© Isothermal Technology
Page 22 of 102
923 milliK manual - issue: 1.10
In some industrial applications a 3-wire connection technique is used in order to reduce the
cost of cabling (important when a large number of PRTs and long cable runs are involved,
such as in a petrochemical plant). In this arrangement, there is a separate current and voltage
sense connection to one side of the PRT element, but the other side has a common
connection:
The milliK measures the voltage developed across the I- wire by measuring the voltage
between pins 4 and 6. Provided the wires are all the same gauge and length (same resistance)
the voltage drop down the I+ wire will be the same, so the milliK can subtract this correction
from the voltage measured between V+ and V- (pins 1 and 6) to determine the voltage across
the PRT element. In this way the milliK is able to perform a 3-wire measurement that is
substantially immune to the resistance of the wires. The common and current leads should be
cut to the same length and connected directly to pins 2 and 3 and a link wires (carrying no
current) should be used to link pins 1 to 2 and pins 3 to 4 in order to minimise uncertainty due
to the limitations of 3-wire measurement (inequalities in the wire resistance).
If you wish to make temporary connections to a 3-wire PRT using intermediate connectors
(typically required for calibrating 3-wire PRTs), then you must ensure that the resistance in
the extension connections to the I+ and I- wires is the same:
This can be achieved by using the same gauge and length of wire for the I+ and I- leads. Use
the lowest resistance wire (that is mechanically suitable) so as to minimise the resistance of
