Fahrenheit (TF) & Celsius (TC) Probe Kit

(c) 1998 EME Systems
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In addition to preassembled probes, EME offers parts to construct probes. The difficulty is that the sensor element must be attached to the end of the cable along with two resistors in a manner that is rugged and will withstand both mechanical abuse and immersion in water and exposure to the elements.

3DX conceptThe solution we have come up with is a three dimensional circuit board that has an "X" cross section. The leads of the sensor go into the arms of the "X" and the 3 or 4 leads of the cable attach at the other end, with the surface mount resistors and capacitor in the grooves. The whole assembly is then encapsulated in a Raychem heat shrink tubing lined with adhesive.

                         @      1k       ,--------,
   + 4-18 volts ---------------/\/\------|        |
                         @               |  LM34  |
   signal 10mV/degF -----------/\/\------|  CAH   |
                         @      3k       |        |
    common -,----------------------------|        |
            |            @               '--------'
            '----------shield-----------

3D circuit boardThe resistor in series with the power supply protects the sensor against ESD and against polarity reversals. The resistor in the signal line serves those same purpose, and also isolates the sensor output from the capacitance of the cable to prevent instability. Observe that the output resistance of the sensor is 3000 ohms. This may need to be taken into account in low-impedence systems. Click here for contsruction details.

The shield may be necessary only in noisy environments and for long runs of cable. The sensor only draws about 70 microamps of current. Therefore ground current errors are minimal. For example, it takes more than 1000 feet of #22 gage wire to give a ground current error equivalent to 0.1 degree Farenheit.

The Raychem MVT1 or ES2000 tubing has an adhesive liner that melts and flows over all the components and fills in the gaps. It also bonds firmly to PVC cable. It can be pinched shut at the open end to form a waterproof seal. A standard heat-shrink gun is required. The tubing then withstands temperatures up to 110 degrees Celsius without melting.

finished temperature probeOnce the probe is assembled it can be calibrated. The highest grade of sensors direct from National Semiconductor have an accuracy of +/- 1 degree Fahrenheit, or +/- 0.5 degree Celsius. Lower grades of sensor may allow +/- 4 degrees F (D grade). The sensor can be calibrated after assembly at one or two or multiple points. The sensor is quite linear, and most of the error tends to be in the zero point, not in the slope. So for <1 degree F error over the range of -17 to +212 Fahrenheit, a single point calibration is all that is required. We also offer the preassembled probes.

These same parts in the X3D kit can also be used to mount other types of sensor to a cable, or to join two cables together with or without resistors/capacitors/diodes in series with the individual lines.

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