Monitoring Temperature and Relative Humidity

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Initial monitoring identifies unforeseen problems and allows adjustments to controlled exhibit spaces and cases. Continuous monitoring assesses the operation of exhibit features that are designed to provide specific environments.

How does one establish a monitoring program?


The program to monitor temperature and humidity must be well planned and have sufficient resources to collect the data and interpret the findings. Considerable information is available on this subject (see "Suggested Reading"). Conservators can help you determine the best locations for the monitors, ensuring collected information is relevant. In general:

  • Locate the monitor close to collection objects
  • Locate away from heating or cooling equipment and sources of vibration
  • Make sure that air flow around the monitor is not restricted
  • Ensure monitor accessibility for maintenance
  • Make sure the monitoring sensors are calibrated


Placing one person in charge of the monitoring program ensures more consistency in implementation and allows this individual to become familiar with the monitoring equipment, locations, and normal results. It is critical to maintain the equipment and check its calibration; faulty or inaccurately calibrated equipment magnifies errors inherent in even the best equipment, resulting in very inaccurate readings.
Exhibit cases can be sample monitored; not every case requires recording equipment if case construction is identical. It is important however, that external factors affecting the case environment be written down including; equipment malfunction, loss of power, an unusual weather event, or high visitation. It is also very helpful to gauge the collected data against outside weather patterns which are available for most locations.
A few considerations for planning the monitoring program are:

  • Recording versus direct read: Units that record the data provide a permanent record for future review. Data from direct read units must be written down.
  • Length of monitoring: Some recording monitors can collect data for up to one year. Most recording units require daily, weekly, or monthly attention.
  • Staff time:" Direct read units may be fine for single applications but require extensive staff time for multiple sites. Computerized data loggers require very little staff time and may therefore be more cost effective.
  • Accuracy: The accuracy and frequency of data required to assess the function of a case designed to hold a tight relative humidity, is greater than that required to determine whether or not an exhibit case is overheating due to poor lighting design.
  • Frequency: Some applications require continuous monitoring, for example initial assessment of a controlled case environment. Periodic information is fine for other purposes, such as determining when to replenish silica gel.



What types of equipment are available to monitor temperature and RH?


The basic options for recording temperature and humidity are: direct read units, and recording hygrothermographs and data loggers. Because most monitoring programs required that the exhibit case not be opened not all of the options listed below will be appropriate.

1. Direct read equipment:
  • Psychrometers use the readings from wet and dry thermometers to determine the relative humidity. When used correctly, readings are very accurate; battery powered units are recommended. Since the climate can vary greatly over the course of a day, however, a single daily reading is of limited use. A psychrometer is not useful in assessing the environment inside an exhibit case since the case would have to be opened to obtain a reading.
  • Color indicator cards are cardboard with strips of cobalt salts that respond to different relative humidities. These inexpensive cards can be useful in tracking general trends—such as exhaustion of desiccant gel or the malfunction of mechanical equipment—but are too inaccurate to assess performance of a controlled exhibit case.
  • Hand-held single read probes and meters vary in their accuracy. They are impractical for exhibit cases unless a port hole is included in the design of the case.
  • Dial thermometers and hygrometers used in exhibit applications are quite common, however, should be calibrated on a regular basis.
  • Inexpensive LED readout thermometers and hygrometers with a built-in memory of high and low readings provide the added benefit of recording the extreme conditions registered by the unit.
2. Recording equipment (in many instances this equipment is only required during the first year of a case's assessment or for periodic assessment):
  • Recording hygrothermographs mark the temperature and humidity on a revolving drum or disk, thus giving a continuous reading that provides very comprehensive information. Units collect data from a week up to several months. A hygrothermograph requires calibration against a psychrometer. Information must be charted on a graph to show long-term trends.
  • Data loggers record temperature and humidity at set time intervals for up to one year. Data is downloaded into a computer program that charts the information. It is very cost effective since little staff time is required. The cost is competitive when a second sensing unit is purchased so that one logger can service two locations (e.g., a unit can be located outside the case with an additional remote sensor located in the case interior).
  • Hand-held electronic probes and meters that record readings are also available. They can be downloaded into a variety of recorders. These types of electronic meters can be turned into remote sensors by a cable connection to an off-site recorder.
  • A remote sensor connected by cable to a chart reader can be very useful for monitoring exhibit cases. The sensor can be fitted into an accessible wall of the case and connected to a chart in a more convenient location. This method is visually unobtrusive and eliminates the need to open a case to retrieve data.
  • A wireless remote transmitter with sensor can be used within cases with the receiver placed at a montoring station in the room.


Low pressure reduces the accuracy of the equipment; correction tables must be used at high altitude (above 900 m), as per the National Oceanic and Atmospheric Administration, National Climatic Data Center. You can also contact your local airport for local climatological data.

TYPE OF SYSTEM DESCRIPTION USE/COMMENTS 1 APPROXIMATE COST MANUFACTURER / SUPPLIER
remote sensor chart recorders probe with cable connected to a chart recorder accuracy ±1°F and ±3% RH; 1-6’ cable; some have alarms for high or low levels; can have multiple recording channels. $640- $2000 Omega Engineering, Inc. or various other firms
hand-held electronic hygrometers and thermometers probe or sensor connected to a hand-held unit with digital display accuracy ±2°F and ±3%RH; operates on batteries; portable; some can be connected to remote recorders. $250 and up Cole-Parmer
psychrometer readings from a wet and dry thermometer are referenced on a char t to give humidity accuracy dependent on operator; use distilled water; wick must be clean. $150 and up Cole-Parmer
indicator cards a combination of cobalt and other inorganic salts that change color in response to relative humidity accuracy ±10%; cheap; useful as a visual indicator of a problem when dataloggers are used. $1 per card Humidial Corp.
dataloggers sensor with a memory chip that records data for future downloading accuracy ±4%RH ; can be left in place for up to one year; no visual reading from the unit itself s o problems may go undetected until downloading. price from a round $1,000 SmartReader 2, TRAK-R Logger or equivalent, Hanwell Wireless
recording hygrothermographs sensors are connected to pens that simultaneously record data on a char t connected to a revolving drum or disk accuracy ±2°F and ±2%RH; allows on the spot reading of data; some units record up to three months; are already available in many institutions. $250 and up Cole-Parmer, Dickson
hygrometers and thermometers small unit that displays either temperature, humidity, or both on a dial or digital display accuracy ±2°F and ±3%RH; calibration must be checked frequently; small enough to be easily hidden. $50 and up Cole-Parmer



Products, Manufacturers, and Suppliers


Mention of a product, manufacturer, or supplier here is for information only and does not constitute an endorsement of that product or supplier. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.

Remote Recorders
Omega Engineering Inc, Bridgeport, NJ, 08014

Data Loggers
ACR SmartReader 2 Temp/RH Logger (optional remote sensor), Cascade Group, Oyster Bay, NY 11771
Handwell wireless logger, Cascade Group, Oyster Bay, NY 11771
TRAK-R Loggers, Central Islip, NY, 11722

Paper Based Hygrometers
Humidity Indicator, Humidial Corp, Colton, CA, 922324
Psychometers/Thermohygrometers, Cole-Parmer Instrument Co., Vernon HIlls, IL, 60061

Suggested Reading


Hitchcock, A. and Jacoby, G.C., 1980. "Measurement of Relative Humidity in Museums at High Altitude," Studies in Conservation, 25:78-86.

Lindblom Patus, B. The Environment - Monitoring Temperature and Relative Humidity, Technical Leaflet, Preservation Manual of the Northeast Document Conservation Center; www.nedcc.org/tleaf22.htm