ATLANTIC CITY, NJ — Humidity and its control have been receiving greater and greater attention, especially their relationship to good or poor indoor air quality (IAQ). So it’s no wonder that an American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) seminar explored “Practical Aspects of Humidity Measurement.”

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.


Lew 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:

  • Economic and health consequences of accuracy;

  • Determining locations that need more humidity control;

  • The sensor’s cycling range, speed, and frequency (these can affect sensor placement);

  • Contaminant load in the air; and

  • Frequency of performed calibration.

    Other sensor characteristics that need to be considered, especially for their usefulness in the field and in troubleshooting situations, include:

  • Whether the installed unit can have a field-calibrated test;

  • Cost and ease of recalibration;

  • Repeatability; and

  • Response time.


    It 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.


    When designing/installing rh control systems, Harriman advised taking a careful look at the facility to spot the prime location(s) for sensor placement:

  • In a medical facility, for example, is it more important to have the sensor in the copier room or the patient room? (The correct answer is the patient room.)

  • In an ice arena, it is more important to get RH values on ice surfaces rather than in the open room, or near walls. (Ice needs a constant dewpoint, while wall RH varies with temperature.)

  • In a museum, it is more important to monitor humidity in museum rooms, near the sensitive collections, than in the supply air duct.

  • In nearly any commercial facility, sensors are best placed on inside walls instead of near doors; the internal wall temperature is more stable.

  • It is a bad choice to put a sensor near a humidifier or dehumidifier. Similarly, if you’re called out to troubleshoot RH conditions, check other aspects of the sensor’s location; for instance, did the company place the coffeemaker in the sensor’s immediate vicinity?

    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.


    Harriman 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:

  • Does the sensor report correct humidity levels? What are levels of accuracy now vs. what they were at the factory, or now vs. six years ago?

  • Can the sensor be calibrated? Does it allow adjustment? Can it be reached by a technician? If it can, “Consider the reality of people,” said Harriman. Has anyone been assigned to do it? If so, do they know they have been assigned? If so, do they know how to do it? And if so, do they actually do it?


    Finally, 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:

  • OA measurements;

  • RH greater than 80% and less than 20%;

  • Supply air RH; and

  • Applications requiring better than ±2% RH accuracy.

    Easier applications include:

  • Indoor air measurements away from the heat source;

  • Those measuring within a range of 40% to 60% RH;

  • Return air measurements;

  • Applications requiring ±5% RH or wider accuracy (“vanilla-grade” office buildings and schools).

    “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).


    Frank 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:

  • Do you want to measure high humidity or low humidity?

  • What is the temperature range?

  • Will the sensor survive?

  • What degree of accuracy is required? Is there an acceptable drift rate (like 1% per year, or 5% per year)?

  • What about repeatability and hysteresis?

  • What response time is required (seconds or minutes)?

  • Is temperature compensation required?

  • Will chemicals or contaminants be present?

  • Is the sensor NIST traceable?

  • Can recalibration be done? Can it be done in the field?

    “These questions come from 10 years of taking these kinds of calls,” said Caporale.

    For installation, he recommended:

  • For space mounting — on an indoor wall 6 feet above floor. Avoid doors, windows, vents, diffusers, heaters, and exterior walls.

  • For duct mounting — mount the unit so the sensor probe is in the center of the duct, away from the cooling coil, Caporale said.

    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