Lake Shore Model 218 Temperature Monitor User’s Manual
3-6
Installation
Four-Lead Sensor Measurement (Continued)
In a four lead measurement, current leads and voltage leads run separately to the sensor. With
separate leads, there is little current in the voltage leads so their resistance does not enter into the
measurement. Resistance in the current leads will not change the current as long as the voltage
compliance of the current source is not reached. When two lead sensors are used in four lead
measurements, the short leads on the sensor have an insignificant resistance.
NOTE:
The Model 218 does not have three-lead measurement capability.
3.3.2.6
Two-Lead Sensor Measurement
Sometimes a crowded cryogenic system forces users to read sensors in a two-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 current 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 current and voltage
together. If the leads contribute 2 or 3
Ω
to a 10 k
Ω
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 uA. For example: a 10
Ω
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.
NOTE:
The Model 218 does not have three-lead measurement capability.
3.3.2.7
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.
• Use four lead measurement whenever possible.
• Do not connect sensor leads to chassis or earth ground.
• Use twisted shielded cable outside the cooling system.
• Attach the shield pin on the sensor connector to the cable shield.
• Do not attach the cable shield at the other end of the cable, not even to ground.
• Run different inputs and outputs in their own shielded cable.
• Use twisted wire inside the cooling system.
• Use a grounded receptacle for the instrument power cord.
• Consider ground strapping the instrument chassis to other instruments or computers.
