Metrology Systems

Primary Leak Calibration System

Metrology Systems
Primary Leak Calibration Metrology Systems

The leak rate capability of the Primary Leak Calibration System is 1 x 10-9 to 1 x 10-3 atm-cc/sec for all gases. VTI provides a calibration traceability back to its own standards. VTI has characterized its Primary Leak Calibration system to have a Calibration and Measurement Capability (CMC) of 4.5% of reading assuming a 95% confidence level.

The VTI System is a six port Delta P Delta V type. All the measurements taken can be traced back to “first principles” of gas flow. They include pressure (vacuum), volume, time, and temperature.

The basic delta P/delta V test sequence begins by allowing the gas from a Unit Under Test (UUT) to collect in a previously evacuated test manifold complete with a high accuracy vacuum gauge and a known calibrated volume.  As the gas collects in the manifold, the pressure rises versus time (the “delta P” portion of the test).  After a sufficient collection time to determine the slope of the rate of rise (ROR) line, the previously evacuated “known” volume is valved in (the “delta V” portion of the test).   The increase in total free volume causes the slope of the ROR line to become less steep, since there is more volume for the gas to fill, and the pressure rises more slowly.

In the math the change in slope due to the “known” change in free volume allows for the calculation of the leak rate exiting the leak being calibrated.  In effect, the change in volume allows for self calibration of the unknown manifold and connection volumes yielding a total volume.  Of course, the temperature of the gas is a factor in calculating the total number of gas molecules entering the volume per unit time, and thus, it must also be measured accurately and kept stable during the testing.

Vacuum Gauge Metrology System (VGMS)

Calibration Pressure Range
1 x 10 -9 Torr to 100 Torr

Pressure Control Gas

Oven Nominal Temperature
150 degrees C

High Vacuum (indication only)
Bayard-Alpert nude ion gauge

High Vacuum (pressure standard)
Spinning Rotor Gauge

Low Vacuum (pressure standard)
1 Torr High Accuracy Baratron

Low Vacuum (pressure standard)
10 Torr High Accuracy Baratron

Low Vacuum (pressure standard)
100 Torr High Accuracy Baratron

Tank Pressure
500 psig Baratron

Rough Pump

Foreline Pump