Recalibration - Time Electronics 7000 User Manual

Time Electronics
7000 RTD Calibrator
8

Recalibration

8.1.1 Overview
The 7000 is designed around a PIC microcontroller that takes its analogue readings from a
24bit sigma delta A/D. To limit the drift in the A/D there is a background calibration of the A/D
at regular intervals. The operation of the 7000 is based on a ratio technique that compares
the voltage across the unknown resistance to the voltage across an internal precision
resistor. The internal precision resistor defines the maximum reading from the instrument as
approximately 2.6 kΩ, and is used as the full scale adjustment (gain trim).
The zero offset adjustment is done by storing calibration factors in the microcontroller's
non-erasable EEProm memory. This memory also contains the Pt100 scale factors.
The EEProm is programmed at manufacture and should not need readjustment. However,
should it become corrupted it will be necessary to return the whole unit to Time Electronics
for re-programming.
8.1.2 Equipment required for re-calibration
1. A precision resistance measuring instrument - such as a Time Electronics 5075 7.5-
digit multimeter. The accuracy of this instrument will define the overall calibration
accuracy. It should have an accuracy of better than 0.01 ohms over the range 0 to
2.6 kΩ. It must also be suitable for 4-wire measurement, and its excitation current
needs to be 1mA in the range 20 to 360Ω. Auto-ranging should be turned off to
ensure the 1mA excitation current is maintained.
2. Stable transfer resistors 20 Ω to 360 Ω.
21
User Manual