Auto and Manual Ranging
The meter powers up in the Automatic Range mode. In automatic mode the meter automatically finds the correct
range in order to produce the best accuracy. However, if it is desired to set the range manually, follow these steps:
1. Power the meter
2. Notice the two (2) digit number to the immediate left of the analog bargraph. This number is the low end of the
presently selected range (see the specifications for the ranges).
3. To change the range, press the UP key to raise the range or press the DOWN key to lower the range. The two
digit number on the left of the bargraph will change with each key-press.
4. An advantage of Manual mode is that it takes less time for the meter to take a reading. In Auto Range mode the
meter must first locate the correct range before displaying a measurement.
LCD Backlighting
Press the BACKLIT key to illuminate the LCD. The backlight will remain on for 5 seconds and then automatically
switch off to preserve battery life.
Auto Power Off
To preserve battery life, this meter has an automatic power off feature. If the unit is not used for approximately 20
minutes, the meter shuts off. To override this function, follow these steps:
1. From a power OFF condition, press and hold the ON/OFF and MAX HOLD keys simultaneously.
2. When `n’ appears on the display, release the MAX HOLD and then the ON/OFF key.
3. The Auto Power Off feature is now disabled. Note that the Auto Power Off feature is re-activated the next time
the meter is powered down.
RS-232 Output
The meter includes an RS-232 PC interface jack. This PC interface allows the meter to store and display readings on
a PC as they are recorded. The interface cable and 407752 software for data acquisition are sold separately.
Detailed instructions are provided with the software.
Analog Outputs
The meter includes an AC and a DC analog output. These outputs are proportional to the displayed sound level and
are ideal for use with chart recorders and dataloggers. The labeled 3.5mm output mini-plugs are located on the
bottom of the instrument.
The DC output is 10mV per dB.
The AC output is 0.707V rms full scale.
Calculate the AC value per dB in the range 0.707*10(dB – max dB of range)/20
Example: 30-80dB range
80dB = 0.707Vrms
70dB = 0.707 * 10(70-80)/20 = 0.707 * 10(-.5) = 0.223Vrms
50dB = 0.707 * 10(50-80)/20 = 0.707 * 10(-1.5) = 0.022Vrms
Calibration
To calibrate the meter, an external calibrator such as the Extech Instruments 407744 or 407766 is required in
addition to a small screwdriver.
Turn the meter on and set the parameters of the 407750 to the following before proceeding:
Response: Fast
Function: A weighting
Range: 50 to +100 dB
Place the calibrator gently over the microphone of the meter. Set the calibrator to output 1kHz sine wave at 94.0dB.
Adjust the calibration potentiometer, located at the bottom of the meter, until the display shows a reading of 94.0 dB.
Battery Replacement
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