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Altitude Compensation for CO2 Sensors

Sky over city

Altitude makes a difference when you are measuring CO2 levels. CO2 sensor modules are calibrated at sea level, and are not designed to automatically compensate for higher altitudes.

CO2 sensors need either a manual offset using a barometer or lookup table for altitude compensation, or a built-in barometric pressure sensor to automatically compensate for the change.

Altitude Compensation: Identifying the Problem

co2 sensor module

A customer called recently and said we had sold him a defective CO2 sensor module. He had calibrated it precisely at 0.0ppm (zero parts-per-million), took it outside, and the sensor read 335ppm.

“Where do you live?” we asked.

“Colorado,” he replied.

It took a few minutes to explain, but in the end the customer understood that altitude makes a difference when you are measuring carbon dioxide levels. CO2 sensor modules are calibrated at sea level. Most are not designed to automatically compensate for changes to CO2 at higher altitudes.

Atmospheric Pressure and CO2

When you think about altitude, it helps to imagine a vertical column of air rising from the earth’s surface to the outer limits of the atmosphere. The gas molecules in air have weight. The weight is heaviest at the earth’s surface, but becomes lighter as you measure it higher up the vertical column. At the top of the column, the weight becomes zero.

atmospheric pressure from a column of air

When talking about the “weight of air” scientists use the term atmospheric pressure. AP is typically measured in hectopascals (hPa), inches of mercury (inHg), or millimeters of mercury (mmHg). At sea level, these are as follows:

  • 1,013.25 hPa
  • 29.921 inHg
  • 760 mmHg

As you go higher in the atmosphere, each of these numbers get smaller. So to correct for atmospheric pressure when measuring CO2 levels, you simply divide the atmospheric pressure at sea level by the atmospheric pressure where you take the CO2 measurement. It doesn’t matter which scale you use; the answer will always be the same ratio. Multiply this fraction by the CO2 measurement at sea level, and you have the new CO2 level at any altitude.

Here’s an example.

The city of Denver, Colorado is approximately 5,000 feet above sea level. At this altitude, the atmospheric pressure is 632 mmHg (barometric pressure table). A CO2 sensor calibrated at 400ppm at sea level would read 400(623/760) or 332ppm.

Here at CO2Meter, our rule of thumb is that the CO2 level at altitude will change about 3% for every 1,000 feet (300m). At 5,000 feet, that would be 400ppm less 15%, or about 340ppm.

This type of correction can have real-world consequences. For example, suppose a grower in Denver wanted to maintain exactly 1,000ppm CO2 in their greenhouse. In order to achieve this, they would have to set the sensor to 15% above 1,000ppm, or about 1,150ppm.

Note that while CO2 sensor modules do not automatically account for changes in altitude from sea level, they can be recalibrated for this. And some CO2 meters have built-in barometers that can compensate in real time. So if you are at high altitudes and need extremely accurate CO2 level readings, you should look for these features before making a purchase. 

Altitude Compensation for Oxygen Sensors

Like CO2 sensors, oxygen sensors also require pressure temperature compensation. The 21.9% oxygen level commonly stated assumes STP, or standard temperature and pressure. Go to Colorado and the oxygen level is 17.3%. Here's a handy chart to show you the differences. Before purchasing an oxygen sensor check whether it is automatically compensated for pressure with a barometric pressure sensor on-board, or if you will need to adjust the oxygen level either by a lookup table or with a separate barometric pressure sensor. 

If you need assistance choosing the correct sensor for your application email us or call us at (877) 678-4259. We'll be happy to answer any of your questions.

Oxygen Sensor Storage Best Practices

Following the manufacturer's proper oxygen sensor storage and operating conditions are critical to long sensor life.

University Research and CO2Meter Sensors

CO2Meter has worked closely with universities nationwide that continue to use our sensor technologies to gather data for scientific research, discovery, and advancements.

Carbon Dioxide Sensors Testing

sensor testing

Carbon dioxide sensor testing is one of the many behind the scene tasks we perform every day at CO2Meter.

I was looking online at a new consumer product designed to monitor CO2. The product was well designed. The sensor inside was small. I was sure our customers would like it.

So I sent an email with a link to Ray Hicks, our Chief Engineer. I got short reply:

“Know the product, it’s poor performing. We tested it, but it’s not stable, I have a dozen of those CO2 sensors in a box on the shelf.”

What many of our clients don't realize is that for every CO2 gas sensor module we sell, there are three more sitting in a dusty bin on a shelf you will never see and we will never sell.

How CO2 Sensors are Discovered

It starts with Travis Lenander, President of CO2Meter or one of our sales engineers who travel worldwide to sensor trade shows. Some of the best sensors designed are first available at SENSOR+TEST in Germany or Smart Sensor in Korea. Our team has visited or exhibited at the AHR Expo in the United States for over a decade to learn about the latest in indoor air quality products.

Once we find a sensor that meets a specific customer need or fills a gap in our product lineup, we will arrange a visit to the manufacturer's production plant where the sensor is assembled. Many sensors are designed in Europe or the United States, but manufactured in Asia. If we are satisfied with the quality control of the production process, the next step is to order a batch of sensors for in-depth analysis.

How CO2 Sensors are Tested

When sensors arrive at our facility, our in-house engineers spend hundreds of hours testing them for accuracy and repeatability. They testing I/O (signal input and output) to verify functionality. If the sensor performs reliably to the manufacturer's stated specifications our engineers will add new code to our free Gaslab® software to make it easy for OEMs to communicate with the sensors during application development.

carbon dioxide sensors testing

But we’re not finished testing yet. Depending on the application, some new sensors are given to a select group of electronics experimenters, researchers  or "early adopters" around the country who integrate them into projects and report back their real-world results. We follow up closely on their progress and report back to our engineers any problems in the field.

Throughout this entire process, our engineers exchange emails with the manufacturer’s engineers to request low-level documentation, undocumented features, or to ask for changes to make the sensors easier to use. It is not uncommon for us to receive binary code to re-flash an entire batch of sensor’s ROM memory before we make them available to customers.

For example, the most common UART (serial port) configuration in asynchronous mode is 9600/8/N/1. If a sensor doesn’t use this industry standard (some don't), we’ll ask the manufacturer to change it before we offer it to our customers. The same goes for I2C, analog voltage or digital I/O outputs.

In addition to testing by our engineers, we continuously collect customer feedback and sent it back to the manufacturers. This cycle benefits everyone as the sensors continue to evolve and improve.

Only after a sensor passes through this gauntlet do we begin to stock it in quantity to make it available for sale online. 

But we're not done yet. We monitor and follow up on every return we get from the field. If we notice a high failure rate, we'll pull the product and repair or replace the defective sensors already in the field.

How CO2 Sensors are Sold

Other companies may offer the same sensor module online, but they have never actually bench-tested it. Everything they know about a sensor is printed on the manufacturer’s data sheet. Instead, they take an order online, pass it on to the manufacturer, let the customer wait 4-6 weeks for delivery, and keep the difference between the retail price and wholesale cost as profit.

At CO2Meter, we pride ourselves in knowing how each of our products work intimately.

And because of our intimate knowledge of the sensors we sell, manufacturers worldwide send their customer requests for technical support to us. We also serve as a liaison between overseas manufacturers and OEM users to make shipping easy and just-in-time delivery dependable. For large-quantity orders, we even negotiate with the manufacturers to get our customers the lowest possible volume pricing.

Why it Matters

All this sensor knowledge helps our customers in three important ways.

  • It makes our customer service team much more knowledgeable about gas sensors when making a recommendation for a project or device.
  • We only use the best sensors available when we design our own products like the CO2 + RH/T Data Logger to test indoor air quality.
  • It gives us very happy customers.

Do We Get it Wrong?

Do we get it wrong? Occasionally yes. Ray tells a story about a CO2 sensor we were very excited about for cell culture incubation and live animal applications. The sensor even briefly appeared on our website, although we didn’t put it in the field. Ultimately we couldn’t trust it, so we took it off the website. It wasn’t until later during a visit to the vendor that we discovered the sensor's production had been sub-contracted to a “back water” factory with no quality control. It was a huge waste of both time and money, but you can still find that sensor on other websites for sale today.

So the next time you see one of our gas sensor modules on the CO2Meter.com website or in a newsletter article, I hope you appreciate the fact that it is one of the best available. We stand behind all the products we sell.

Hydrophobic Filters on CO2 Data Loggers in High Humidity

Hydrophobic filters use on our portable diffusion data loggers allows accurate measurement of CO2, in high humidity applications.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

Still need Help? Talk to an Expert.

We'll be happy to help you find the right product!

Call us at 877.678.4259.

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