In-Situ TROLL 9500 Operators Manual - Manual (Page 150)

145
Multi-Parameter
Water Quality TROLL ®
TROLL 9500 Operator’s Manual 0095110 rev. 007 01/09
APPENDIX
ELECTRONIC DRIFT AND DEVICE RECALIBRATION
The electronics of the Multi-Parameter TROLL 9500 will experience
accuracy drift over time. This drift applies to all channels and is addi-
tive to the initial calibration accuracy. Electronic drift does not apply to
user-calibrated sensors because the drift is compensated for during
calibration.
The system wide long-term drift is dominated by the stability of the
voltage reference used in the device. Other components exhibit long-
term drift of a much smaller magnitude.
The equation used to calculate drift over time is:
D = K
where:
D is the drift in ppm
K is the long term stability coefficient listed as ppm / 1000 hours
but actual units are ppm
t is the time expressed in hours
Using this information, the maximum drift over time is shown in the
table on this page.
DETERMINING DENSITY
The density of water in a well can be determined using an accurate
tape measure and a pressure transducer. A change in pressure is
recorded between two points as the transducer is lowered into the
water and the corresponding change in depth is recorded using the
measuring tape. Density is then calculated using a simple formula.
This method is valid if the pressure and depth measurements are
accurate and the water within the well is homogeneous throughout the
entire depth of the well.
TIP: The recommended frequency of factory recalibration of
the MP TROLL 9500 depends upon the amount of drift a
user is willing to tolerate. For example, if a drift of 0.025% is
acceptable, then the recalibration period is 6 months; if a drift of
0.05% is acceptable, then the recalibration period is 2 years.