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Calibration of Humidity Alarm 4 - 20 mA Output Circuit

For 6995 Mk IA Dehydrator


Figure 1. E to I Board

The humidity alarm 4 - 20 mA output signal is calibrated using the zero and span set screws on the E to I board, as follows:

  1. Open the control panel and locate the E to I board.
  2. On the E to I board, locate the adjusting screws in the potentiometers at the right side of the board. The zero screw is on the top potentiometer and is marked P2; the span screw is on the bottom potentiometer and is marked P1.
  3. Turn the purge selector valve (Tech Manual Figure 6-1, 3) from Normal to Isolate.
  4. Turn the calibration dump valve (Figure 6-1, 4) from Normal to Calibrate. You will hear a rush of air to atmosphere behind the panel.
  5. Close the metering valve at the bottom of the calibration flowmeter (Figure 6-1, 5). This allows starting with dry air from the dehydrator outlet.
    6. The dew point sample connection uses a quick-disconnect air coupling. Be sure the connection is tight. A loose coupling can fly apart, causing injury.
  6. Attach a frost point/dew point monitor per MIL-M-24144D to the dew point sample port (Figure 6-1, 6). This gives an external measurement of dew point for comparison with the controller.
    7. NOTE: Figure 2 shows the linear relationship between dew point temperature and your 4 - 20 mA response. If you can read degrees directly, you won't need Figure 2.

    Figure 2. mA Output vs. Dew Point Temperature
  7. Using the metering valve in the calibration flowmeter, establish and measure a low dew point (-50° F ± 10°) in the outlet air stream. Adjust the zero screw until the output circuit at the PLC reads within 5 - 10° of this value (or until your mA reading is 4 mA ± 0.5 mA).
  8. Using the metering valve, establish a dew point approximately 15° F higher than the last one. Measure this dew point. Adjust the span screw until the output circuit reads within 5 - 10° F of this value.
  9. Repeat step 8, working up to about a 0° F dew point (20 mA output), adjusting the span potentiometer each time.
  10. Repeat steps 7 through 9 as necessary.
  11. Disconnect the frost point/dew point monitor and return all valves to their original positions.
Last updated: 03/19/2008
Webmaster: Al Friend
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