainless steel cap to filter larger particles and dust and also to protect the sensor from mechanical damage. For a more detailed tutorial, beyond the scope of this article, please see reference 1: the Honeywell application note, “Operating Principle and Construction of Zirconium Dioxide Oxygen Sensors.”

Measurement

Honeywell’s oxygen sensors contain several output connections:

Two heater connections—the heater requires a specific voltage to ensure the correct operating temperature at the cell.

Three cell connections—a reversible DC constant current source is applied between PUMP and COMMON in order to create the electrochemical pumping action. The resultant Nernst voltage is sensed between SENSE and COMMON.

For more details regarding cell sense signal levels and setting voltage trip levels in external conditioning circuitry for response time, compensation for temperature dependence, sensitivity/slope, full step-by-step measurement and calibration procedure, initial sensor drift and active burn-in and examples, see reference 1, Honeywell application note, “Operating Principle and Construction of Zirconium Dioxide Oxygen Sensors.”¹

Interface electronics

When design time is tight (as we all know is the usual case), Honeywell has a series of three oxygen sensor interface boards that will give accurate oxygen measurement with their sensors. If it’s a custom design that is needed for the sensor control and conditioning, then the following will describe the basic building blocks necessary to create a fully functional analog interface to a microcontroller.

Heater control

The sensor will require an adjustable dc voltage, such as 4 Vdc, for example, depending upon the sensor cap. This voltage creates the correct operating temperature for the sensing cell. Because the heater inside the sensor has low resistance, a high current will be needed of at least 2 A or more so keep lines short from this voltage supply and minimize its output noise. Monitor longer lines if needed with a sensing feedback circuit close to the heater to avoid a large voltage drop.

Control circuit voltage regulation

The input supply voltage available externally will need to be stepped down and controlled. (Buck switching regulator, linear regulator, or transformer isolation if needed by using a discrete design or a modular DC/DC converter.)

Start-up delay

The sensor will need to warm up for at least 60 seconds before the control circuitry becomes activated because ZrO₂ only becomes operational above 650°C and as temperature decreases below this level the cell impedance increases dramatically. It is imperative that the sensing cell is not pumped when cold! Doing so may damage the sensor since the constant current source will try to drive whatever voltage is necessary. This delay is usually implemented in the microcontroller software but may also be done in hardware.

Constant current source

A typical 40-µA DC constant-current source is required to drive the pump side of the sensing cell. A nice, low impedance drive op amp circuit will usually be the best choice here.

Constant current source reversal¹

Connection of the constant current source between the PUMP and COMMON has to be able to be reversed whenever either of the reversal voltages is met. Analog switches are ideal for this.

Output amplification and filtering

Since the sensed Nernst voltage is in the mV range, it is practical to amplify this to a more sensible level. Input impedance of the chosen op amp should be as high as possible to avoid loading the cell. Consider an FET input amplifier. Input offset should be less than 0.5 mV. Noise on this buffered, amplified signal should be filtered with a low-pass filter that has a cut-off frequency of 250 Hz. DO NOT filter the low mV Nernst voltage directly as this may load the cell. To improve common mode noise rejection a small capacitor like 10 nF can be placed across the input terminals of the op amp in a low-cost effort, or a difference amplifier or even an instrumentation amplifier may be considered in a more critical application with high noise environment.

Pump reversal voltage and comparison

The amplified sense signal needs to be compared to voltage references which are the specified pump reversal voltages in the datasheet scaled by the same gain factor as the output amplifier. Each time the upper or lower reference is met, the constant current source should be reversed.¹

This part of the circuit should always start up in the condition that applies the constant current source between PUMP and COMMON as this begins the evacuation necessary to start the pumping cycle, i.e. PUMP should be positive with respect to COMMON.

Signal conditioning¹

A suitable microcontroller will be required to monitor the amplified sense signal and continually calculate t_d or t_p. See Equation 3 where t_d = (t₁ –t₂) + (t₅ – t₄). Averaging will reduce the natural sensor noise with the amount of averaging set to suit the response time needs of the application. Adaptive filtering is the best solution where the amount of averaging is changed depending upon the amount of variation in the calculated values.

Output conditioning

The microprocessor output should be scaled or transformed into the required output, i.e. voltage, current loop, serial interface, etc. In some cases, this may necessitate the use of a digital-to-analog converter (DAC) and output drive circuitry for such interfaces as 4-20 mA and such. Filtering and output resolution should also be taken into consideration.

Summary

The designer would well be advised to carefully use the manufacturer’s datasheets for the sensor in conjunction with reference 1, the Honeywell application note, “Operating Principle and Construction of Zirconium Dioxide Oxygen Sensors.”¹ A reliable and accurate system will be your reward.

References

1. Honeywell application note, “Operating Principle and Construction of Zirconium Dioxide Oxygen Sensors.”