We have done Biofeedback experiments in postmenopausal women and we have the equpment to perform Temperature measurements (2 Channels), Skin resistance and some other Parameters.

Hi David, psychophysiological measures have been a main component to all of my work which has included simulators, VR, desktop, and "real life" scenarios.

The primary measure I have used has been EDA (skin conductance) and more recently heart rate measures from PPG (primarily HRV). 

I currently recommend the Empatica E4 for a relatively robust sensor which can be worn during VR use (including HMDs).  Using a wrist sensor like the E4, which also grabs temperature and accelerometer data, allows you to correct for physical movement to a certain degree if necessary (temp for physical exertion, accel for movement artifacts).

I would agree with your use of both HRV and EDA.  EDA should be a strong measure for relaxation, assuming you're not having participants perform other tasks which have been shown to confound data.  E.g., if you're interested in relaxation (operationalized as arousal via EDA) then you would not want to have them perform excessive physical activity, extremely difficult mental exercises, or place them in sickness inducing virtual environments.  HRV then can be used to help clarify some of those findings as it measures both sympathetic & parasympathetic responses and you can use the EDA data as your window into just the sympathetic.

Depending on your experimental setup, you could also grab things like accelerometer/gyroscope data from your HMD to look for head movement (I would anticipate lower movement = more relaxed, but I have not done this work before). 

Additionally, I would suggest using some post-treatment self-report to help add another measure.  Just from a quick scholar search, there was a relative study from 1975 (in link below).  Also attaching my MS thesis which discusses EDA as a measure of mental effort and sickness.

Happy to discuss further if you'd like! Good luck!

http://psycnet.apa.org/journals/ccp/43/5/595/

Thesis Exploring mental effort and nausea via electrodermal activit...

Hi David, I'm currently coming to the end of a study I'm doing in developing virtual reality exposure therapy to benefit those who stutter and to treat social anxiety..

I've been using the Affectiva Q Sensor, but as it is a very outdated piece of equipment, it has not been very effective. The only results it gives is a constant slow increase in temperature and EDA.. (no use to me)

However, as Chase has mentioned above.. These devices are great for your final results to show the results in how the physiological measurements changed during different parts of the VR experience.

I would not recommend the Affectiva Q Sensor, but I'd highly recommend a newer device.. Also to back up what Chase has said, EDA and the participant's heart rate measurements will be of most use to you..

Hope this helps

Thank you all very much for your helpful input :-)