Omega Engineering CYD201, Operator'S Manual

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OMEGA Model CYD201/CYD208 User’s Manual
Installation 2-3
3. Keep leads as short as possible.
4. Use twisted-pair wire. Use Duo-Twist™ wire (or equivalent) for two-wire,
or Quad-Twist™ wire (or equivalent) for four-wire applications.
5. Thermally anchor lead wires.
2.4.1 Two-Lead Vs Four-Lead Measurements
In two-lead measurement, the leads that measure sensor voltage also carry
the current. The voltage measured at the instrument is the sum of the
temperature sensor voltage and the IR voltage drop within the two current
leads. Since heat flow down the leads can be critical in a cryogenic
environment, wire of small diameter and significant resistance per foot is
preferred to minimize this heat flow. Consequently, a voltage drop within the
leads may exist.
Four-lead measurement confines current to one pair of leads and measures
sensor voltage with the other lead pair carrying no current.
2.4.1.1 Two-Lead Measurement
Sometimes system constraints dictate
two-lead measurement. Connect the
positive terminals (V+ and I+) together
and the negative terminals (V– and I–)
together at the instrument, then run
two leads to the sensor.
Expect some loss in accuracy; the
voltage measured at the voltmeter equals the sum of the sensor voltage and
the voltage drop across the connecting leads. The exact measurement error
depends on sensor sensitivity and variations resulting from changing
temperature. For example, a 10
lead resistance results in a 0.1 mV
voltage error. The resultant temperature error at liquid helium temperature is
only 3 mK, but, because of the lower sensitivity (dV/dT) of the diode at
higher temperatures, it becomes 10 mK at liquid nitrogen temperature.
2.4.1.2 Four-Lead Measurement
All sensors, both two-lead and four-lead devices,
can be measured in a four-lead configuration to
eliminate the effects of lead resistance. The exact
point at which the connecting leads solder to the
two-lead sensor normally results in a negligible
temperature uncertainty.
I+
V+
I–
V–
Two-Lead
Measurements
I+
V+
I–
V–
Four-Lead
Diode