Lake Shore 372, Руководство пользователя, Страница: 48 / 214

36 c
HAPTER
2: Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
2.6.6 Interference
Between Two Model
372s The digital phase sensitive detectors in the Model 372 effectively cancel noise from
external sources, but they do have some difficulty canceling noise at or very near their
own measurement frequencies. An odd operating frequency was intentionally cho-
sen to minimize the possibility that outside noise sources would be the same fre-
quency as the instrument. One source of noise that can be very close to the
measurement frequency of a Model 372 is another Model 372. When two or more
instruments are operated with leads closely coupled in the same Dewar it is possible
for them to beat against each other and cause small oscillations. If this happens, the
measurement frequency of one instrument can be changed slightly to avoid interfer-
ence. Because of this, it is also recommended to keep the control input and measure-
ment input excitation frequencies set to different values when being used
simultaneously.
2.6.7 Vibration Noise Noise can be generated in lead cables when they are moved or vibrated. This noise
can be related to the triboelectric effect or changes in capacitance between conduc-
tors in the cable. A sudden movement of measurement lead cables can overload the
instrument momentarily when it is on the most sensitive ranges. Cables carrying
small signals should be fastened down without putting strain on the connections. For
similar reasons, physical vibration of the Model 372 during sensitive measurements
should be avoided.
2.7 Measurement
Error This section discusses measurement error.
2.7.1 Warm Up and
Temperature Drift When the Model 372 is first turned on it should be allowed to warm up for at least 30
min before use to allow its internal temperature to stabilize. Warm up is an exagger-
ated form of temperature drift because the temperature change inside the enclosure
is larger and faster than would be experienced in a laboratory setting. Temperature
drift, change in reading with change in ambient temperature, is small in the Model
372 and seldom a significant factor in absolute accuracy. Measurement repeatability
is often dominated by temperature drift and temperature change should be consid-
ered if the instrument is being installed in a rack. If possible, precision measurement
instruments should not be placed in the same rack with large power supplies or other
equipment that can change the rack temperature quickly. Instrument racks should be
well ventilated to prevent the build up of heat.
2.7.2 Resistor
Installation Installation is an important part of resistor measurements at cryogenic tempera-
tures. The measured resistor must be at the temperature of its surroundings for the
measurement to be valid. Some installation suggestions for temperature sensors are
given in section 2.9. These suggestions are applicable to any resistor.
2.7.3 Resistor Self-
Heating and Thermal
Resistance A resistor self heats when the electrical power being put into the measured resistor is
large enough to warm the resistor above the temperature of its surroundings. Electri-
cal power gets into the resistor from both the necessary excitation current and unde-
sirable noise (section 2.6). Errors from self-heating are dangerous because they can
easily go undetected. One strategy for detecting these errors is to understand the
thermal resistance of the resistor and predict its reaction to excitation and noise.
Self-heating (
,
T
sh
) is a product of the resistor’s thermal resistance (R
t
) expressed in
kelvin per watt (K/W), and excitation power (P
s
) expressed in watts.
,
T
sh
= R
t
P
s
; P
s
can be calculated as excitation current squared, times measured resis-
tance (P
s
= I 2 R).