geokon’s 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.
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Figure 1: Model 3810A Addressable Thermistor String
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Figure 2: Model 3810A Addressable Thermistor