Campbell CR3000 Micrologger, Руководство оператора

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Section 7. Installation
274
•
PRTs are not usually manufactured ready to use for most CR3000 PRT
setups.
This section gives procedures and diagrams for many circuit setups. It also has
relatively simplified examples of each circuit type and associated CRBasic
programming.
7.7.17.1 Measurement Theory (PRT)
RTDs (resistance temperature detectors) are resistive devices made of platinum,
nickel, copper, or other material. Platinum RTDs, known as PRTs (platinum
resistance thermometers) are very accurate temperature measurement sensors.
This discussion f
ocuses on the 100 Ω PRT. Apply the following principles to
other RTDs.
•
A PRT element is a specialized resistor with two connection points.
Most PRTs are either 100 Ω or 1000 Ω. This number is the resistance
the PRT has at 0 °C.
•
The resistance of a PRT increases as it is warmed. Industrial standards
define how PRTs respond to temperature; see PRT Callendar-Van Dusen
Coefficients
(p. 300).
•
There are many ways to measure a PRT with a CR3000 datalogger.
When using Vx terminals , the most direct route is to measure a four-
wire PRT in a three-wire half bridge.[ When using IX terminals , the
most direct route is to connect a four wire PRT directly to the CR3000.
Other ways to measure a PRT are listed in TABLE: PRT Measurement
Circuit Overview
(p. 274).
•
Better excitation accuracy results if the highest possible excitation is
used. Better measurement resolution results if the voltage output range
from the PRT spans the analog-input voltage range of the CR3000.
Better measurement accuracy occurs when the output signal can be kept
as large as possible. Procedures in the following example balance these
best practices.
•
A feature of PRT measurements is the ratio RS/RS
0
, where RS is the
PRT resistance now and RS
0
is the PRT resistance at 0 °C. RS/RS
0
makes it easy to apply the results of an ice-bath calibration to a
temperature measurement. For jobs that do not need veryhigh accuracy,
skip the calibration and assume that PRT resistance at 0 °C is either 100
Ω or 1000 Ω.