Standby Current; Three-Wire Offset - Fluke Hart Scientific 1575 Super-Thermometer User Manual

optimum current depends on the probe. Most 25Ω SPRTs and 100Ω PRTs and
RTDs work well with 1.0 mA. High-temperature 0.25Ω SPRTs may require 10
mA. Thermistor probes use a much smaller current, typically 0.01 mA. Consult
the specifications for your probe for the recommended driving current.
TIP: Program the data window to show the current so you can easily see
that it's set correctly (see Section 7.5.3).
The range and resolution with which the current can be set will depend on the
reference resistance and the input resistance. The actual current may differ
slightly from the setting because of limited resolution and accuracy of the cur-
rent source. Current may also be limited if the resistance is large.
7.4.3.5

Standby Current

This option is only enabled when the 1590 is used with the 2590 Scanner. It
sets the standby current applied to the probe while another is being measured.
Refer to Section 11, 2590 Scanner, for instructions on using the standby cur-
rent feature of the scanner.
7.4.3.6

Three-wire Offset

The three-wire offset (3W OFFSET) parameter is used when measuring tem-
perature using two-wire or three-wire RTDs. It adjusts the measured resistance
to compensate for lead resistance. This is not necessary with four-wire probes
since the lead resistance is canceled automatically.
CAUTION: When using four-wire probes the 3W OFFSET value must be
set to zero or the measurement may not be accurate.
When using three-wire or two-wire probes, the 1575/1590 is unable to directly
cancel the lead resistance. The measured resistance will be too high by the
amount of resistance of one lead if it's three-wire or two leads if it's two-wire.
The true resistance can only be calculated by subtracting the lead resistance er-
ror from the measurement. The exact amount of resistance to subtract must be
entered for the 3W OFFSET parameter.
The lead resistance error of a three-wire probe can be determined by measuring
the resistance between the two common leads and dividing by two. Connect the
probe to the input with one of the matched wires connect to C1 and P1 and the
other connected to C2 and P2. Set up the measurement to measure resistance
(you may use the 100Ω reference and 1 mA of current).
NOTE: Be sure to divide the measured lead resistance by two to obtain
the correct 3W OFFSET value for a three-wire probe.
7 Front Panel Operation
Probe Menu
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