Campbell-scientific CR3000 Micrologger Instrukcja Użytkownika Pobierz pdf (Strona 269)

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Section 7. Installation

269

Example PRT Specifications:

• Alpha = 0.00392 (PRTType 2)

Excitation Current

Excitation current should be optimized such that when the sensor is at its

maximum-expected resistance (maximum-expected temperature), the voltage

across the resistor is close to, but does not exceed, the maximum allowed by one

of the CR3000 analog-input voltage ranges. Excitation should be limited to avoid

too much self heating. Self heating is not a major concern, but is evident if

excitations are repeated frequently, especially with excitation currents greater than

1000 µA.

Total maximum output of all PRTs in series must be less than 5000 mV,

otherwise the Input Limit

and current excitation drive range of the CR3000 may

be exceeded.

In practical terms, if limiting current to 1000 µA to avoid self heating, limiting

input channels to those on the CR3000 wiring panel, and keeping total voltage

output to 5000 mV, the CR3000 will accommodate (14) 100- PRTs or (4)

1000- PRTs. The use of multiplexers will allow more PRTs to be measured.

Contact a Campbell Scientific applications engineer for assistance when using a

multiplexer to measure additional PRTs in a current-loop configuration. These

practical limits are arrived at using Ohm’s Law:

V=I*R

where,

V=Voltage(milliVolts,mV)

I=Current(milliAmps,mA)

R=Resistance(Ohms,Ω)

For 100- PRTs:

Keeptotalvoltageto≤5000mV

V=I*R

V=14*(1mA*100Ω)

V=14

00mV

Conclusion:Limitisimposedbytheavailablenumberofdifferential

inputchannels.

For 1000- PRTs:

Keeptotalvoltageto≤5000mV

V=I*R

V=4*(1mA*1000Ω)

V=4000mV

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