Hi Mario! Thanks for participating in our blog. The value you are searching for is called “response time”.

There are two parts to this. The first part is the mechanics or electronics of the sensor itself – what is the lag time necessary to get any reading, convert it from analog to digital, store it local memory (if attached to a data logger), and then send or display the data. Usually, this is so fast it is negligible, but will have added time (measured in fractions of a second) if the device is in a sleeping state to save battery life, then must wake up to take a reading.

The second part is the physics of the measurement at the sensor element itself. In this case we are talking about temperature in this blog. How fast a bare sensor element can respond in controlled conditions is a known value for our products, but we don’t share it because it is meaningless (and misleading) in application. For instance, the bare sensor is usually placed in a metal sleeve, or coated with water-proofing. Then it is placed on the data logger, sometimes inside the logger, sometimes on a probe. And then the protected sensor is then placed in the actual measurement environment, where there are a ton of variables that will affect response times, such as airflow, temperature differential, rate of temperature change, mounting location, etc.

In your question though, you asked specifically about gases and particles. Those work quite differently than temperature sensors, so I’ll discuss them separately.

For gases concentrations, we can just talk about humidity, because that’s what is being measured – the concentration of gaseous H2O (also known as water vapor). A bare humidity sensor with have a very quick response time. But you never want your humidity sensor unprotected. It is usually behind a filter, made of plastic or metal, which can further slow the response. And then the placement of the sensor in or out of airflow will also affect the ability to detect changes in an actively controlled environment. And if you are measuring relative humidity, the measurement is relative to temperature, so the physics of temperature change (as discussed above) must also be considered. For other gases, I am not personally familiar with the sensors we use at Vaisala, but I should work similarly.

For particles, the response time for a valid measurement isn’t based on the sensor (though these sensors are very fast electronically as they depend on light, often lasers). Usually a particle count measurement is presented as the number of particles of a given size per volume of air, say per cubic foot or per cubic meter. In order to achieve a reportable or meaningful number, the particle counter must process the required volume of air, and this takes time for the vacuum pump to pull the air volume through the particle counter. In the USA, the standard is per cubic foot, and the devices are usually designed to sample one cubic foot of air per minute, and one cubic meter of air every 35 min.

I hope this helps. Let me know if you have any further questions.


Best Regards,

Paul Daniel
Sr. GxP Regulatory Expert