34
c
HAPTER
3:
Installation
Model 335 Temperature Controller
3.5.6 Two-Lead Sensor
Measurement
There are times when crowding in a cryogenic system forces users to read sensors in a
2-lead configuration because there are not enough feedthroughs or room for lead
wires. If this is the case, plus voltage to plus current and minus voltage to minus cur-
rent leads are attached at the back of the instrument or at the vacuum feedthrough.
The error in a resistive measurement is the resistance of the lead wire run with cur-
rent and voltage together. If the leads contribute 2
)
or 3
)
to a 10 k
)
reading, the
error can probably be tolerated. When measuring voltage for diode sensors, the error
in voltage can be calculated as the lead resistance times the current, typically 10 µA.
For example: a 10
)
lead resistance times 10 µA results in a 0.1 mV error in voltage.
Given the sensitivity of a silicon diode at 4.2 K, the error in temperature would be only
3 mK. At 77 K the sensitivity of a silicon diode is lower so the error would be close to
50 mK. Again, this may not be a problem for every user.
Connectors are also a big source of error when making 2-lead measurements. Con-
nector contact resistance is unpredictable and changes with time and temperature.
Minimize interconnections when making 2-lead measurements. Refer to FIGURE 3-6
for an image of a 2-lead sensor measurement.
2-lead sensor measurement
+
+
–
–
I
V
V
I
3.5.7 Lowering
Measurement Noise
Good instrument hardware setup technique is one of the least expensive ways to
reduce measurement noise. The suggestions fall into two categories: (1) do not let
noise from the outside enter into the measurement, and (2) let the instrument
isolation and other hardware features work to their best advantage. Here are some
further suggestions:
D
Use 4-lead measurement whenever possible
D
Do not connect sensor leads to chassis or earth ground
D
Use twisted shielded cable outside the cooling system
D
Attach the shield pin on the sensor connector to the cable shield
D
Do not attach more than one cable shield at the other end of the cable
D
Run different inputs and outputs in their own shielded cable
D
Use twisted wire inside the cooling system
D
Use twisted wire for heater leads
D
Use a grounded receptacle for the instrument power cord
D
Consider ground strapping the instrument chassis to other
instruments or computers
FIGURE 3-6
