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Connecting Sensors

Sensor Types

There are two commonly used sensor types for the NFCS - LM61 based sensors and 50k Ohm thermistors. Other types are possible, but these two cover most applications. They are electrically quite different, but the NFCS can accomodate either type on any channel.

The LM61 sensors work over a range from about -20°F to 212°F. They are fairly inexpensive and in the NFCS they provide a temperature resolution of better than a tenth of a degree Centigrade.

As configured for the NFCS, thermistor based sensors cover the range from about 40°F to over 300°F (limited by melting of the cable insulation). Because thermistors are highly nonlinear, they provide much better resolution at lower temperatures. At room temperature their resolution is better than a tenth of a degree, but at boiling the resolution is only about a quarter of a degree Centigrade. They are smaller than the LM61 sensors and can fit in tight spaces.

Physical I/O Tab

Sensors are configured using the 'Physical I/O' tab on the NFCS web interface. This tab displays all of the possible inputs and outputs for the NFCS, and allows you to specify which inputs and outputs you will use in your application. Sensor inputs are listed at the top, and are of type 'AI' (analog input).

Because there are so many inputs and outputs, the NFCS allows you to specify which ones you're going to use. Only selected inputs and outputs are visible outside the 'Physical I/O' page. In the NFCS, the process of selecting a physical I/O involves three steps:

  1. Identify the panel and connector number on the outside of the controller where your sensor or other device is plugged in. For instance, the sensor input panel at the top left side of the NFCS is labeled 'P1' and the top left connector is numbered '1'. We'd refer to this as 'Panel 1, Connector 1'.
  2. Find the corresponding line in the 'Physical I/O' page. In our example, the first line on the 'Physical I/O' page corresponds to panel 1, connector 1. To use a physical I/O in the NFCS, you must select it by clicking the 'Create Element' link in the rightmost column of the 'Physical I/O' page. In this example, an element has already been created and assigned the name 'P1 Channel 1'.
  3. For each sensor, make sure that the correct sensor type is selected. The most commonly used sensor in the NFCS is the LM61.
  4. Finally, go to the 'Data Elements' tab and give your sensor a meaningful name.

As each sensor is added, check to see that a reasonable temperature is displayed. Holding the sensor between your fingers should result in a quick rise in displayed temperature.

Calibration

The NFCS allows calibration of both gain and offset for each sensor. In most cases, calibration is not necessary. If calibration is desired, an offset calibration is usually all that's needed. Offsets are simply added to the measured temperature. Offsets are stored internally in Centigrade, but are displayed in the current units.

To get the highest possible accuracy, gain calibration can be done as well. To perform gain calibration, you need to establish two known temperatures that are as far apart as possible while remaining within the sensor's measurement range. The easiest way to do this is to use an ice bath and boiling water. Immerse the sensors in an agitated (stirred) bath of water packed with crushed ice. This will be very close to 0°C (32°F). Water at a vigorous boil will be very close to 100°C (212°F). If you're at a high altitude, you'll have to correct for altitude effects.

Download the sensor calibration spreadsheet. Follow the instructions in the spreadsheet - enter the actual low and high temperatures (use a reference thermometer if you have one, otherwise use freezing and boiling). Enter the actual readings for each sensor at low and high temperatures. The blue cells in the spreadsheet will then give you the values for gain and offset for each sensor. Enter those values in the Physical I/O tab.

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