Application and Technical Data (cont.)
Calibration
Occasionally, the interchangeability tolerance listed for a sensor is
unacceptable for an application. On those occasions there are
three types of calibrations available. Thermo Sensors maintains a
laboratory with equipment traceable to the National Bureau of
Standards to furnish these calibrations.
1. If there is only one temperature of interest, or the interest is
over a narrow range, elements can be selected by calibration to have a closer interchangeability tolerance of no more than .25% of the tem
perature. This selection calibration makes field adjustment of
the measurement instrument unnecessary when elements are
changed in critical applications.
2. Elements can be calibrated at a particular temperature of interest.
Also if there are temperatures of interest over a narrow
range, a two or three point calibration will provide the user with
information for interpolation within that range so that the measurement
instrument can be adjusted.
3. a complete computer generated resistance/temperature table
can be provided for a platinum sensor over the range of 0°C to
its maximum useful temperature. These tables can be provided
in °F or °C with temperature increments from .01° to 1° as specified
by the customer.
Matched Pairs
When using two platinum sensors to measure the differential temperature
of chilled water, standard ± 0.12% elements can produce
an error as much as 0.6°C. Usually this is unacceptable in critical
applications. In those cases, sensors having a differential of down
to ± 0.6°C at 0°C or .25% of a selected temperature can be furnished
by calibration at additional cost.
Lead Wire Compensation
Since the readout instrumentation for sensors is normally remoted
considerable distances from the sensors themselves, it is
important to consider and usually eliminate additional resistance
imposed by the connecting wires. It is important that only the
resistance change of the sensor be measured. To eliminate any
change in lead resistance due to ambient changes, either a three
or four wire connection should be used.
Most industrial applications are served well by using a three wire
system while the four wire is common in laboratory environments.
Diagrams and comments relating to the four commonly used
wiring systems are shown below.
2-Wire System
At balance RB will equal RT + 2RL giving an error equal to the
two leads of the sensor connection. Depending on lead length
and wire size the error may be negligible or profound.
3-Wire System
Industrial sensors commonly use this three wire connection system.
For this system to be effective, all the leads (RLA, B, C)
should be very near the same length and of the same gauge.
At balance:
RB = RLB = RT + RLA
RT = RB + RLB - RLA
Any error would be to the magnitude of the difference in resistance
of RLA and RLB which should be negligible assuming the
leads are the same length and gauge.
4-Wire System
This system provides precision measurements. By switching the
pairs of leads and averaging, you arrive at a value from which the
lead resistance, thermal emf's in the leads and resistance
changes in the leads due to ambient variation has been eliminated.
Switch Position a At balance:
RB + RLC = RT + RLA
RT = RB + RLC - RLA
Switch Position B
At balance:
RB + RLA = RT + RLC
RT = RB + RLA = RLC
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