Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1088111
32 | February 2019 ufacturers wanted to ensure that the instruments can report accurately even during high particulate events. For air quality applications this requirement can be cru- cial. There are many conditions where particle counts (and thereby mass) can soar, so having an instrument that can operate in high concentration environments is significantly more important. So, having some measure of the worst- case environment in which an instrument can operate accu- rately should be specified. 7. Sampling flow rate This requirement stipulates that the instrument must main- tain the sampling flow within +/-5% of a target flow rate. Typi- cally if the instrument is unable to do so it would report a flow error. Early instruments often didn't control or even measure the flow rate. In such cases, if the flow failed the instrument would blithely report erroneously low readings since less or no air was moving through the instrument. For both cleanroom and air quality applications, ensuring that the flow rate is tightly controlled or monitored is critical in order to report reasonably accurate results. 8. Sampling time This requirement states that the sampling period be accu- rately controlled. In this day and age of microcontrollers with crystal con- trolled clocks this is a simple requirement to meet and is seldom an issue for instrumentation. 9. Response rate This requirement states that when moving an instrument from a dirty environment to a clean environment that the instrument quickly respond and start reporting the clean air condition. For cleanrooms this can be critical since some instru- ments are moved regularly between dirty and clean envi- ronments (when doing spot checking with a handheld instru- ment for example) and the user wouldn't want to have to wait an extended period for the instrument to be useable in a clean environment after sampling in a dirtier environment. For air quality applications this can also be important even for instruments that are mounted in fixed locations. You want an instrument to recover reasonably quickly from an event with elevated particulates and report correctly once that condition has been corrected. 10. Calibration interval This requirement states that the manufacturer specify a suggested calibration interval for the instrument and per- form at least size calibration, size resolution and counting efficiency and sampling flow rate calibration. the actual environment. There needs to be the means of val- idating that sensors report a reasonably accurate value with- out externally tampering with the data, to make them appear to be working. 4. Size resolution This requirement states that the maximum width of the cali- bration histogram (area between the yellow lines) be 15% or less of the height bin for a "user" specified channel. Essentially what we're trying to control here is to keep the particle height bins reasonably tightly grouped so that we have significant separation between calibrated sizes and can tell the difference between particles of similar sizes. For cleanroom applications this improves your ability to re- solve intermediate sizes and size particulates more accurately, which is very important for many manufacturing processes. For air quality applications sizing particulates accurately allows you to more accurately estimate mass. 5. False count rate This requirement states that the manufacturer must mea- sure the particle counts in clean air and that they should be statistically below some small threshold. For cleanroom applications this is critical in that manufac- turers often rely on an instrument to measure particulates in very clean environments so small numbers of "false" counts can be a big problem. There are always false counts, for example, cosmic rays are continually bombarding the earth and account for small numbers of counts. There are also other factors that can create such. The manufacturer must take measures to ensure that these are below a small statis- tically derived threshold. For air quality applications the false count rate is much less important, in that the instruments typically operate in environments with large numbers of particulates. But if these false counts become a significant percentage of the actual counts then that error can impact the quality estimation. In sensors that measure aggregate mass this error manifests as a fixed baseline within the sensor (a minimum mass con- centration) which gets added to any mass estimation error. 6. Maximum particle number concentration This requirement states that the manufacturer must speci- fy the maximum particle concentration that the instrument can operate in while still reporting counts within 10% of the actual counts. For cleanroom applications this figure is typically of little use, since the environments are typically very clean, but it was added since some manufacturing environments where these instruments are used are much dirtier and these man-