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Here the external conditions around the vacuum system and the operating conditions inside (e.g. working pressure, composition of the gas content) are important.. Sensors should only be installed vertically with the vacuum flange at the bottom to keep condensates, metal flakes and filings from collecting in the sensor or even small components such as tiny screws and similar from falling into the sensor and the measurement system. The hot incandescent filaments could also bend and deform improperly and cause electrical shorts inside the measurement system. This is the reason behind the following general rule: If at all possible, install sensors vertically and open to the bottom. It is also very important to install measurement systems, if possible, at those points in the vacuum system which will remain free of vibration during operation.
The outside temperature must be considered and it is necessary to avoid hot kilns, furnaces or ovens, or other sources of intense radiation which generate an ambient temperature above the specific acceptable value for the measurement system. Excessive ambient temperatures will result in false pressure indications in thermal conductivity vacuum sensors.
The gauges used for pressure measurement in vacuum technology will certainly work under “dirty” conditions. This is quite understandable since a vacuum system does not simply produce low pressures but has to run processes in chemistry, metallurgy or nuclear physics at low pressures. Here, depending on the nature of the process, considerable quantities of gases or vapors will be pumped either continuously or intermittently; these can pass into the measurement systems and – due to surface reactions or deposits – can falsify the pressure measurements considerably. This is true for all types of vacuum gauges whereby, high-sensitivity, high-accuracy measurement systems are particularly sensitive to contamination. One can attempt to protect the measurement systems against contamination by providing suitable shielding. This, however, will often lead to the pressure registered by the measurement system – which is indeed clean – deviating considerably from the pressure actually present in the system.
It is not fundamentally possible to keep the measurement system in a vacuum gauge free from contamination. Therefore, it is necessary to ensure that:
These two conditions are not easy to satisfy by most vacuum gauges in practice. . Dirty THERMOVAC sensors will show a pressure which is too high in the lower measurement range since the surface of the hot wire has changed. In Penning vacuum gauges, contamination will induce pressure readings which are far too low since the discharge currents will become smaller. In the case of ionization vacuum gauges with hot cathodes, electrodes and the tube walls can be contaminated which, under certain circumstances, will result in a reduction of dielectric strengths. Here, however, the measurement systems can usually be baked out and degassed by passing a current through or by electron bombardment, quite aside from the fact that ionization vacuum gauges are often used in the ultrahigh vacuum range where it is necessary to ensure clean operating conditions for other reasons.
In all those measurement instruments which use the ionization of gas molecules as the measurement principle (cold-cathode and hot-cathode ionization vacuum gauges), strong magnetic leakage fields or electrical potentials can have a major influence on the pressure indication. At low pressures it is also possible for wall potentials which deviate from the cathode potential to influence the ion trap current.
In vacuum measurement systems used in high and ultrahigh vacuum, it is necessary to ensure that the required high insulation values for the high-voltage electrodes and ion traps also be maintained during operation and sometimes even during bake-out procedures. Insulation defects may occur both in the external feed line and inside the measurement system itself. Insufficient insulation at the trap (detector) lead may allow creep currents – at low pressures – to stimulate overly high pressure value readings. The very low ion trap currents make it necessary for this lead to be particularly well insulated. Inside the measurement sensors, too, creep currents can occur if the trap is not effectively shielded against the other electrodes.
An error frequently made when connecting measurement sensors to the vacuum system is the use of connector piping which is unacceptably long and narrow. The conductance value must be kept as large as possible. The most favorable solution is to use integrated measurement systems. Whenever connector pipes of lower conductance values are used, the pressure indication, may be either too high or too low. Here measurement errors by more than one decade are possible! Where systems can be baked out it is necessary to ensure that the connector pipe can also be heated.
The measurement cables (connector cables between the sensor and the vacuum gauge control unit) are normally 2 m long. If longer measurement cables must be used – for installation in control panels, for example – then it will be necessary to check the situation for the influence on the pressure reading. Information on the options for using overlength cables can be obtained from our technical consulting department.