Water activity meter reading over 1

  • Thread starter fedaykin
  • Start date
  • #1
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Good evening. I recently was taught how to use our new water activity meter, and I noticed something strange.
This meter uses changes in the reflectance of a mirror.

Whenever we'd bring the sample up to temperature, the measurement would reach a low point, then at about 0.5° below our set temperature of 25°, the measurement would hit about 1.02 or 1.03. I'm curious if this is a quirk of the meter itself, or if there is a physical phenomenon underlying this. The underlying sample was a tomato paste with aw measured at about 0.99.
 

Answers and Replies

  • #2
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When you say 'water activity' are you referring to the refractive index of water?
 
  • #3
Borek
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Nope, water activity is a measure of the saturation of the water vapor, used in food sciences: https://en.wikipedia.org/wiki/Water_activity

Not that I knew it, had to google.
 
  • #4
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Which device are you using to get those measurements?
How accurate is the measurement in general?
Could be a simple accuracy issue, but it is odd that the device software doesn't filter it.
 
  • #5
DrDu
Science Advisor
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Nope, water activity is a measure of the saturation of the water vapor, used in food sciences: https://en.wikipedia.org/wiki/Water_activity

Not that I knew it, had to google.

Well, that's pretty much the definition of activity in thermodynamics.

Sounds like a dew point measurement.
 
Last edited:
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  • #6
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Thank you for all the posts here.
I've decided the readings above one are probably from hysteresis, probably due to a difference in frequency of sampling temperature and mirror fogginess.
The makers of the device (it's an Aqualab 4) could assume that people would only use the result at a stable temperature.
Since the device has a target temperature, it simply waits for the measurement to stabilize to a certain accuracy within range.

Perhaps they display a preliminary value for convenience of the end user.
 

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