R-DAS Thermocouple Expansion Board

Model 1.3 and 1.3H Thermocouple Sensor Boards for RDAS Flight Computers

The Glitch Laboratories Model 1.3 temperature sensor board for R-DAS Flight Avionics is designed to be used in high power rockets as a means of gathering temperature data.  It has a very fast response time, on the order of a tenth of a second, is small, and light.  Without offset, the board produces a voltage resembling temperature in °F for both type T and K thermocouples (i.e., 45°F produces a nominal signal level of 0.45 volts). The board operates from a single-sided power supply, so it cannot produce a negative voltage for a negative temperature.  This is circumvented by adding 1 volt to the output offset (so in the example above 45°F actually produces a nominal value of 1.45 volts.). Allowing for the non-linearities of thermocouple response, this allows measurement of temperatures below -100°F (-73°C).

The model 1.3 board is perfect for measuring atmospheric temperatures while the model 1.3H board is best suited for measuring skin friction heating, nozzle surface, combustion chamber wall temperatures below 1300 deg F.

Overview

A thermocouple consists simply of the junction of two pieces of wire made from different metals. Change the temperature around the junction, and a DC voltage appears at the end of the wires. A small DC voltage. The Model 1.3 Thermocouple Sensor Board is the result of experimentation over more than two years with various circuits that could stably amplify this small input voltage into a larger, more convenient output voltage.  Many different configurations were tried, but Glitch finally settled on a circuit based on the Analog Devices AD595 thermocouple sensor chip.  The Glitch Lab Thermocouple Sensor Board allows the user to route this output voltage to one of the six recording analog input channels of the RDAS.  The RDAS also supplies power to run the sensor.

Features

The specifications for the Model 1.3 are listed in Table 1. At 1.5”, the board is slightly wider than an RDAS Compact, is 2” long, and weighs (unconnected) 0.5 oz. The Model 1.3 can amplify the output from any thermocouple. It is designed to provide an output voltage from a type T or K thermocouple input that is easily deciphered in the field: The voltage out is 1 volt plus the temperature. More accurate results can be had by calibration and conversion of the output with standard thermocouple formulas.  This can be done by setting up a spreadsheet in Excel or similar program (tedious) or by using the DisplayData Program from Glitch Laboratories that is provided with every Model 1.3. Use of this software is described in Appendix B.  This software also has the ability to read and/or convert and display data in English and Metric units.

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