Campbell CR23X Spécifications télécharger pdf (Page 209)

SECTION 13. CR23X MEASUREMENTS

13-9

NOTE: Excitation transients are eliminated

if excitation leads are contained in a shield

independent from the signal leads.

The size of the peak transient is linearly related

to the excitation voltage and increases as the

bridge resistor, R

, increases. Table 13.3-4

shows measured levels of V

for 1000 foot

lengths of three Belden wires used in Campbell

Scientific sensors. Values are given for R

equal to 1 kohm and 10 kohm. Table 13.3-4 is

meant only to provide estimates of the size of

excitation transients encountered; the exact

level will depend upon the specific sensor

configuration.

Equation 13.3-7 can be solved for the maximum

lead length, L, permitted to maintain a specified

error limit. Combining Equations 13.3-7 and

13.3-4 and solving for L gives:

L = -(R

+ (t/ln(V

)))/R

[13.3-15]

where V

is the measurement error limit.

EXAMPLE LEAD LENGTH CALCULATION

FOR 107 TEMPERATURE SENSOR

Assume a limit of 0.05°C over a 0°C to +40°C

range is established for the transient settling

error. This limit is a reasonable choice since it

approximates the linearization error over that

range. The output signal from the thermistor

bridge varies nonlinearly with temperature

ranging from about 100 µV/°C at 0°C to 50

µV/°C at 40°C. Taking the most conservative

figure yields an error limit of V

= 2.5 µV. The

other values needed to calculate the maximum

lead length are summarized in Table 13.3-5 and

listed below:

1) V

~ 50 mV, peak transient at 2 V excitation

2) V

~ 2.5 µV, allowable measurement error

3) t = 450 µs, CR23X input settling time

4) R

= 1 kohm, 107 probe source resistance

5) C

= 3.3 nfd, CR23X input capacitance

6) C

~ 42 pfd/ft., lead wire capacitance

Solving Equation 13.3-15 gives a maximum

lead length of:

L ~ 1003 ft., error ~ 0.05°C

Setting the allowable error at 0.1°C or

approximately 5 µV, the maximum lead length

increases to:

L ~ 1085 ft., error ~ 0.1°C

13.3.4 SUMMARY OF SETTLING ERRORS FOR

CAMPBELL SCIENTIFIC RESISTIVE

SENSORS

Table 13.3-5 summarizes the data required to

estimate the effect of lead length on settling

errors for Campbell Scientific's resistive

sensors. Comparing the transient level, V

, to

the input range, one suspects that transient

errors are the most likely limitation for the 107

sensor. The sensors in the WVU-7 are the

same as in the Model 107 (the lead wire is

different), but the signal leads for the WVU-7

wet- and dry-bulbs are not subject to excitation

transients because they are shielded

independently from the excitation.

TABLE 13.3-5. Summary of Input Settling Data For Campbell Scientific Resistive Sensors

Sensor Belden Ro Cw τ

ττ

τ* Input

Model # Wire # (kohms) (pfd/ft.) (us) Range(mV) V

(mV) V

(mV)**

107 9661 1 42 45 10 2500

227 9661 0.1-1 42 5-45 1000 500

237 9661 1 42 45 50 5000

034A 9721 1-6 41 1-222 5000 5000

0-90

* Estimated time constants are for 1000 foot lead lengths and include 3.3nfd CR23X input

capacitance.

** Measured peak transients for 1000 foot lead lengths at corresponding excitation, V

1 2 ... 204 205 206 207 208 209 210 211 212 213 214 ... 373 374

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