Welch Allyn TM 262 Auto Tymp - User Manual - Manual (Page 31)

Revised 4/11/08 2-13
Installation
with the 5.0cm 3 opening on the test cavity. To keep a record of this test cavity calibration check,
simply press the Print All button on the front panel of the instrument.
Since sound pressure will vary with altitude and barometric pressure, some variation from the
0.5, 2.0 and 5.0 cm
3 readings may be observed. Your instrument is carefully calibrated at our
factory, which is at approximately 250 feet above sea level. If you are located at an elevation of
1000 feet or higher, your instrument may need to be recalibrated to account for your elevation
(see the next topic). It is not necessary to recalibrate for barometric pressure changes on a daily
basis. Keep in mind that a change in barometric pressure (i.e., from low to high or vice-versa)
will slightly affect the test cavity readings.
Altitude adjustment
The altitude calibration adjustment allows you to ‘correct’ the ear canal volume (ECV)
measurement and test cavity volume measurement for variations due to altitude. Because the
instrument is a pressure sensitive device that makes measurements relative to ambient air
pressure, changes in air pressure due to weather or altitude will affect the ECV readout of the
instrument. The slight pressure change resulting from changing weather conditions will usually
yield volume readouts with ±0.1 cm
3 of the expected cavity value, but pressure changes due to
altitude can shift these cavity values by as much as 30%. These changes in pressure do not
affect the accuracy of the compliance measurement system in any way. However, you may
prefer that the instrument gives ECV values as they would appear at sea level. The altitude
calibration mode allows you to adjust the Auto Tymp without the services of a qualified Welch
Allyn representative.
Table 2: Altitude correction.
Altitude (ft.) Altitude Table (cm 3 )
0 - 1,500 2.0
2,000 - 3,500 2.1 ±0.1
4,000 - 6,000 2.2 ±0.1
6,500 - 7,500 2.3 ±0.1
8,000 - 9,000 2.4 ±0.1
9,500 - 10,000 2.5 ±0.1