The humidity chamber is a sample environment that allows to independently control the sample temperature and the relative humidity of the air inside the closed chamber, while the sample can be exposed to the X-ray beam. The sample inside the chamber equilibrates with the air inside and takes up (or looses) water. In the case of lipids (for which the chamber was originally designed), this will change the lamellar spacing and thus allows to perform swelling experiments to determine phases. The chamber uses thermoelectric modules (TEC or Peltier elements) in a feedback loop with a Proportional-Integral-Differential (PID) control electronics to control the different variables. The control electronics communicates with a PC using a serial connection (RS232) through which all the control parameters are accessible and the current state of the variables can be read.
The relative humidity of the air inside the chamber is controlled by regulating the temperature of a small water reservoir inside the chamber (raising the water temperatures will increase, lowering it will decrease the amount of water vapor in the air and thus the relative humidity). In total there are three different control variables that need to be taken care of independently:
The sample temperature is controlled by regulating the temperature of the aluminum sample base, which is mounted to the goniometer head. The base was designed for flat samples (about 10mmx30mm in lateral size), but can in principle take also other kinds of samples. The thermal contact between the base and the sample can be enhanced by using thermal conductive paste which also helps to keep the sample in place. Close to the sample on the aluminum base the temperature sensor (AD590) is mounted. It measures the actual temperature and gives feedback to the control electronics. The temperature of the base is controlled using one TEC. The heatsink of the Peltier element is a custom made water chilled plate that also serves as a mount to of the chamber base to the goniometer head
To stabilize the environment inside the box and isolate it from the influence of the outside (i.e. hutch) air temperature, the walls of the box are regulated using two Peltier elements run in series (one on top, the other on the bottom of the box). Particularly at high relative humidity inside the chamber it is necessary to keep the box at elevated temperatures (typically Tsample +10C) to prevent condensations on the box walls or windows.
water bath temperature / relative humidity
The relative humidity of the air inside the chamber is controlled by regulating the temperature of the water in a small container inside the chamber. It's actually the difference between the temperature of the water and the sample temperature that determines the relative humidity of the air the sample is exposed to. But instead of controlling the water temperature we use a humidity sensor (HIH3006 or HIH3010) as a feedback device for the PID control to directly control the relative humidity. The sensor is mounted close to the sample and kept at the same temperature by thermal contact to the sample base. The water bath temperature is then regulated using one TEC module on the back side of the box in such a way to obtain a certain relative humidity of the air close to sample. Alternatively the water bath temperature can be controlled directly, which is advantegeous for experiments at relative humidity values close to 100%, due to the increasing insensitivity of the rH-sensor at high humidity.
For each of these variables there is a separate independent PID-control loop with a set of control parameters that allows to optimally control the sample environment inside the chamber. The chamber was originally designed for lipid diffraction and grazing incidence scattering measurements but can in principal also be used for other types of experiments.