Lake Shore Model 218 Temperature Monitor User’s Manual
Installation
3-5
3.3.2.2
Sensor Lead Cable
The sensor lead cable used outside the cooling system can be much different form what is used
inside. Between the instrument and vacuum shroud, heat leak is not a problem, but error and noise
pick up need to be minimized. Larger conductor, 22 to 28 AWG stranded copper wire is
recommended because it has low resistance yet remains flexible when several wires are bundled in
a cable. The arrangement of wires in a cable is also important. For best results, twist voltage leads,
V+ and V– together and twist current leads I+ and I– together. Cover the twisted pairs of voltage
and current leads with a braided or foil shield connected to the shield pin of the instrument. This
type of cable is available through local electronics suppliers. Instrument specifications are given
assuming 10 ft of sensor cable. Longer cables, 100 ft or more, can be used but environmental
conditions may degrade accuracy and noise specifications.
3.3.2.3
Grounding and Shielding Sensor Leads
The sensor input measurements are NOT isolated from earth ground. Do not ground sensor leads
outside of the instrument.
Shielding the sensor lead cable is important to keep external noise from entering the measurement.
A shield is most effective when it is near the measurement potential so the Model 218 offers a
shield that stays close to the measurement. Connect the sensor cable shield to the input connector
shield pin. Do not terminate the shield at the opposite end. Do not connect the shield to earth
ground on the instrument chassis or in the cooling system. Please note, the shell of the connector is
in contact with the chassis so the cable shield should never touch the outer shell of the connector.
If a commercial cable is used in which the outer shield is tied to the connector shell, do not
terminate the shield at the sensor end or connect it to a shield pin in the connector.
3.3.2.4 Sensor
Polarity
Lake Shore sensors ship with instructions that indicate which sensor leads are which. It is important
to follow these instructions for plus and minus leads (polarity) as well as voltage and current when
applicable. Diode sensors do not operate in the wrong polarity. They look like an open circuit to the
instrument. Two lead resistors can operate with any lead arrangement and the sensor instructions
may not specify. Four-lead resistors may depend more on lead arrangement. Follow any specified
lead assignment for four lead resistors. Mixing leads could give a reading that appears correct, but
is not the most accurate.
3.3.2.5
Four-Lead Sensor Measurement
All sensors, including both two-lead and four-lead can be measured with a four-lead technique.
Four-lead measurement eliminates the effect of lead resistance on the measurement. If it is not
taken out, lead resistance is a direct error when measuring a sensor.
Summary of Contents for 218S
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