Fluke Hart Scientific 1575 Super-Thermometer, Руководство пользователя

Страница: 18 / 156

temperature measurements. It also offers other advantages that contribute to the
value of this instrument. Consider the following issues.
2.2.2.1 Lead Resistance
Measurements using an electrical sensor can be affected by the resistance in the
connecting wires or leads. Resistance also exists in the patented DWF connec-
tors and the junction between the wires and patented DWF connectors. In com-
monly used two or three-wire measurement circuits these resistances and their
variability causes errors of 0.1 to 1.0°C.
The 1575/1590 uses a four-wire circuit that completely eliminates the effects of
lead resistance. In this scheme, often referred to as a Kelvin circuit, the sensor
is driven with current from one set of wires and the resulting EMF is sensed
with a different set of wires. The signal is passed to an amplifier with a very
high input impedance that draws negligible current from the sensor. As a result,
no measurable voltage develops along the EMF sensing wires. The 1575/1590
will accurately measure the resistance of sensors even in the presence of lead
resistance that can be as high as 10
Ω
.
2.2.2.2 Thermoelectric EMF
A resistance sensor such as a PRT contains several junctions between wires of
different metals. These act like thermocouples generating small electric poten-
tials called thermoelectric EMFs. Unless rejected in some way, the thermoelec-
tric EMFs can interfere with the sensor EMF and degrade the accuracy of the
measurement. There are three different techniques that can be used to cancel
thermoelectric EMF.
Some resistance bridges apply AC driving current and use sensing circuits that
detect only the AC signal, rejecting the DC EMFs. This technique is very effec-
tive at eliminating thermoelectric EMF errors but can lead to other errors.
Reactance, leakage, and eddy currents become much more significant with AC.
A different technique sometimes used in DMMs is to periodically switch off
current to the sensor and measure the thermoelectric EMF directly. The prob-
lem with this is it leads to self-heating errors as the sensor warms and cools
from the varying current.
The 1575/1590 uses a third technique. Two separate measurements are made
and the driving current is simply reversed for the second measurement. Ther-
moelectric EMF causes errors that are opposite in the two measurements. In es-
sence, averaging the two measurements cancels the errors. This technique is
very effective at eliminating errors from thermoelectric EMF while avoiding
the AC related errors and self-heating problems of the other methods. In fact,
it’s so effective in the 1575/1590 that the instrument is able to achieve an un-
certainty of 0.00025°C or better with no observable error caused by thermo-
electric EMF.
1575/1590 Thermometer Readout
User’s Guide
8