EVALUATION OF A LOW COST, HIGH CAPACITY ELECTRONIC THERMOCOUPLE SELECTION UNIT

Authors: Thomson, Steven; Brazil, Brian

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2002
Publication Date: 7/1/2002
Citation: THOMSON, S.J., BRAZIL, B.L. EVALUATION OF A LOW COST, HIGH CAPACITY ELECTRONIC THERMOCOUPLE SELECTION UNIT. APPLIED ENGINEERING IN AGRICULTURE. 2002. 18(4):505-511.

Interpretive Summary: Many research projects rely on accurate and stable temperature measurement. Devices are available commercially to allow temperature sensors to be read from multiple locations. However, if many channels (on the order of 100 or above) need to be read, cost of these systems can be prohibitive. An inexpensive portable device that uses electronic switches instead of more expensive electromechanical relays was evaluated to allow computer- controlled selection of multiple thermocouples. Tests were performed to document possible interaction between channels, verify proper operation at very low temperatures, and check for system drift. Statistical analysis of results indicated good channel isolation. The system performed well at low temperatures and system drift was minimal, verifying the viability of this design and associated software for selection and acquisition of temperature from multiple thermocouples. The electronic switches afford 60-fold cost savings over conventional relays used in some commercial units, allowing high capacity circuitry to be built cheaply.

Technical Abstract: Two laboratory experiments and one field experiment were conducted to evaluate a new, portable temperature acquisition and multiplexing (MUX) system that has the ability to read many thermocouples at very low cost per channel. Although commercial units are available to multiplex thermocouples at reasonable cost, switching many channels (on the order of 100 or more) can become cost-prohibitive. For our studies, an eight- channel system was built and was designed to be easily expandable. The system is a modified version of one already documented, which uses CMOS electronic switches instead of electromechanical relays to select channels. Laboratory experiments were conducted to evaluate relative temperature responses of readings from several channels, and to evaluate the influence of channels on readings from other channels by placing the sensors in different thermal environments. Results of analysis indicated that there were no significant interactions between thermocopules and channels, but that channels reading the same ambient environment were significantly different from each other. Results also indicated crosstalk between channels was not apparent, which is consistent with previous observations. In an experiment for ozone-based water purification, thermocouples were placed in a thermal environment down to -160C to verify the system's integrity and to examine amplifier characteristics at low temperatures. The MUX and amplifier worked well for measuring temperatures down to -160C and appeared to be temporally stable from spot checks on system drift. The system described should be suitable for measuring relative temperature differences, and will be useful in facilitating research that requires intensive temperature measurement at low cost.