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Front Panel Description
The UNITS button is used to toggle between the two primary display modes. Upon
powering up, the display indicates altitude in feet and airspeed in knots. Pressing the
UNITS button will switch the display to indicate altitude in feet and vertical speed per
minute in feet. Pressing the UNITS button again will toggle back to the original display.
The LEAK CHECK will begin a 120 second timer, and the display will also show the
gain or loss of altitude or airspeed experienced during the displayed time period.
If the Leak Check button is pressed while in the altitude/airspeed mode, the leak check
will enter the airspeed leak test mode.
If the Leak Check button is pressed while in the altitude/VSI mode, the leak check will
enter the altitude leak test mode.
If the operator wishes to select a different length of testing time, the Leak Check button
may be pressed repeatedly, which will cause a reduction in the timer counter in 30-
second increments. The timer will not toggle above 120 seconds nor under 30 seconds.
Once the time is selected, press the Units button to start the timer.
The EXIT button is used to exit out of the leak check mode.
The LIGHT button will toggle the display backlight ON or OFF.
Altitude - The altitude is measured based on a reference point of 29.92 (sea level). This
is pre-set at calibration and cannot be changed by the operator. The altitude is displayed
in increments of 2 feet up to 30,000 feet, and 5 feet at altitudes above 30,000 feet.
Airspeed – The airspeed is calculated based on the pressure differential between the
static sensor and the pressure sensor. The two sensors are identical but independent. A
pressure difference between the two sensors will not cause damage, even if they are at a
high vacuum on one sensor and a high pressure on the other. (Either sensor may be
damaged if its pressure exceeds 20 psi.)
If the pitot pressure is lower than the static pressure, the airspeed reading will show
“InverseP”. An inverted pressure situation will not damage the TST-621 instrument, but
it may damage instruments in the aircraft!
If the airspeed exceeds 500 knots, or if the altitude range exceeds 36,000 feet, the
airspeed display will blank or indicate “ ---- ”. This condition will not damage the TST-
621 instrument, but the display serves to indicate that the pressure differential has
exceeded the altitude or airspeed calculations.
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