Ode and Mike,
As you indicate, the range that we would use the PWT is at the lower end of the
meter's range. The accuracy, when calibrated at full scale, along with the
linearity of the response, makes the error large when compared to the smaller
readings at the low end of the scale.
I would suggest, then, that we increase the accuracy of the readings in the
range that we will be using them. The way to do this is to calibrate the meter
at the maximum uS reading that we are likely to encounter, say, between 25 to
30 uS. This will throw off the accuracy at readings substantially above this
level, but will increase the accuracy between 0 and 30 uS. So, unless the
linearity of the meter's response between 0 and 30 uS is particularly bad, we
would approach an accuracy of near +/- 2% of full scale, with full scale being
20 or 30 uS (our new "full scale" value). If the linearity between 0 and 30 uS
is particularly bad, then we will not achieve this accuracy, and the accuracy of
the meter with normal calibration will also be bad; in short the meter would be
useless for our purposes...
Dan
Re: CS>Hanna Meter Model Number
* From: Mike Monett (view other messages by this author)
* Date: Sat, 16 Oct 2004 06:48:15
Ken, thanks for the good info, especially on controlling
temperature!
As far as getting two units to agree, the spec is +/-2% of full
scale. This means +/-2ppm, which means one unit could read 4 ppm
higher or lower than another and they would still be in spec. A
slight difference in temperature would increase the discrepancy.
The problem is the units are design for higher conductivity than we
typically get in cs, so our readings are at the bottom 20% of the
range. This is the least accurate portion. Here's the specs from
Hanna's site:
Range : 0.1 to 99.9 uS/cm
Resolution : 0.1 uS/cm
Accuracy (@20C/68F) : +/-2% Full Scale
http://www.hannainst.com/products/testers/pwt.htm
Although individual units may differ by more than we would like, one
would hope they would be consistent and repeatable. As far as I can
determine from searching the archives, most people seem to feel the
units are repeatable. I know you have posted seeing different
results, but it is not clear what caused them.
Best Wishes,
Mike Monett
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