Campbell Scientific CR3000 Micrologger - Manual (Page 295)

Section 8. Operation
295
8.1.2.9 Self-Calibration
Read More! Related topics can be found in Offset Voltage Compensation
(p. 287).
The CR3000 self-calibrates to compensate for changes induced by fluctuating
operating temperatures and aging. Without self-calibration, measurement
accuracy over the operational temperature range is worse by about a factor of 10.
That is, over the extended temperature range of -40°C to 85
°C, the accuracy
specification of ±0.12% of reading can degrade to ±1% of reading with self-
calibration disabled. If the temperature of the CR3000 remains the same, there is
little calibration drift with self-calibration disabled.
Note Self-calibration requires the CR3000 to have an internal voltage standard.
The internal voltage standard should periodically be calibrated by Campbell
Scientific. When high-accuracy voltage measurements are required, a two-year
calibration cycle is recommended.
Unless a Calibrate() instruction is present in the running CRBasic program, the
CR3000 automatically performs self-calibration during spare time in the
background as an automatic slow sequence
(p. 143),
with a segment of the
calibration occurring every 4 seconds. If there is insufficient time to do the
background calibration because of a scan-consuming user program, the CR3000
will display the following warning at compile time: "Warning when Fast Scan x is
running background calibration is disabled".
The composite transfer function of the instrumentation amplifier, integrator, and
analog-to-digital converter of the CR3000 is described by the following equation:
COUNTS = G * Vin + B
where COUNTS is the result from an analog-to-digital conversion, G is the
voltage gain for a given input range, and B is the internally measured offset
voltage.
Automatic self-calibration only calibrates the G and B values necessary to run a
given CRBasic program, resulting in a program dependent number of self-
calibration segments ranging from a minimum of 6 to a maximum of 91. A typical
number of segments required in self-calibration is 20 for analog ranges and 1
segment for the panel temperature measurement, totaling 21 segments. So, (21
segments) * (4 s / segment) = 84 s per complete self-calibration. The worst-case is
(91 segments) * (4 s / segment) = 364 s per complete self-calibration.
During instrument power-up, the CR3000 computes calibration coefficients by
averaging ten complete sets of self-calibration measurements. After power up,
newly determined G and B values are low-pass filtered as follows.
Next_Value = (1/5) * New + (4/5) * Old
This results in
• 20% settling for 1 new value,
• 49% settling for 3 new values
• 67% settling for 5 new values
• 89% settling for 10 new values
• 96% settling for 14 new values