DigiVac 200 Pirani Manual de instrucciones - Página 7
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SERVICING - GAUGE TUBE CLEANING.
In many cases, a gauge tube may become fouled with oil or other foreign matter. It is often possible to
restore the functionality of contaminated probes with cleaning. If the contaminant is known, the tube
should be filled with a fluid that is known to be a solvent to that contaminant. As an example, ether is
often effective in removing residues of some oils. Commercial carburetor cleaners are very powerful
solvents and are highly effective against some contaminants.
After cleaning with solvents, the gauge tube should be completely dried or flushed with a volatile solvent
to assure that it is dry prior to re-installing it. If this is not done, contamination of the system may result.
6.1 FACTORY REPAIR AND CALIBRATION.
The vacuum gauge assembly is designed to provide years of trouble-free service, and the liberal internal
use of plug-in components make it easily repairable. No field servicing of the unit is recommended,
other than replacement of the gauge tube, but factory servicing and calibration are available at a nominal
cost and turn-around times of 24 hours are typical.
6.2 FIELD CALIBRATION.
Each DIGIVAC vacuum gauge controller is calibrated to the particular vacuum gauge sensor that is
shipped with the unit. While changing the gauge tube is possible, it will result in a slightly different
reading as all gauge tubes are not created equal. Calibration for this unit must be performed by
DigiVac.
Before re-calibrating the instrument, it should be ascertained that the instrument is in fact incorrect. In
many cases, the problem will be with a tube that is fouled, or a system that is operating improperly. It is
recommended that a spare tube be kept on hand and stored in a clean, dry place. Then, in cases of
suspect readings, the tube should be changed before proceeding further.
7.0 NOTES ON CALIBRATION.
The DIGIVAC is calibrated in nitrogen, which has thermal properties virtually identical to air. If you are
using a Pirani vacuum gauge, other gases will affect the readings by an amount proportional to the
thermal conductivity of the gases. In most cases, the gases present in a vacuum system will be air,
nitrogen, or oxygen, and no appreciable errors will occur.
Certain other gases, however, have thermal conductivity significantly greater than air and will cause the
instrument to read higher than the actual amount of pressure. Examples of such gases are water vapor,
fluorocarbon refrigerants, and acetone. Conversely, other gases have thermal conductivity significantly
lower than air and will cause the instrument to read lower than actual pressure. Examples of such gases
include helium, oxygen and to a lesser extent, CO2.
When interpreting readings using gases other than air, it should be borne in mind that the DIGIVAC
reads Millitorr, which is a measure of absolute pressure - that is the opposite of vacuum. Thus, a lower
numerical reading actually is a higher level of vacuum. For more information, refer to section 8.0.
Note that isolated transducers like capacitance manometers are independent of gas type.
8.0 UNDERSTANDING TORR.
The DIGIVAC and many similar instruments are calibrated in microns or "millitorr." It is appropriate to
discuss what microns are and to relate microns to other measures of pressure and vacuum.
DIGIVAC 201 series Instruction Manual Page 7