1. Infrared Temperature Sensors

Infrared sensors have been used extensively to determine the temperature of objects. They are non-contacting, which means that the current through the rail should not be a factor. Furthermore, because the parameter being directly measured is the infrared radiation being emitted by the rail, the electromagnetic field should not be a factor. They are commercially available, and in general, not overly expensive. This technology is frequently used in the electric distribution industry to determine the temperature of live power lines because they are not sensitive to electromagnetic fields.

2. Thermocouple Screening

Existing research yielded information relating to the shielding of thermocouples from the electromagnetic fields associated with the rail gun. If possible, the chief advantage to this system would be cost. Thermocouples are widely available and cheap, although customized shields range in the hundred dollar price range. Depending on the type of shielding used, however, there may be significant “thermal inertia,” slowing the transfer of heat from the rail to the thermocouple. Too high of a magnetic field may not be entirely saturated by the shield but may allow for a calibration equation to be derived for accurate temperature measurement. 

3. Fiber-Optic Temperature Sensors

Although fairly new, these sensors are becoming more and more popular. Like the infrared devices above, these fiber-optic sensors detect the radiation emitted from a body, and then relate it to its temperature. Essentially, they consist of a spectroscope which is connected to a fiber-optic filament. The filament conducts the radiation from the desired source to the sensor on the spectrometer where it is measured. The advantages of this setup are manifold. By using the fiber-optic filament to gather the radiation, interference from other radiation sources is avoided. Also, the fiber optic filament can be fairly long, allowing the sensitive electronic equipment in the spectrometer to be remotely located and also, potentially shielded from the electromagnetic field. There are a number of companies which already produce devices of this type, many of which have adequate temperature ranges for our purposes. There are also many which can take measurements rapidly enough (some up to 1000Hz) to provide detailed information on the temperature of the rails over its short firing sequence. Finally, these devices are generally very accurate.  The disadvantages of fiber-optic temperature sensors are basically their cost. Most fiber-optic sensors are priced over $1000 and cannot be support by the budget for this project.

4. Ultrasonic & Acoustic Sensors

A relatively new technology used in the temperature measurement industry involves taking sound waves and measuring their speed in a material. The speed of the wave is related to the temperature of the material, thus providing a way to determine temperature. While these systems have shown great promise both in terms of accuracy and temperature range, they are still being developed, and are not widely available. Although we haven’t found any commercially available devices, we are assuming that they are likely to be relatively expensive and also beyond project budget.

1. Shielded Thermocouple:

The shielded thermocouple design consists of two parts that need to be both designed and manufactured.  A type K thermocouple is fabricated from 24 AWG gage, grade PFA, twisted and shielded thermocouple wire obtained from OMEGA Engineering, Inc.  The exposed thermocouple bead then is surrounded by a MuMetal shield manufactured by the MuShield Company to the specifications outlined to the right..

The thermocouple consists of a positive NiCr alloy lead and a negative NiAl alloy lead each of nominal wire diameters ranging from 1.4-2.4 mm in diameter.  The junction of the two leads result in a bead where temperature measurements will be taken.  The thermocouple’s shield is made from MuMetal, a ferromagnetic material comprised of approximately 80% Nickel, 20% Iron, and trace amounts of Molybdenum. MuMetal is a specialty material specifically made for blocking magnetic fields.  It has a permeability rating of 200,000, the highest of any material. The thermocouple leads travel through the MuMetal shield and out the opposite end towards a data acquisition unit.  The higher the magnetic permeability of the material the more magnetic field it can absorb.  Maximum permeability is achieved when the flux density in the shield is 2500. Choosing a 1 in. diameter with a thickness of 2.5 in. in a 5000 gauss field (what we can produce) an inside flux density of 2500 is achieved.  This flux density gives the material a maximum permeability.

2. Infrared Temperature Sensors:

An OMEGA OS522 OMEGASCOPE® handheld IR thermometer will be used to investigate what effect, if any, a magnetic field might have on an infrared temperature sensor. The OMEGA 522 has an LCD readout on the rear of the rear of the thermometer. It also features an RS232 output as well as an analog output. The thermometer which will be used in the experiments is equipped with a laser sighting attachment.  The sensor is 8.6” x 6.6” x  2.0”, and weighs about one pound. 

Unfortunately BAE's rail gun is not accessible to our team. However, a number of simple experiments have been developed to test the viability of our designs within a comparably disruptive magnetic field. Our field will be created through the purchase of dual 4" x 2" x .5" neodymium (NdFeB) Grade N42 magnets. The surface field is 5120 Gauss with Brmax = 13,200. The magnets will be mounted parallel on opposing side to a 1 ¼ " thick steel plate which has a 1" diameter hole drilled through till ¼" material remains. This plate will represent the rail of the rail gun. In the center of this hole there will be an estimated 1 Tesla magnetic field. With this magnetic field available our team will carry out temperature measurement experiments to verify capabilities of above designs and find possible calibration equations for recording data within strong magnetic fields.