OMEGA Model CYD201/CYD208 User’s Manual
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.
2.4.1Two-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.
2.4.1.1Two-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.
I+
V+
Two-Lead
Measurements
V–
I–
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.
2.4.1.2Four-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.
