User`s manual

50 cHAPTER 4: Operation
Model 335 Temperature Controller
When Current Reversal is On, the sensor excitation current is a 10 Hz square wave (5 Hz
for NTC RTD on the 100 K range). This square wave excitation generates a small electro
-
magnetic noise signal in the sensor cable, which can be picked up by sensitive measure-
ment equipment in the system. Turning Current R
eversal off will eliminate this noise at
the cost of introducing thermal EMF voltage errors into the sensor measurement.
Menu Navigation:
Input SetupQInp
ut (A, or B) QCurrent Reversal (Off or On)
Default: On
Interface Command: INT
YPE
4.4.6 Thermocouple
Sensor Input Setup
(Model 3060 Only)
When a Model 3060 thermocouple option is installed in the Model 335, a thermocou-
ple option becomes available under the Sensor T
ype parameter in the Input Setup
menu. The standard diode/RTD sensor inputs can still be used when the thermocou-
ple option is installed, but the thermocouple and standar
d inputs cannot be used
simultaneously. Refer to section 8.12 to install the Model 3060.
Thermocouples include a variety of commercial (such as E, K, T) and specialt
y types
such as cryogenic (Chromel-AuFe). Standard curves are included in the Model 335 for
the types listed in TABLE 4-6. Other types can be used as long as an appropriate tem-
perature response curve is loaded as a use
r curve. Representative thermocouple
specifications are given in TABLE 1-2. The Model 335 provides one thermocouple
range and no excitation because thermocouples do
not require it. Internal room tem-
perature compensation is included for conv
enience (section 4.4.6.2) and should be
calibrated before use. Room temperature com
pensation is enabled by default, but
can be turned off if external compensation is being used.
Menu Navigation:
Input SetupQInp
ut (A or B)QSensor Type (Thermocouple)
Interface Command: INT
YPE
4.4.6.1 Internal Room Temperature Compensation
Room-temperature compensation is required to
give accurate temperature measure-
ments with thermocouple sensors. It corrects for the temperature difference
between
the instrument thermal block and the curve normalization temperature of
0 °C. An external ice bath is the most accurate form of com
pensation, but is often
inconvenient. The Model 335 has internal room-temperature compensation that is
adequate for most applications. The internal compensation can be turned on or off by
the user. It operates with any thermocouple type that has an appropriate tempera-
ture response curve loaded. Room-temperature compe
nsation is not meaningful for
sensor units measurements.
Room temperature compensation should be calibrated as part of every installation 
(section 4.4.6.2).
Menu Navigation:
Input SetupQInp
ut (A or B)Q
ENTER
QRoom Compensation (Off or On)
Default: On
Interface Command: INT
YPE
4.4.6.2 Internal Room Temperature Compensation Calibration Procedure
Factory calibration of the instrument is accurate
to within approximately ±1 K. Differ-
ences in thermocouple wire and installation technique create errors greater than the
instrument e
rrors. The best accuracy is achieved by calibrating with the thermocou-
ple actually being used, because it eliminates most sources of error. If that is not pos-
sible, use a thermocouple made from the same wire.