Campbell CR800 Series, Operator'S Manual

Page: 320 / 598

Section 8. Operation
320
• 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 CR800.
•
In difficult cases, settling error can be measured as described in
Measuring Settling Time
(p. 320).
Measuring Settling Time
Settling time for a particular sensor and cable can be measured with the CR800.
Programming a series of measurements with increasing settling times will yield
data that indicate at what settling time a further increase results in negligible
change in the measured voltage. The programmed settling time at this point
indicates the settling time needed for the sensor / cable combination.
CRBasic example
Measuring Settling Time
(p. 320)
presents CRBasic code to help
determine settling time for a pressure transducer using a high-capacitance
semiconductor. 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 pressure. Reviewing CRBasic Programming —
Details
(p. 119)
may help in understanding the CRBasic code in the example.
The first six measurements are shown in table First Six Values of Settling Time
Data
(p. 321).
Each trace in figure
Settling Time for Pressure Transducer
(p. 321)
contains all twenty PT() mV/V values (left axis) for a given record number, along
with an average value showing the measurements as percent of final reading (right
axis). The reading has settled to 99.5% of the final value by the fourteenth
measurement, which is contained in variable PT(14). This is suitable accuracy
for the application, so a settling time of 1400 µs is determined to be adequate.
Measuring Settling Time
'This program example demonstrates the measurement of settling time using a single
'measurement instruction multiple times in succession. In this case, the program measures
'the temperature of the CR800 wiring panel.
Public RefTemp 'Declare variable to receive instruction
BeginProg
Scan (1,Sec,3,0)
PanelTemp (RefTemp, 250) 'Instruction to make measurement
NextScan
EndProg measures the settling time of a sensor measured with a differential
'voltage measurement
Public PT(20) 'Variable to hold the measurements
DataTable (Settle,True,100)
Sample (20,PT(),IEEE4)
EndTable