Campbell Scientific CR5000 Measurement and Control Module - Manual (Page 60)

Section 3. CR5000 Measurement Details
3-6
1. The signal must rise to its correct value.
2. A small transient caused by switching the analog input into the
measurement circuitry must settle.
3. When a resistive bridge measurement is made using a switched excitation
channel, a larger transient caused when the excitation is switched must
settle.
3.3.1 Minimizing Settling Errors
When long lead lengths are mandatory, 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 CR5000.
3.3.2 Measuring the Necessary Settling Time
The CR5000 can measure the time required for a particular sensor/cable
configuration to settle. This is done by allowing the signal to settle the
minimum amount of time and then making zero integration measurements
every 100
µ
s. Looking at the series of measurements it is possible to see the
settling of the sensor signal. By counting the number of measurements before
the signal settles the correct settling time can be determined (100
µ
s per
measurement plus an extra 100
µ
s if the measurement is made on the 20 mV
range where the default settling time is 200
µ
s).
This technique for measuring settling time can only be used with
instructions that measure only one voltage per repetition. The
settling time to use with instructions that make more than one
measurement can be determined with an instruction that uses
only one measurement. For example, for the BrFull6W
determine the settling time with BrFull; for BrHalf3W use
BrHalf.
The following example demonstrates measuring the settling time for a
differential voltage measurement. If you are not yet familiar with CR5000
programming, you may want to read Section 4 before trying to follow the
example.
The series of measurements on the sensor is made with one instruction.
Measurement instructions have a repetitions parameter that allow one
instruction to repeat the measurement on a number of channels. There is also
the capability to repeat measurements on the same channel. When
NOTE