Thermal fluctuation is a temperature change in a given environment over a period of time. The primary cause of these changes is natural temperature variation throughout the day, although there are man-made heat sources as well.
- Air conditioning
- Body heat
- Daily variation
- Machinery and appliances
Any change in the basic environment can degrade repeatability of measurements, but thermal change has a few specific effects. Gas and materials expand with the addition of heat and constrict when cooled. This effect is not noticeable during everyday life, but when taking nanoscale measurements it can play a big role. A difference of even a few degrees can change the size and shape of a sample and also alter the characteristics of the instrument itself. Without monitoring or controlling temperature fluctuation, inaccuracy can creep into measurements.
Another particular effect of temperature variation is unique to biological applications. Tissues and cells are very sensitive to temperature. As temperatures change, the behavior and characteristics of biological materials alter. Additionally, cells being imaged ex vivo can perish if the temperature goes too high or too low. Biological research requires strict control of thermal fluctuation.
Temperatures change gradually, so it can be exceedingly difficult to diagnose a thermal fluctuation issue. Additionally, the inaccuracy caused by thermal effects can be minor, so it is easy for these issues to go undetected for weeks or months. Multiple measurements must be taken throughout the day, over several days, and then correlated to thermometer readings to diagnose a thermal issue with certainty.
Oftentimes, researchers will have limited control over the temperature change in their lab. Rooms with windows are subject to sunlight, which causes temperatures to rise throughout the day. The heat from sunlight will transfer into a room even when the blinds are closed. When buildings have HVAC systems, the temperature may be set building-wide, beyond the controls of the individual researcher. HVAC systems themselves can introduce their own fluctuations, as cooling cycles may be irregular or introduce rapid cooling that can frustrate measurements.
Whenever possible, keep precision instrumentation away from sources of thermal fluctuation. If possible, conduct measurements in a room with no windows. Limit exposure to radiators, fans, and air conditioning ducts. Ask that a thermostat be placed in the lab so that researchers can control the temperature levels in their lab independently. Basements can be a great place to escape the sunlight and lessen the effects of intra-building temperature gradients.
If you can’t control the location of your instrument, you can accomplish some degree of control by monitoring temperature levels. Install a thermometer so that you can at least note the temperature along with your measurements. Try to take similar measurements at similar points in the day, to ensure constant temperature levels across measurements which will be compared to each other. Some instrument makers offer temperature feedback features which will control for temperature changes in the software or system itself. Otherwise, you may be able to manually correct for the fluctuation using your own calculations.
Enclosing the instrument is another way to moderate the effects of thermal fluctuation in the room. Creating a smaller environment around the instrument will insulate the instrument from temperature changes in the larger room. For very sensitive applications, it is recommended to purchase an active temperature control system. These systems continuously monitor temperature levels and adjust accordingly using heaters or coolers. Some acoustic enclosures and soundproof hoods, such as those offered by Herzan, are offered with temperature stability features as an option.