Every sensor that measures a variable value like a gas sensor requires calibration. Calibration is the process of matching a sensor’s output to a known standard. If the two do not match, the sensor’s output is adjusted. If they match, the sensor is considered “calibrated.”
For example, when you set your home thermostat to 68 degrees you expect it to actually turn the furnace on at 68 degrees. But how does a thermostat know what “68 degrees” is? The simple answer is “the thermostat was calibrated.”
For gas sensors there are 3 types of calibration:
- Span (2-point) calibration
- Single point calibration
- Automatic Background Calibration
Span or 2-point calibration is typically performed at the factory after a gas sensor is manufactured. To perform a span calibration, a gas sensor is exposed to 2 gases, one with no target gas, and one with a known amount of the target gas.
Span calibration begins by the sensor being exposed to a pure inert gas like nitrogen or argon. In the case of oxygen sensor calibration, because the gas contains no oxygen, the sensor’s reading for 0% oxygen is recorded in the sensor’s EPROM memory. Next, the sensor is exposed to a known percentage of gas, often the highest level for which the sensor is rated. For example, a 25% oxygen sensor would be calibrated with 25% oxygen. After being exposed to the 2nd known gas, the sensor response is again recorded in the sensor's memory.
Once these 2 points are known, a linear response to the gas concentration between the 2 points can be assumed. This is known as a calibration curve. In theory, any gas level readings between the 2 calibration points should fall on the line. In reality, some gas sensors do not have a linear response to different amounts of the target gas. In these cases, the manufacturer may perform 4 or more point span calibration to create a curved response line instead of a straight line.
During span calibration a second procedure known as “zero-point adjustment” is carried out. Zero-point adjustment negates “zero drift” by creating an offset to the sensor’s actual reading when exposed to 0% gas. For example, If at 0% oxygen a sensor reads “0.01%” when exposed to no oxygen, a negative offset of “-0.01%” is stored in the sensor’s memory and applied to all readings.
It is important not to confuse calibration with zero-point adjustment. While zero-point adjustment gives a more accurate sensor reading, span calibration is first required to match the sensor’s response to a known target gas.
Single Point Calibration
Single point calibration is used when only one measurement point of a gas sensor is needed. It is useful in cases where maximum accuracy is less important than total cost. This is especially important when the sensor cannot be easily removed or taken out of service for bench top recalibration, or where there is not trained staff to perform the procedure.
In most cases single point calibration is achieved in fresh air. Fresh air contains approximately 78% nitrogen, 20.9% oxygen, 0.9% argon and 400ppm (parts-per-million) of carbon dioxide. All other trace gases should be less than a few parts per million. Knowing this, any gas sensor exposed to fresh air should match these readings. Any deviation is evidence of too much (or in the case of oxygen too little) gas.
While single point calibration may be used in the field, it is only possible because the gas sensor was originally exposed to span calibration after manufacturing. Once the calibration curve is stored in the sensor’s internal memory, single point calibration is used like the zero-point offset.
For example, if a CO2 sensor uses single point calibration when the “calibrate” button is pressed on the device in fresh air, the sensor’s software assumes the current stable reading is 400ppm CO2 and adjusts the curve with an offset value. If a carbon monoxide sensor uses single point calibration in fresh air, pressing the calibrate button tells the sensor that there is no CO present and to save any offset to the zero point.
Automatic Background Calibration
Manufacturers of early CO2 sensors used in buildings to measure occupancy or indoor air quality (IAQ) levels realized the difficulty of calibrating wall-mounted gas monitors. Removing the units from the wall to bench calibrate was expensive, required trained staff, and with budget cuts calibration schedules were often ignored.
To solve the problem of indoor air quality CO2 sensor calibration in the field, Senseair AB of Sweden developed a technique known as Automatic Baseline Calibration (ABC). The theory behind ABC is that at some point each day a room is unoccupied. The CO2 level should return to 400ppm, the same as outdoor air. By storing the lowest CO2 readings taken over several days in EPROM memory, an offset to 400ppm could be calculated, then added or subtracted from the actual CO2 reading displayed.
While it is called automatic background calibration, in reality it is automated version of single-point calibration. And like single-point calibration, a span calibration curve was originally saved in the sensor's memory at the factory. For maximum accuracy, even devices that use ABC should be calibrated over time.
The advantage of automatic background calibration is that the CO2 sensor is self-zeroing over the life of the sensor. The disadvantage is that if the sensor never “reads” normal 400ppm air, over time it will display inaccurate CO2 levels. Learn more about ABC calibration here.
How often should a sensor be calibrated?
The more accurate a gas level reading required, the more often the sensor should be calibrated. However, after working with hundreds of sensors and applications over the years, here are some general guidelines:
- Scientific Experimentation – Before each test
- Personal Safety – Monthly to yearly (daily bump-test recommended by OSHA)
- Greenhouse – After each growing season
- Manufacturing – Bi-annually to annually
- Indoor Air Quality – Annually or longer if ABC is used
Because these are only guidelines, calibration schedules may also be dictated by experimental protocols or by particular industrial standards. When deciding on a calibration schedule you should consult your specific sensor requirements for your application.