Campbell Scientific CR3000 Micrologger - Manual (Page 269)

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-Ω PRTs or (4)
1000-Ω 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-Ω PRTs:
Keep total voltage to ≤ 5000 mV
V = I * R
V = 14 * (1 mA * 100 Ω)
V = 1400 mV
Conclusion: Limit is imposed by the available number of differential
input channels.
For 1000-Ω PRTs:
Keep total voltage to ≤ 5000 mV
V = I * R
V = 4 * (1 mA * 1000 Ω)
V = 4000 mV