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Applications
The twisted wires cancel induced noise from magnetic sources while the shield protects from
induced voltage noise.
Shielding of signal leads is recommended when high resolution or high accuracy is required.
Without shielding noise pickup can be as much as 50 µV. Depending on thermocouple type this
can amount to 1°C or to many degrees for low-output thermocouples. The shield should be
connected to the guard terminal of either the binding posts or the connector.
Shielded, twisted-pair wiring is recommended when noise-generating sources are near cable
runs. Typical noise sources include motors, generators, and electronic equipment emitting high
levels of pulse noise. Even the electromagnetic interference (EMI) from fluorescent lights,
especially those with electronic ballasts, can add noise when in close proximity to either the
thermocouple or dc wiring.
Grounding
The output of the Model 1140A is referenced to an internal floating ground. This ground is
isolated from the input power, chassis, and the remote interface. It is usually sufficient to connect
the low output terminal on the Model 1140A's front panel to the input ground or common of the
instrument that the Model 1140A is feeding. Sometimes it may be desirable to use a separate
wire (other than the thermocouple leads) to connect to an instrument ground on the bench.
For safety reasons and for lowest noise, it is important that the chassis of the Model 1140A be
connected to an earth ground, as is normally provided by the ac power cord. The Model 1140A
contains a power line filter, which diverts noise to earth ground.
ITS-90 and IPTS-68
These temperature scales are defined by the National Institute of Standards Technology.
Although the higher-accuracy ITS-90 is the current temperature scale recommended by NIST,
many older instruments still use the IPTS-68. To satisfy these needs, Ectron provides both.
When using IPTS-68, the thermocouple types that are normally based on NIST Monograph 175
are instead based on older NIST standards: Types B, E, J, K, R, S, and T use Monograph 125 and
Type N uses Monograph 161.
Offset
Available both in thermocouple and in linear voltage operation, an offset can be added or
subtracted from the output. This is normally used when a thermocouple has a known offset;
adding this offset allows the Model 1140A to provide higher-accuracy measurements. When an
offset is set, OFFSET is noted in the lower middle right of the display, with the value underneath.
The available range of offset is ±5°C for all thermocouple types or ±11.000 V dc for dc voltages.
The thermocouple offset, accessible through the THERMOCOUPLE → THERMOCOUPLE OFFSET
menu, is intended to be an offset at the temperature of the Model 1140A terminals, and is not an
offset at the simulated (or measured) temperature.
The offset applied is computed using the Seebeck coefficient for the current thermocouple type,
at the temperature of the front-panel terminals in use. As such it can be used to compensate for
an error of a thermocouple calibrated at the environment of the Model 1140A, and is not
intended to compensate for thermocouple errors at other temperatures.
5-4
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