
Lake Shore Model 325 Temperature Controller User’s Manual
3-6
Installation
3.4.7
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:
Use 4-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 more than 1 cable shield at the other end of the cable
Run different inputs and outputs in their own shielded cable
Use twisted wire inside the cooling system
Use similar technique for heater leads
Use a grounded receptacle for the instrument power cord
Consider ground strapping the instrument chassis to other instruments or computers.
3.5
THERMOCOUPLE SENSOR INPUTS (Model 325-TX Only)
The information in this section is for a Model 325 configured at the factory with one or two thermocouple sensor inputs;
being Model 325-T1 or -T2. Sensor connection is important when using thermocouples because the measured signal is
small. Many measurement errors can be avoided with proper sensor installation.
CAUTION:
Do not leave thermocouple inputs unconnected. Short inputs when not in use.
3.5.1
Sensor Input Terminals
Attach sensor leads to the screws on the off-white ceramic terminal blocks. Each block has two screw terminals; one
positive (on the I+
/
V+ side of the connector), one negative (on the I–
/
V– side of the connector). See Figure 3-4.
The current and voltage references silkscreened on the back panel are for the diode/resistor connectors. For
thermocouples, the positive (+) wire goes to the left-side terminal and the negative (–) wire to the right-side terminal.
Remove all insulation then tighten the screws on the thermocouple wires. Keep the ceramic terminal blocks away from
heat sources including sunlight and shield them from fans or room drafts.
Common Thermocouple Polarities
Positive (+)
Negative (–)
Type K (Nickel-Chromium vs. Nickel-Aluminum)
Chromel (YEL)
Alumel (RED)
Type E (Nickel-Chromium vs. Copper-Nickel)
Chromel (PUR)
Constantan (RED)
Type T (Copper vs. Copper-Nickel)
Copper (BLU)
Constantan (RED)
Chromel-AuFe 0.03%
Chromel
Gold
Chromel-AuFe 0.07%
Chromel
Gold
Figure 3-4. Thermocouple Input Definition and Common Connector Polarities
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