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26
c
HAPTER
2:
Cooling System Design and Temperature Control
Model 350 Temperature Controller
Once the total uncertainty is calculated, the noise or instrument measurement reso-
lution can be factored in. The instrument noise is calculated using the measurement
resolution listed in TABLE 1-3. The temperature equivalent noise is calculated by
dividing the measurement resolution by the sensor’s sensitivity at the temperature of
interest. For a typical CX-1010 at 100 mK the measurement resolution from
is listed at 10
)
. This results in 18 µK of noise (10
)
÷ 558,110
)
/K = 18 µK).
Knowing the noise and uncertainties of the sensors at different excitations can help
in the selection and design of the overall measurement.
Note that in TABLE 2-1, the self-heating is only a rough estimate as the thermal resis-
tance, electrical resistance, and sensitivity are all strong, nonlinear functions of tem-
perature and will change rapidly as you self-heat away from the zero power
resistance. Conversely, self-heating is a reproducible error; so, if self-heating can be
accurately calculated or measured, the self-heating offset can be calibrated out.
The optimum excitation is a balance between minimizing self-heating errors (low
excitation) and maximizing resolution and accuracy (large excitation). TABLE 2-1
illustrates the tradeoffs that need to be made when selecting a sensor and excitation
current. Lowering excitation current can dramatically decrease the error due to self-
heating, but sometimes reducing the current can actually increase the overall uncer-
tainty of the measurement.
As seen in TABLE 2-1, the best uncertainty varies among the three sensors and their
corresponding excitation ranges. The best values are shaded to guide the eye. The CX-
1010 has a high self-heating error at 30 nA, so it is likely that a lower current will be
preferable. The GR-50 sensor can be driven with up to 300 nA, but lower currents sig-
nificantly reduce the self-heating. The GR-50’s total uncertainty drops to its lowest
level at 30 nA, but if the excitation is further reduced to 10 nA the total uncertainty
increases due to the contribution of the instrument accuracy. The RX-102B illustrates
the tradeoff even clearer; the total uncertainty almost doubles from 30 nA to 10 nA.
Summary of Contents for 350
Page 4: ...Model 350 Temperature Controller ...
Page 6: ......
Page 14: ...Model 350 Temperature Controller ...
Page 28: ...14 cHAPTER 1 Introduction Model 350 Temperature Controller ...
Page 53: ...Examples of PID control 2 13 4 ManualOutput 39 www lakeshore com FIGURE 2 4 ...
Page 58: ...44 cHAPTER 2 Cooling System Design and Temperature Control Model 350 Temperature Controller ...
Page 118: ...104 cHAPTER 5 Advanced Operation Model 350 Temperature Controller ...
Page 178: ...164 cHAPTER 7 Options and Accessories Model 350 Temperature Controller ...
Page 192: ...178 cHAPTER 8 Service Model 350 Temperature Controller ...