Specifications
Section 4. Sensor Support
4-11
TABLE 4.2-7. CRBASIC Measurement Settling Times
Settling
Time
Entry
Input
Voltage
Range
Integration
Code
Settling
Time*
0 All 250 ms 450 ms (default)
0 All _50Hz 3 ms (default)
0 All _60Hz 3 ms (default)
>100 All All μs entered
*Minimum settling time required to allow the input to settle to CR1000 resolution specifications.
A finite settling time is required for CR1000 voltage measurements for the
following reasons:
1. A small switching transient occurs when the CR1000 switches to the
single-ended or differential channel to be measured.
2. When switched voltage excitation is used in a bridge measurement, a
relatively large transient on the signal conductor may be induced by
capacitive coupling from the nearby excitation conductor in the cable.
3. Long 50 or 60 Hz integrations require a relatively long reset time of the
internal integration capacitor before the next measurement due to dielectric
absorption.
4.2.6.1 Minimizing Settling Errors
When long lead lengths are required the following general practices can be
used to minimize or measure settling errors:
1. DO NOT USE WIRE WITH PVC INSULATED CONDUCTORS. PVC
has a high dielectric which extends input settling time.
2. Where possible, run excitation leads and signal leads in separate shields to
minimize transients.
3. 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 CR1000.
4.2.6.2 Measuring the Necessary Settling Time
Settling time for a particular sensor and cable can be measured with the
CR1000. Programming a series of measurements with increasing settling times
will yield data that indicates at what settling time a further increase results in
negligible change in the measured voltage. The programmed settling time at
this point indicates the true settling time for the sensor and cable combination.
EXAMPLE 4.2-1 presents CRBASIC code to help determine settling time for a
press
ure transducer with 200 feet of cable. The code consists of a series of
full-bridge measurements (BrFull) with increasing settling times. The pressure
transducer is placed in steady-state conditions so changes in measured voltage
are attributable to settling time rather than changes in the measured pressure.