Linearity; Temperature Compensation; Level Sensing - Xylem YSI 2500 User Manual

displayed and printed as a "PL shift". While establishing a stable calibration, the 2500 Analyzer will run 5 calibrations
before aborting calibration for a sensor. The flexible parameter selection allows the user to disable this error mode.
In summary, by the default enzyme sensor calibration settings, recalibration will occur after every 5 samples or 30
minutes, after a calibration shift of 2% or greater, or after a sample module temperature drift of more than 1°C. After 5
attempts without successfully calibrating, the instrument aborts calibration for that sensor.
11.5

Linearity

As discussed earlier, an enzyme sensor consists of an electrode and an enzyme membrane. As a membrane ages, its
response becomes non-linear (shown in Figure 11-3 below).
Under optimal conditions the enzyme sensor response depends on diffusion limitation of the substrate. When the
substrate can diffuse at a greater rate than the enzyme can turnover product, enzyme kinetics defines the response and
nonlinearity is a symptom. This occurs as an enzyme membrane ages.
It is necessary to periodically check sensor linearity. YSI offers linearity standards for all of the recommended calibration
values.
11.6

Temperature Compensation

The sensitivity of the sensors, in the 2500 Analyzer, varies with temperature changes. The sample module contains a
temperature probe that monitors the fluid temperature. The sample results are temperature corrected for the difference in
temperature between the sample and the calibration.
11.7

Level Sensing

The 2500 Analyzer employs level sensing on the Sipper and, optionally, in the waste, calibrator and buffer bottles.
The Sipper level sensor detects the sample surface and then travels into the sample about 3 millimeters (1/8 inch). This
controlled immersion depth permits the use of sample tubes/plates that are filled to different heights without significant
carry-over between samples.
The Sipper and Arm Assembly should never be touched while the unit is in operation.
The calibrator and supply bottles are monitored for low levels and the waste bottle is monitored for high level.
Aging Enzyme Membrane Response
Figure 11-3
11-3