Anything worth measuring is worth measuring right, and to do that periodic calibration is essential. In HVAC, the primary reason to measure CO₂ is to optimize ventilation and realize energy savings. According to a recent European study, demand-controlled ventilation (DCV) can reduce energy use by 20–50% in public buildings. If you’re not measuring correctly, you might be losing out on some or even all of those potential savings.
While there’s no single right answer to this question, “never” is always the wrong answer! A good starting point is to ask these two questions
Do not accept automatic drift compensation as a “good-enough” alternative – all sensors have some drift. You can further ask:
Always start with a shorter inspection interval and increase it gradually. Your actual field inspection data is the best way to determine the right inspection interval for your instrument.
A good strategy may be to first perform field checks against the reference instruments, as these are relatively fast to perform. Depending on the observed differences in the field check, a simple guideline for adjustment in HVAC applications could be:
• if the observed difference is 2%RH or less, do nothing;
• if the difference is 2–4%RH, make a one-point field adjustment; and
• if the difference is more than 4%RH, send the device for service or change the measurement module.
In HVAC the measured conditions typically only vary a little, meaning single-point field adjustment is usually the right choice. However, if the whole measurement range of the instrument is used or if the field check indicates a large correction is needed, multi-point adjustment is the right choice as something might be wrong with the instrument. Multi-point adjustment is more time consuming and expensive as it usually requires moving the instrument to a laboratory, but it will tell you if the device is working properly over its intended range.
For many basic applications Vaisala’s HVAC products are essentially maintenance free. The specified long-term stability is usually so good that adjustments are rarely needed, although a field check is always recommended. In more demanding applications, where traceability is required to maintain certifications, you can choose to carry out field checking and any necessary adjustments yourself.
With some of our products you can check or adjust relative humidity or CO2 readings against a handheld instrument or, in the case of carbon dioxide, against gas bottles. The easiest solution is to purchase field-replaceable measurement modules that come with a calibration certificate; these measurement modules can be easily exchanged in minutes.
Learn more about our handheld humidity and temperature meter HM70 or take a look at our calibration services.
A free webinar on maintaining the HVAC CO2 sensors the easy way will be held on 15th May. Register for the webinar on our web page.
The Vaisala CARBOCAP® Carbon Dioxide Probe GMP252 is an intelligent, stand-alone, ppm-level probe. It's intended for measuring CO2 in agriculture, refrigeration, greenhouses, demanding HVAC applications, and for plant growth chamber manufacturers.
Vaisala HUMICAP® HM70 Handheld is designed for demanding humidity measurement in spot-checking applications. It is also ideal for field checking and calibration of Vaisala's fixed humidity instruments.