CO2 Sensor Response Time
12Pages

APPLICATION NOTE AN005: CO2 Sensor Response Time ABSTRACT GSS offer CO2 sensors with different gas sampling methods. The gas is either diffused into the measurement chamber typically through a PTFE membrane, or pumped into the chamber using a flow through adaptor. Apart from some gas flow around the sensor, the response time of diffusion sensors is dominated by the digital filter setting and the pore size of the membrane filter. Diffusion sensors are used for applications that do not require a fast response, or where a slow response is beneficial such as environmental control systems. However, there are many applications that require faster real-time gas measurements. GSS have designed a family of sensors with gas flow through adaptors optimised for responsiveness and CO2 measurement accuracy at flow rates from 50ml/minute to >6l/minute. This application note explores flow through gas sampling and the effects of flow rate on response time. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 9 March 2021 Copyright © 2021 Gas Sensing Solutions Ltd.

Gas Sensing Solutions Ltd. Page | 2 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 9 March 2021 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN005: CO2 Sensor Response Time INTRODUCTION GSS manufacture Carbon Dioxide sensors with two different types of gas sampling methods, diffusion and flow through. Different sampling methods are needed to optimise the gas measurement method to meet the varying application requirements. TERMINOLOGY The response time of a sensor is usually quoted as the time to reach a proportion of the target value. For example, T90 30s indicates the sensor will reach at least 90% of the final value within 30s of the measured property being changed. DIFFUSION Diffusion of the gas into the...

APPLICATION NOTE AN005: CO2 Sensor Response Time MEASUREMENT SPEED GSS sensors use a technique called Non-Dispersive Infra-Red (NDIR) where the gas concentration is determined by the amount of light absorption by the target gas in measurement chamber. There are three factors that affect sensor response time; the gas flow into the measurement chamber, the amount of digital filtering done by the sensor, and the measurement rate. For GSS diffusion sensors with a measurement rate of 2Hz, the limiting factor is the diffusion rate through the membrane. For sensors with a gas flow port, the...

APPLICATION NOTE AN005: CO2 Sensor Response Time DIGITAL FILTER SETTING GSS sensors (excluding CozIR-Blink) implement a digital filter that allows the user to select whether to read raw CO2 data or filtered CO2 readings from the sensor. This allows the user to trade the response time with reading noise, depending on application requirements. The graph below in figure 1 shows the effect of the filter when exposed to a step change. The raw reading (Z) changes from 400 to 1500ppm on the next measurement sample. With a digital filter setting of 16, T90 is a period of 18 readings. For a...

APPLICATION NOTE AN005: CO2 Sensor Response Time Figures 2 and 3 below show the combined effect of gas exchange and filter setting in a real-life example. Response vs filter setting Gas reading (%) Figure 2: Filter Response vs Filter Setting Graph (2s gas pulse) Figure 3: Filter Response vs Filter Setting Graph (step change) Gas Sensing Solutions Ltd. Page | 6 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 9 March 2021 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN005: CO2 Sensor Response Time FLOW RATE For a sensor where the sample is being passed through the sensor, the flow rate is critical to the response time. As a rule of thumb, to fully exchange a gas volume in a chamber, 5 times the volume of gas needs to pass through the chamber. However, much like the filter settings above, this is not a linear function. If the gas volume is only exchanged once, the sensor will only be able to respond with ~80% of the final reading. These figures are only an estimate, and much depends on the type of flow system used. Figure 4: Response...

APPLICATION NOTE AN005: CO2 Sensor Response Time Figure 5: Response Time Graph (step change) Figure 6: Response Time 20Hz vs 50Hz At lower flow rates, the flow rate dictates the response time. Figure 6 shows the response time for SprintIR-6S (20Hz sample rate) and SprintIR-R (50Hz sample rate). Gas Sensing Solutions Ltd. Page | 8 For regular updates, sign up at https://gassensing.co.uk Revision 1.0, 9 March 2021 Copyright © 2021 Gas Sensing Solutions Ltd.

APPLICATION NOTE AN005: CO2 Sensor Response Time CONCLUSION GSS has designed a range of CO2 sensors specifically for applications that require fast response times. The response time of the sensor depends on several factors including the magnitude of the gas change to be measured, the measurement rate, sensor gas volume and digital filter settings. The SprintIR® range of sensors are optimised to support measuring CO2 in near real-time at flow rates from 50ml/minute to greater than 6ml/minute with better than ±70ppm +5% of reading accuracy. This makes them well suited for medical monitoring...

APPLICATION NOTE AN005: CO2 Sensor Response Time IMPORTANT NOTICE Gas Sensing Solutions Ltd. (GSS) products and services are sold subject to GSS’s terms and conditions of sale, delivery and payment supplied at the time of order acknowledgement. GSS warrants performance of its products to the specifications in effect at the date of shipment. GSS reserves the right to make changes to its products and specifications or to discontinue any product or service without notice. Customers should therefore obtain the latest version of relevant information from GSS to verify that the information is...

APPLICATION NOTE AN005: CO2 Sensor Response Time ADDRESS Gas Sensing Solutions Ltd. Grayshill Road Cumbernauld G68 9HQ United Kingdom Page | 11 For regular updates, sign up at https://gassensing.co.uk Copyright © 2021 Gas Sensing Solutions Ltd.