Microcantilever-direction of deflection

[engineer23]

Microcantilever---direction of deflection

I was reading a paper on a microcantilever-based hydrogen sensor. Here's how the sensor works (as I understand it):
The cantilever is coated with Palladium film. In the presence of Palladium, diatomic hydrogen splits into H atoms, which are absorbed by the cantilever. The stress imposed by this absorbance bends the canitlever, which changes the capacitance. Thus, percent hydrogen is a function of capacitance.

I would expect the deflection to be downward. However, the plots in the article show that the values for cantilever deflection (as measured by an optical sensor) are positive. For example, for 10% H2 concentration, the measured deflection is ~.4 micrometers.
For 0.4% H2 concentration, the deflection is 1.2 micrometers.

Can someone explain this behavior? The diagram in the article that shows how the device works has the cantilever bending downward (which I assume would be a negative value of deflection), but the plots show positive deflection values.

Thanks for any help!

 

[mgb_phys]

Are they just using the absolute value for the deflection?
Since you presumably can't have less than zero hydrogen it's only going to bend one way and it might just be easier for them to quote deflection from the initial position rather than an absolute Z value

 

[engineer23]

I think you may be right. The diagram definitely indicates deflection is downard, so I think the values they report are deflection from initial position (absolute).

The only other thing that bothers me is that deflection appears to decrease with increasing hydrogen concentration. This doesn't make sense to me...it seems that the more H2, the more absorbed H, and the greater the deflection. Also, the uncoated (and thus unloaded) beam is reported to have a negative deflection.

The paper is by D.R. Basalt and appears in Sensors and Actuators B, vol. 88 (2003) pp. 120-131.