# Tactile pressure sensor and drift. Reading the datasheet

I am looking at the specs of tactile pressure sensor and I can't understand what this means:

"Drift: <3.3% per logarithmic time scale"

How should I understand this? Does this sensor drifts a lot or this is actual good. I am looking for cheap way for measuring wight changes over a long period of time.

Cheers Mitko.

• A link to the actual part or (better) its datasheet would be useful
– user16324
Commented Oct 31, 2014 at 12:12
• Here you go: tekscan.com/pdf/FLX-A401-force-sensor.pdf Commented Oct 31, 2014 at 12:15

There may be some relaxation mechanism, the way a dent will appear in a carpet if you leave a chair leg sitting on it too long. This specification places an upper limit on the effect that will have on the reading.

So if you read 100N force after 1 second, the reading may have drifted by 5% or 5N (or less) after 10 seconds, 10N (or less) after 100 seconds, 15N after 1000 seconds and so on. (I am assuming they mean log(base 10) but the datasheet is not explicit on that point)

It's not obvious if there is a lower limit to this behaviour (i.e. 15% difference between 1ms and 1 second) but I would assume so, down to the 5 us specified settling time.

Another question is : does the same drift apply when you then remove the weight? (the chair leg depression disappears eventually after you move the chair). If I were contemplating this sensor the first thing I would do is get my hands on one and characterise it with actual measurements. It may be better than the spec, bearing in mind that future production may vary within these limits.

Whether this is good or bad depends on whether it meets your needs, and how well it compares with other sensor technologies within your budget.

• So if I understand correctly, the sensor will drift less and less with time. So if we continue the carpet analogy at first the carpet will compress a lot, but with time it will settle in a state and will drift less. Commented Oct 31, 2014 at 13:03
• For any one weight reading, yes.
– user16324
Commented Oct 31, 2014 at 17:08