MODEL 3810A ADDRESSABLE THERMISTOR STRING
| INTRODUCTION |
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1.
INTRODUCTION
Model 3810A thermistor strings are temperature sensors that combine NTC
thermistors with digital Modbus RTU communication for ease of setup and use.
Each thermistor sensor contains a small printed circuit board assembly (PCBA)
with a thermistor and circuitry for communication and analog to digital
conversion. These sensors are assembled with cable using customer-specified
spacing. Then each sensor is encapsulated in a protective housing, creating a
finished thermistor string.
Thermistors are semiconductors that behave as resistors with a high (usually
negative) temperature coefficient of resistance: they are thermal resistors. The
thermistor beads are made from a mixture of metal oxides encased in epoxy or
glass. The beads are small in size and extremely robust, with a high degree of
stability over a long life span. The accuracy and interchangeability of the
thermistor bead used on the 3810A is ±0.07 °C, within the temperature range of
0 °C to 50 °C.
The cable consists of two twisted pairs, each wrapped with aluminized Mylar foil
and a single drain wire. There are four conductors: power, ground, and the two
differential RS-485 lines for communication. A short-circuit between any of the
four conductors will render all sensors in the string inoperable. Likewise, a break
in the cable would render inoperable all sensors below the break. However,
because the 3810A uses a bus topology, failure of any single thermistor sensor
does not affect any of the other sensors.
The sensors are polled individually via industry standard Modbus RTU (Remote
Terminal Unit) commands.
FIGURE 1:
Model 3810A Addressable Thermistor String
FIGURE 2:
Model 3810A Addressable Thermistor
