BB ELECTRONICS CATALOG (2010)

Data Acquisition – Analog and Digital Remote I/O
Temperature: Temperature inputs come in two variations,
Resistance Temperature Detector (RTD) or Thermocouple.
RTD: Simply put, an RTD is sensor used to measure temperature by
correlating an elements resistance with temperature. RTD elements
consist of a length of fine wire coiled around a ceramic or glass core.
The wire is made of a pure material with a defined resistance.
Platinum is the most popular and accurate. Copper is also used in
some instances. A Pt100 sensor has a resistance of 100 ohms at 0°C
and is by far the most common type of RTD sensor. A Pt1000 has
1000 ohms resistance at 0°C. Copper RTD’s are usually built at 10
ohms (CU10). Because of this low resistance, measured temperatures
spans must be large to get an adequate signal. Therefore, CU10 RTD
use is limited to devices such as motors or transformers.
2-Wire RTD - The lowest cost method is to keep the signal
conditioner close enough to the sensor so the copper wire
resistance change does not introduce an excessive error in the
measurement.
3-Wire RTD - Lead compensation with 3 wires requires 2
conductors attached to one end of the RTD and a single conductor
on the other end. This method requires the 3 lead wires to track
each other in resistance change versus ambient temperature. It is
effective for a total copper lead resistance equal to about 10% of the
RTD value. The error introduced increases proportionally as the
lead wire length increases.
Analog Signals
Level – the analog level gives important information,
such as the intensity of a light source, pressure in a
chamber, or temperature in room.
5V
4-Wire RTD - A 4 wire input provides the most accurate
measurement. Excitation current flows through 2 wires and the
other 2 wires are used to measure the voltage across the RTD. The
wire leads have no influence on the accuracy of the measurement.
Thermocouple: Based on the principle that when two dissimilar
metals are joined, a predictable voltage will be generated that
relates to the difference in temperature between the measuring
junction and the reference junction (connection to the measuring
device). The selection of the optimum thermocouple type (metals
used in their construction) is based on application temperature,
atmosphere, required length of service, accuracy and cost.
Thermocouple Type Temperature Range Temperature Range
IEC Color Code (Short Term) (Continuous)
E – Purple -400 to +900°C 0 to +800°C
J – Black -180 to +800°C 0 to +700°C
K – Green -180 to +1300°C 0 to +100°C
T – Brown -250 to +400°C -185 to +300°C
Frequency – Analog signals can be characterized by their frequency.
When frequency is important, acquisition speed must be considered.
2 Hz
Remote I/O
Serial Ethernet USB Software
Signal
Conditioners
Shape – the shape of an analog signal can be as important as the level. By measuring
the shape, the DC value, peak, and slope can be determined. This is of particular
interest when the value changes rapidly. Sounds and vibration are some applications.
Digital Signals
ON
State – The state of a digital signal is essentially the level of
the signal [on or off, hi or low]. A common application is
monitoring the condition of a switch [open or closed].
OFF
Rate – The rate of a digital signal is determined by measuring how it changes state with respect
to time. A common application is determining how fast a motor shaft spins. Unlike Analog
Frequency, digital rate measures how often a signal occurs and an algorithm in not required.
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