Omega Engineering CYTX231 Series, Руководство пользователя

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Omega Model CYTX231 User’s Manual
2-2 Installation
2.3 SENSOR INSTALLATION RECOMMENDATIONS
Refer to the Omega Product Catalog for installation details and sensor
specifications. Call Omega for copies of application notes or sensor
installation questions.
Below are general recommendations on sensor
installation:
1. Do not ground the sensor.
2. Shield leads and connect shield wire to SHIELD on screw terminal
connector only. Do not connect shield at other end of cable.
3. Keep leads as short as possible.
4. Use twisted-pair wire. Use Omega CYW4 wire (or equivalent) for
two-wire, or four-wire applications.
5. Thermally anchor lead wires.
2.3.1 Two-Lead versus 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.
2.3.1.1 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
Ω
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 diode’s lower
sensitivity (dV/dT) at higher temperatures, it becomes 10 mK at liquid
nitrogen temperature.
I+
V+
I–
V–
Two-Lead
Measurements