Campbell Scientific CR3000 Micrologger - Manual (Page 293)

Section 8. Operation
293
• When relatively large resistances are measured (> 1000 ohms), or relatively
long cable lengths are used (> 50 foot) with sensors requiring current
excitation, a 0.1-µf feed-forward capacitor should be placed between IX and
IXR to prevent excessive ringing
(p. 465).
With this capacitor present a
minimum of 3 ms is recommended for the SettlingTime parameter in the
measurement instruction. The capacitor simply connects between the chosen
IX output terminal and the IXR return terminal. The capacitor has no
polarity.
8.1.2.8.2 Measuring the Necessary Settling Time
Settling time for a particular sensor and cable can be measured with the CR3000.
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 true settling time for the sensor and cable combination.
CRBasic example Measuring Settling Time
(p. 293)
presents CRBasic code to help
determine settling time for a pressure transducer utilizing a high-capacitance
semi-conductor. 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 the section Programming
(p. 114)
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. 294).
Each trace in figure Settling Time for Pressure Transducer
(p. 294)
contains all twenty PT() mV/Volt 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.
CRBasic Example 65. Measuring Settling Time
'Program to measure 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
BeginProg
Scan (1,Sec,3,0)
BrFull (PT(1),1,mV20,1,Vx1,1,2500,True,True,100,250,1.0,0)
BrFull (PT(2),1,mV20,1,Vx1,1,2500,True,True,200,250,1.0,0)
BrFull (PT(3),1,mV20,1,Vx1,1,2500,True,True,300,250,1.0,0)
BrFull (PT(4),1,mV20,1,Vx1,1,2500,True,True,400,250,1.0,0)
BrFull (PT(5),1,mV20,1,Vx1,1,2500,True,True,500,250,1.0,0)
BrFull (PT(6),1,mV20,1,Vx1,1,2500,True,True,600,250,1.0,0)