Measurement Computing CIO-EXP-GP - Manual (Page 24)

6 CONFIGURATION FOR RTD MEASUREMENTS
An RTD is a temperature sensor that consist of a resistive element, usually a length of wire encased in a
sheath. Various wire materials are used with platinum being the most common. There are three types of
hookups: two-wire, three-wire, and four-wire. An excellent source of information on RTDs and how to
select one for your application may be found in the OMEGA Engineering catalog.
6.1 Channel Selection
The General Configuration section describes the channel selection, setting the jumper and verifying the
installation and operation of the CIO-EXP-GP with your data acquisition board. Configure your boards
as described in that section before continuing with this section.
6.2 VEXC JUMPER Select
There is a set of jumpers near the 37-pin connectors labeled “VEXC SEL”, which stands for channel
excitation voltage select. These jumpers connect the on-board excitation voltage to one of the A/D board
channels so that it may be measured. Measurement Computing Corp. does not use a measurement of the
excitation voltage in any of its software. You do not need to set this jumper if you are using the
CIO-EXP-GP with Measurement Computing Corp. software, or with packages such as Labtech Notebook
which use the Universal Library. Use this jumper only with software from other manufacturers that
specifically require it.
6.3 CJC Jumper Selection
There is a set of jumpers near the 37 pin connector labeled “CJC SEL”, which stands for cold junction
compensation select. Remove this jumper. There is no cold junction compensation used with bridge
sensors.
6.4 Powering the CIO-EXP-GP
The General Configuration section describes the power selection options for powering the CIO-EXP-GP
itself. Configure your boards as described in Powering the CIO-EXP-GP in the General Configuration
section before continuing with this section.
6.5 Determining the Appropriate Gain
To accurately measure a voltage, the full scale of the signal should be matched to the full range of the
input circuit. (Most DAS boards have an input range of ±5V, which is the native range of the analog to
digital converter at the heart of the board. Some DAS boards include amplification on the input circuit to
allow the signal to be amplified to make better use of the resolution of the A/D.) For example, an input
signal which varies between 0 and 1 volt would only be using 1/10th of a ±5V A/D converter's
resolution. By switching the input signal of the DAS board to unipolar (no negative voltage) and
amplifying the sign wave signal by 5, the entire range of the A/D converter is used and a higher
resolution measurement may be made. By adding this gain and selecting this range, the resolution on a
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