Application reports

Question: How do you clean vacuum measuring devices? What equipment and tools are required?

Answer: The cleaning of vacuum instruments - or more precisely, vacuum gauges - must be done carefully - with regard to the particular gauge -. Success is not guaranteed and for some gauge types, the replacement of the sensor can be more economical than cleaning.

Background:By a vacuum gauge we mean vacuum measurement instrument, which measures absolute pressure.

 According to the definition, 'Absolute Pressure' is the pressure of the gas per unit area exerted force (eg Newton / cm² ). It is caused by the collisions of gas molecules on the vessel wall. At atmospheric pressure, the highest pressure recognised or considered in vacuum technology, the force is 1 kp / cm² = 9.81 N / cm².

To try and give a physical explanation of the above definition:
Imagine placing a 1 kilogram hammer gently on the tip of your little finger, you will have a feeling of pressure, take this as atmospheric pressure.

When evacuating a container from atmospheric pressure, the vacuum pressure drops off quickly by several orders of magnitude, and thus in the same way the force per unit area or force on you little finger.

Now imagine replacing the hammer with a business card, and then you have the mbar feeling for the force per cm² at about 1mbar. For such small pressures, it requires intricate mechanical and sometimes electrical amplification to measure such tiny forces.

NOTE: To feel these tiny forces, nature has equipped our skin with tiny hairs.

Direct pressure measurement as described above- as force per unit area - fails in high vacuum because the small forces of the gas are completely covered by other forces. Therefore other gas properties are used for indirect pressure measurement.

Examples:

• Heat conduction in thermal Vacuum measuring tubes, e.g. Pirani gauge
• Gas friction, for example in the ball friction vacuum gauge and vacuum meter in quartz crystal.
• Ionization, for example in Cold and Hot cathode vacuum gauges.

These indirect methods require very delicate measurement systems to capture the small effects as signals and to show on an electrical display. So before you roll up your sleeves and start running before you can walk, please read our brief overview:

Vacuum gauges are sensitive instruments; and hence are inherently incompatible with coarse cleaning procedures.

Finally, we consider the real culprit, the dirt to be removed.

Vacuum gauges are usually in operation for a long time before cleaning is required. The time is measured in weeks, usually months or even years.

Let us use car washing as an example; we can compare the difference between a car requiring cleaning as:

• (A) a 5 km tear along on a dusty dirt road and
• (B) a six-week holiday covering 5000 motorway miles through Scandinavia.

You will probably have guessed already. Cleaning a vacuum gauge is almost always similar to case (B). That is, it's not about loose dust that you can just rinse off quickly. Rather, it is "grown" deposits and grime that have built up over long periods of time. To stay with the car example, imagine the insects that you collect on a long holiday trip on the front of your car.

Sometimes high temperatures and the formation of reactive particles in some Vacuum gauges, lead to chemical reactions of gases and vapours on the surfaces of the sensor elements. These chemical reactions impair the thermal and electrical insulating layers, adhering firmly and impairing the function of the indirect measurement principles. The cleaning of a vacuum gauge means the removal of these layers.

We thank Mr Ronald Wittig of the BEHR Company for suggesting this tip of the month.

In the next episode of our "Tips of the Month" we actually go on to the cleaning methods of the respective types of gauges.