Lake Shore 234, User Manual

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Lake Shore Model 234/234D User’s Manual
2-4 Installation
2.3.4 Proper Shielding Techniques
Since the Model 234 excitation voltage is so low, shield resistance sensor
leads to avoid inducing measurement errors due to AC noise. Use twisted
pair cable, one pair for current and one for voltage, with an overall shield.
The most common shield configuration connects the shield to the Model 234
SHIELD connection (measurement ground potential) at one end and leaves
the other end open. The other end of the shield is left open since most
vacuum jackets, or feedthrough bulkheads, are at earth ground potential.
Connecting the shield at both ends produces a ground loop that adversely
affects measurement.
Some systems can benefit from continuous shielding, such as those in which
the shield is carried as a conductor through the bulkhead, rather than
terminated at it, or those in which the vacuum jacket is not at earth ground
potential. Try different shielding configurations if “noisy” readings are
encountered.
2.4 SENSOR MEASUREMENT
The Model 234 analog control loop maintains a constant excitation voltage of
5 mV or 10 mV across the sensor. Multiple scale resistors (see Paragraph
5.4) convert the resulting sensor current to voltage. The scale resistor with
the best measurement resolution is selected. Once the proper scale is
determined, the voltage across the scale resistor is measured. The
excitation voltage is then reversed. The new reading is combined with the
previous reading to eliminate any thermals present and the sensor
resistance and log resistance is calculated.
With no CalCurve present, the output is the log of the resistance. Place the
T/R switch in the R position. If the I/V OUT switch is in the I position (4 - 20
mA out), the output is 4 mA for the log(R) equal to 0 (1
Ω
) and 20 mA for the
log(R) equal to 6 (1 M
Ω
). A resistance of 1 k
Ω
results in an output of 12 mA
(log(R) equal to 3). If input resistance is greater than 400 k
Ω
, output is 0
current. A resistance greater than 400 k
Ω
is considered an open and 0
current is used as an alarm indication for a 4 - 20 mA loop. If the I/V OUT
switch is in the V position (0 - 20 mA out), output is 0 mA for the log(R) equal
to 0 (1
Ω
) and 20 mA for the log(R) equal to 6 (1 M
Ω
). A resistance of 1 k
Ω
results in an output of 10 mA (log(R) = 3). (For the Model 234D only,
although the transmitter output is still log (R), the display is simply R.)