Lake Shore 350, User Manual

Page: 80 / 192

66 c
HAPTER
4: Operation
Model 350 Temperature Controller
4.4.1 Negative
Temperature
Coefficient (NTC)
Resistor Sensor Input
Setup NTC resistor sensors include Cernox™, Rox, Thermox and others detailed in TABLE 4-7.
More detailed specifications are provided in TABLE 1-2. When configured for NTC RTD,
the Sensor Excitation setting becomes available in the Input Setup menu. This setting
allows a choice of 1 mV or 10 mV sensor excitation voltage. This setting determines
the sensor voltage at the full scale of any given resistance range. The 1 mV voltage
level is designed to minimize self-heating effects at the sensor when used at very low
temperatures (below approximately 300 mK). The 1 mV voltage level, however,
results in approximately 10 times more noise in the sensor reading than the
corresponding resistance range at the 10 mV level, reducing measurement
resolution. Therefore it is recommended that the 10 mV level be used for applications
that do not require temperature measurement below about 300 mK.
The excitation current for the NTC RTD sensor type can vary between 10 nA and 1 mA,
depending on resistance range and sensor excitation level chosen. When autoranging
is enabled, the range will be automatically selected so that the excitation voltage is at
or below the sensor excitation setting. This keeps the power dissipated in the sensor
at a minimum, while still providing enough signal to provide accurate measurements.
In very low temperature applications, it is recommended to turn autorange off and
use a higher resistance range (lower current) to minimize the power dissipated in the
sensor, and therefore reduce self-heating effects.
For example, for a Cernox resistor that reads 10 k
)
at 150 mK, the autorange
algorithm would select the 10 k
)
range, but manually selecting the 100 k
)
range
provides 10 times less current, and therefore, 100 times less power into the sensor,
which may be required to significantly reduce the effects of self-heating. Current
Reversal is also enabled by default in order to compensate for thermal EMF voltages.
Refer to section 4.4.4 for details on the Thermal EMF Compensation (Current
Reversal) feature. Refer to section 2.7 for details on self-heating effects.
Menu Navigation:
Input Setup
Q
Input (A, B, C, or D)
Q
Sensor Type
Q
NTC RTD (Cernox)
Input Setup
Q
Input (A, B, C, or D)
Q
Sensor Type
Q
NTC RTD (Rox)
Interface Command:
INTYPE
4.4.2 Positive
Temperature
Coefficient (PTC)
Resistor Sensor Input
Setup PTC resistor sensors include the platinum and rhodium-iron sensors detailed in

TABLE 4-7. More detailed specifications are provided in TABLE 1-2. The excitation cur-
rent for the PTC RTD sensor type can vary between 1 uA and 1 mA, depending on resis-
tance range. A resistance range selection is available in order to achieve better
reading resolution. When autoranging is enabled, the range will be automatically
selected so that the excitation voltage is below 10 mV. Refer to section 4.4.3 for
details on manually selecting the range. Current Reversal is also enabled by default in
order to compensate for thermal EMF voltages. Refer to section 4.4.4 for details on
the Thermal EMF Compensation (Current Reversal) feature.
Menu Navigation:
Input Setup
Q
Input ( A, B, C, or D )
Q
Sensor Type
Q
PTC RTD (Platinum)
Interface Command:
INTYPE
4.4.3 Range Selection The Model 350 is equipped with an autoranging feature that will automatically
select the appropriate resistance range for the connected resistive temperature
device. In some cases it may be desirable to manually select the resistance range. To
manually select a resistance range, set the Autorange parameter to Off, then use the
Range parameter to select the desired range. Autorange will be On by default when-
ever the Sensor Type parameter is set to PTC RTD or NTC RTD. Autorange is not avail-
able for the Diode sensor type.