User`s manual

4.4.4 R a nge S e l e c t i o n 49
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4.4.4 Range Selection
The Model 335 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 NT
C RTD. Autorange is not avail-
able for the Diode sensor type.
Menu Navigation:
Input SetupQInp
ut (A, or B) QSensor Type (NTC RTD, NTC RTD)QAutorange (Off or On)
Input SetupQInp
ut (A, or B) QSensor Type (Diode)Q Range (See table below)
Default: On
Interface Command: INT
YPE
Range and sensor power
Sensor Type Available Range Settings Maximum Sensor Power Sensor Excitation
Diode
2.5 V (Silicon) 25 µW (at 10 µA excitation) 10 µA, 1 mA
10 V (GaAlAs) 100 µW (at 10 µA excitation)
10 µA, 1 mA
PTC RTD (Platinum)
10 ) 10 µW
1 mA
30 ) 30 µW
100 ) 100 µW
300 ) 300 µW
1 k) 1 mW
3 k) 3 mW
10 k)
10 mW
NTC RTD (Cernox™)
10 ) 10 µW 1 mA
30 ) 2.7 µW 300 µA
100 ) 1 µW 100 µA
300 ) 270 nW 30 µA
1 k) 100 nW 10 µA
3 k) 27 nW 3 µA
10 k) 10 nW 1 µA
30 k) 2.7 nW 300 nA
100 k) 1 nW 100 nA
4.4.5 Thermal
Electromotive Force
(EMF) Compensation
To keep power low and avoid sensor self heating, the sensor excitation is kept low.
There are two major problems that occur when measuring the resulting small DC
voltages. The first is external noise entering the measurement through the sensor
leads, which is discussed with sensor installation (section 2.4). The second is the pres-
ence of thermal EMF voltages, or thermocouple v
oltages, in the lead wiring. Thermal
EMF voltages appear when there is a temperature gradient across a piece of voltage
lead. Thermal EMF voltages must exist because the sensor is almost never the same
temperature as the instrument. They can be minimized by careful wiring, making
sure the voltage leads are symmetrical in the type of metal used and how they are
joined, and by keeping unnecessary heat sources away from the leads. Even in a 
well-designed system, thermal EMF voltages can b
e an appreciable part of a low 
voltage sensor measurement.
The Model 335 can help with a thermal compensation algorithm. The instrument will
automatically reverse the polarity of the cu
rrent source every other reading. The
average of the positive and negative sensor readings will cancel the thermal EMF
voltage that is present in the same polarity, regardless of current direction. This cor-
rection algorithm is enabled by default for RTD sensor types, but you can turn it off
using the Current R
eversal parameter.
The Current Reversal parameter defaults to On an
y time the Sensor Type parameter is
changed to PTC RTD or NTC RTD.
TABLE 4-7