Campbell CR7 Manuel d'instructions télécharger pdf (Page 132)

SECTION 13. CR7 MEASUREMENTS

13-8

Table 13.3-6 summarizes maximum lead

lengths for corresponding error limits in six

Campbell Scientific sensors. Since the first

three sensors are non-linear, the voltage error,

V

e

, is the most conservative value

corresponding to the error over the range

shown.

MINIMIZING SETTLING ERRORS IN NON-

CAMPBELL SCIENTIFIC SENSORS

When long lead lengths are mandatory in

sensors configured by the user, the following

general practices can be used to minimize or

measure settling errors:

1. When measurement speed is not a prime

consideration, Instruction 4 (Excite, Delay

and Measure) can be used to ensure ample

settling time for half-bridge, single-ended

sensors.

2. An additional low value bridge resistor can

be added to decrease the source

resistance, R

o

. For example, assume a YSI

non-linear thermistor such as the model

44032 is used with a 30 kohm bridge

resistor, R'

f

. A typical configuration is

shown in Figure 13.3-7A. The

disadvantage with this configuration is the

high source resistance shown in column 3

of Table 13.3-7. Adding another 1K

resistor, R

f

, as shown in Figure 13.3-7B

lowers the source resistance of the CR7

input but offers no improvement over

configuration A because R'

f

still combines

with the lead capacitance to slow the signal

response at point P. The source resistance

at point P (column 5) is essentially the

same as the input source resistance of

configuration A. Moving R

f

' out to the

thermistor as shown in Figure 13.3-7C

optimizes the signal settling time because it

becomes a function of R

f

and C

w

only.

Columns 4 and 7 list the signal voltages as a

function of temperature with a 5V excitation for

configurations A and C, respectively. Although

configuration A has a higher output signal (5V

input range), it does not yield any higher

resolution than configuration C which uses the

±150 mV input range.

NOTE: Since R

f

' attenuates the signal in

configurations B and C, one might consider

eliminating it altogether. However, its

inclusion "flattens" the non-linearity of the

thermistor, allowing more accurate curve

fitting over a broader temperature range.

TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors

Sensor Maximum

Model # Error Range V

e

(µV) Length(ft.)

107 0.05

o

C0

o

C to 40

o

C 5 965

1

207(RH) 1%RH 20% to 90% 500 1950

3

WVU-7 0.05

o

C0

o

C to 40

o

C 5 850

2

024A 3

o

@ 360

o

1390 250

2

227 - - - 2000

3

237 10 kohm 20k to 300k 500 1900

3

1

based on transient settling

2

based on signal rise time

3

limit of excitation drive

1 2 ... 127 128 129 130 131 132 133 134 135 136 137 ... 185 186

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