Campbell Scientific CR3000 Micrologger - Manual (Page 292)

Section 8. Operation
292
The CR3000 delays after switching to a channel to allow the input to settle before
initiating the measurement. The
SettlingTime parameter of the associated
measurement instruction is provided to allow the user to tailor measurement
instruction settling times with 100 µs resolution up to 35000 µs. Default settling
times are listed in table CRBasic Measurement Settling Times
(p. 292),
and are
meant to provide sufficient signal settling in most cases. Additional settling time
may be required when measuring high-resistance (high-impedance) sensors and /
or sensors connected to the datalogger by long leads. Measurement time of a
given instruction increases with increasing settling time. For example, a 1 ms
increase in settling time for a bridge instruction with input reversal and excitation
reversal results in a 4 ms increase in time for the CR3000 to perform the
instruction.
Table 59. CRBasic Measurement Settling Times
Settling
Time
Entry
Input
Voltage
Range
Integration
Code
Settling
Time
1
0 All 250 200 µs (default)
0 All _50Hz 3 ms (default)
0 All _60Hz 3 ms (default)
>100 All X 2 μs entered
1
Minimum settling time required to allow the input to settle to CR3000 resolution specifications.
2
X is an integer >100.
A settling time is required for voltage measurements to minimize the effects of the
following sources of error:
• A small switching transient occurs when the CR3000 switches to the single-
ended or differential channel to be measured.
• A relatively large transient may be induced on the signal conductor via
capacitive coupling during a bridge measurement from an adjacent excitation
conductor.
• 50-Hz or 60-Hz integrations require a relatively long reset time of the internal
integration capacitor before the next measurement due to dielectric
absorption.
8.1.2.8.1 Minimizing Settling Errors
When long lead lengths are required the following general practices can be used
to minimize or measure settling errors:
• Do not use wire with PVC-insulated conductors. PVC has a high dielectric,
which extends input settling time.
• Where possible, run excitation leads and signal leads in separate shields to
minimize transients.
• When measurement speed is not a prime consideration, additional time can be
used to ensure ample settling time. The settling time required can be
measured with the CR3000.