Practical RH Measurement For Drier Customers
In general, a top-notch humidity sensor won’t do building occupants any good if it’s placed badly or isn’t calibrated regularly. And poor humidity control can have serious economic effects on restaurants, retailers, museums, and others that rely on specific ambient conditions for patron and/or product comfort and stability.
SITE VARIABLESLew Harriman, principal of Mason-Grant Co., Portsmouth, NH, discussed “Measurement Issues in Commercial Buildings.” The gist of his report is that for many applications, a humidity sensor’s operating principle and “factory accuracy” are less important than its site-dependent variables. These include:
Other sensor characteristics that need to be considered, especially for their usefulness in the field and in troubleshooting situations, include:
RANKING IMPORTANCEIt may be useful for those diagnosing rh problems, and for those designing systems to control them, to rank the importance of humidity according to the economic impact adverse conditions could have.
“Soft” economic consequences: There is no choice in occupancy; buildings include schools and office buildings. When humidity goes awry, occupants “may be uncomfortable, but tough luck for them,” said Harriman, tongue in cheek; they have no options to go elsewhere.
Less-soft economic consequences: These are patron-choice occupancy facilities, such as restaurants and retail shops. “When people are uncomfortable due to temperature and/or humidity, they do not want to shop or try on clothes,” pointed out Harriman. Poor humidity can directly relate to a loss in revenue for the owner.
Hard economic consequences: These locations include museums, hospitals, supermarkets, and ice arenas, where poor humidity control can damage collections, impair health, ruin produce, or cause fogging and pooling.
CONTROL DECISIONSWhen designing/installing rh control systems, Harriman advised taking a careful look at the facility to spot the prime location(s) for sensor placement:
Contractors and engineers also need to consider the contaminant load of the air, which will have an effect on the frequency with which the rh sensor needs to be recalibrated. Therefore, consider the contaminant load of outdoor air (OA) vs. indoor air (OA carries more particulates and gases); also consider whether to place upstream or downstream of filters, and whether or not you would be placing it near a pollutant source, such as on a kitchen wall or above a heated therapy pool.
MORE ON CALIBRATIONHarriman said the ability and ease with which an RH sensor can be recalibrated should be a large consideration in sensor choice. Questions to ask include:
RH APPLICATIONSFinally, Harriman broke down sensor applications into three categories: tough, easier, and “out of the question.”
Tough applications are those in which it is expensive to monitor RH accurately. These include:
Easier applications include:
“Out of the question” applications are those that you should not agree to, because of the difficulty of getting a reasonably accurate measurement (such as trying to get accurate measurements without proper, regular sensor calibration).
MORE QUESTIONS TO ASKFrank Caporale, of General Eastern Instruments, Woburn, MA, discussed “Common Problems and Solutions.”
“I’m not here to give you the answers,” he said, “but to give you the questions to ask to get the right product for the application.
“Each application should be carefully analyzed,” he continued. Ask the following questions about each application:
“These questions come from 10 years of taking these kinds of calls,” said Caporale.
For installation, he recommended:
He added that more frequent calibration may be needed if the RH sensor encounters condensation, dirt, and/or contaminants. Generally, such units should be checked at least once a year.
Publication date: 07/15/2002