Humidity indicator cards are also available. These use moisture sensitive salts that change colour as the relative humidity alters. These can be very useful for low cost monitoring, especially within display cases and storage boxes, as long as you check them regularly.
These cards do not record the relative humidity levels that have occurred, but only show what the current level is. Separate temperature cards are needed if you want to check the temperature.
Having a pretty good knowledge of the general conditions inside your museum display and storage areas is fundamental to caring for your collections. Being able to accurately measure the temperature and relative humidity is not absolutely necessary in order to take steps to control fluctuations or extremes, but it is helpful in identifying your problem.
Simple and relatively cheap thermometers and hygrometers are readily available in a range of formats. Small versions are handy for use in showcases. Generally speaking, the higher the price the more accurate the instrument. For routine purposes, a high level of accuracy is not necessary, and cheaper, less complex instruments will do the job. These instruments have the drawback that they only give spot measurements at the time of inspection, and they do not provide a record of conditions over a period of time. Still, if used as part of a planned program to gain an understanding of the conditions in your museum over several seasons, they are very useful.
To gain a good sense of the conditions (hot, dry, damp) across all seasons, in various parts of your display and storage areas, it is a good idea to implement a monitoring program that runs for at least a year, so that conditions in all seasons are understood.
The aim of such a program is to provide you with accurate information about the general environmental trends in your museum. Take measurements in numerous parts of the building, representative areas as well as near windows and doorways. This will make you aware of any damp areas or ‘hot spots’ that may be less than ideal for collections, as well as indicating which locations provide the most suitable conditions.
A well designed monitoring program will:
- aid in comparing external and internal conditions, in order to assess the building’s effectiveness in buffering the collection from the external climate
There are a number of other devices that are used to measure temperature and relative humidity. The following sections describe a few that are frequently mentioned in collection care and museum management manuals.
One device that you have probably seen in larger museums and galleries is the recording thermohygrograph. This is a fairly expensive device that operates on a pretty simple principle – that organic materials absorb and lose moisture under different conditions. Thermohygrographs use a bundle of human hairs connected to a lever. When the relative humidity rises the hairs absorb moisture and lengthen, and vice versa, moving the lever accordingly. The main advantage of a thermohygrograph over a standard thermometer and hygrometer is that it can be wound up (or battery operated) to measure continuously. The instrument can be set to measure over a day, a week or a month and it provides a read out of the temperature and relative humidity conditions on a graph.
We do not recommend you purchase a thermohygrograph unless you have, or are about to install, an air-conditioning system. We recognise that this will not be the situation for most museums, and we are not necessarily recommending air-conditioning systems as the ideal. The information about thermohygrographs is included here simply for your interest, and because they are commonly referred to in museum management and collection conservation literature. If you are considering installing air-conditioning we recommend you research the subject of museum environments in considerably more detail than is provided in this topic.
Electronic dataloggers use digital and computer technology to perform the same function as thermohygrographs. They can be set up to continuously record the museum environment. The data recorded is then downloaded to a computer. They have a number of advantages, mainly associated with their capacity to collect a lot of data over time, and the computer program provides the flexibility to present the information in various formats. They are also expensive and, like thermohygrographs, are really only recommended if you are in a position to significantly alter your museum by installing air-conditioning or other means of environmental control.
A whirling, or sling, hygrometer is another instrument you may come across in the literature. It consists of two ordinary thermometers. One is unmodified and termed the dry bulb, which measures the normal air temperature. The other is termed the wet bulb, because it incorporates a fabric sleeve over the mercury end, which is kept wet.
To use, the hygrometer is whirled around in the air. As air passes over the wet bulb it cools due to evaporation, giving a lower temperature reading. By comparing the two temperatures after the instrument has been whirled, with the use of standard published charts, you can calculate the relative humidity.
These are very accurate instruments, and are used to calibrate other devices such as thermohygrographs. There is, however, significant potential for human error during the several steps in the process that can lead to erroneous readings.