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I am assuming that you mean data loggers, which record digitally, not thermohygrographs which record on paper. The time interval for recording T and RH varies depending upon the specific environment, the information you want to get from the data logging, the type of logger, and the desired frequency of downloading. If you are monitoring a fairly stable environment and wish to use the data to understand the general environment of the room within which it is placed, a time interval of 15 minutes is common. Usually, data loggers are downloaded once a month, assuming that your logger has adequate memory to record for this long. This normally will be indicated in the software program used to launch the logger once you select a time interval.

If your logger is newer and has a large memory (will hold more than 6 months data at the selected interval), when you download the first month, choose the option that allows the logger to continue to log, rather than erase the memory and start again. The second month, do the same. At the six month point, stop the logging, erase the memory, and re-launch the logger. In this manner, you will have progressively growing data files, which at the 6-month point, holds half a year's data. This will allow much easier data evaluation for longer time frames. For example, plotting a full year's data would require only two 6-month files, rather than twelve 1-month files. If your logger does not have adequate memory for much over 1-month's recording, then reset it and erase the memory every month.

One month is commonly chosen as the downloading interval since it is long enough to have a reasonable amount of data for evaluation, yet allows you to find problems with logger failure (usually human error or dead batteries) fairly quickly with the risk of loosing only 1 month's data. If you downloaded every six months, you would be risking losing a full 6 months of data before a problem was discovered. Also, if you are actually using the data to find environmental control problems or evaluate system effectiveness, you need to look at the data on a reasonably frequent time frame. For general monitoring, 1 month is a good compromise between convenience and usefulness. However, there are situations when more frequent downloads may be appropriate. For example, I am currently looking at data for a client on a weekly basis as we evaluate a recent mold outbreak in storage.

Where you place the logger is critical. Within any given room, the temperature and RH vary considerably. It is not unusual to have more than a 20% difference in RH. The highest RH generally is found at the floor along exterior walls in the heating season and at the floor along the interior walls during the cooling season. Corollarily, the lowest RH generally is found at the ceiling along interior walls during the heating season and at the ceiling along exterior walls during the cooling season. Other areas of the room will have RH levels somewhere in between these. So, both the height of the data logger from the floor and where it is placed in the room will affect which readings are recorded. You want to place the logger where it will provide the information you want. For example, if the goal of logging was to determine if you may encounter mold growth during the summer, you would place the logger on the floor on an interior wall, assuming the room was above ground. If it was below ground, the logger would be placed on the floor at an exterior wall. If, on the other hand, you wanted to find out how well an environmental system was keeping the temperatures and relative humidities the same in two different rooms, you want to be sure the loggers are the exact same distance from the floor in both rooms, and in similar locations in each room, for example, the middle of an interior wall at 4' 0" from the floor. You may also want to determine how great the environmental variations are within a specific room. In this case, you would want loggers at the floor and ceiling on both exterior and interior walls, as well as at the center of the room. For general on-going monitoring of RH and T, place the logger at the center of the room if possible, or if not, on an interior wall. Locate it above the floor at approximately the average height of the collections. For a historic house museum, where furniture and other decorative objects are displayed, the distance from the floor may be 2 1/2 feet, but for an art museum, the distance may be 5 or 6 feet from the floor. And, of course, keep the loggers away from heat or cooling sources, including sunlight and artificial lights! Good luck and have fun.

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