How to calibrate vacuum gauges
Definition of terms
Since these terms are often confused in daily usage, a clear definition of them will first be provided:
Calibration inspection refers to comparison with a standard in accordance with certain statutory regulations by specially authorized personnel (Bureau of Standards). It is sometimes known as a factory calibration. If the outcome of this regular inspection is positive, an operating permit for the next operation period (e.g. three years) is made visible for outsiders by means of a sticker or lead seal. If the outcome is negative, the instrument is withdrawn from operation.
Calibration refers to comparison with a standard in accordance with certain statutory regulations by specially authorized personnel (calibration facility). The result of this procedure is a calibration certificate which contains the deviations of the readings of the instrument being calibrated from the standard. Calibration facilities carry out this calibration work. One problem that arises is the question of how good the standards are and where they are calibrated. Such standards are calibrated in calibration facilities of the German Calibration Service (DKD). The German Calibration Service is managed by the Federal Physical-Technical Institute (PTB). Its function is to ensure that measuring and testing equipment used for industrial measurement purposes is subjected to official standards. Calibration of vacuum gauges and test leaks within the framework of the DKD has been assigned to Leybold, as well as other companies, by the PTB. The required calibration pump bench was set up in accordance with DIN 28 418 and then inspected and accepted by the PTB. The standards of the DKD facilities, so-called transfer standards (reference vacuum gauges), are calibrated directly by the PTB at regular intervals. Vacuum gauges of all makes are calibrated on an impartial basis by Leybold in Cologne. A DKD calibration certificate is issued with all characteristic data on the calibration.
The standards of the Federal Physical-Technical Institute are the so-called national standards. To be able to guarantee adequate measuring accuracy or as little measurement uncertainty as possible in its calibrations, the PTB largely carries out its measurements through the application of fundamental methods. This means, for example, that one attempts to describe the calibration pressures through the measurement of force and area or by thinning the gases in strict accordance with physical laws. The chain of the recalibration of standard instruments carried out once a year at the next higher qualified calibration facility up to the PTB is called “resetting to national standards”. In other countries as well, similar methods are carried out by the national standards institutes as those applied by the Federal Physical-Technical Institute (PTB) in Germany. Fig. 3.17 shows the pressure scale of the PTB. Calibration guidelines are specified in DIN standards (DIN 28 416) and ISO proposals.
Examples of fundamental pressure measurement methods (as standard methods for calibrating vacuum gauges)
a) Measuring pressure with a reference gauge
An example of such an instrument are the Cpacitance Diaphragm gauges, with the reference versions of these types of gauge able to measure with incredible precision down to 10-4 mbar. (see page on Direct pressure measurement). Below this level, SRG and hot cathode gauges are typically used as the reference (see page on Indirect pressure measurement)
b) Generation of a known pressure; static expansion method
On the basis of a certain quantity of gas whose parameters p, V and T are known exactly – p lies within the measuring range of a reference gauge such as a U-tube or McLeod vacuum gauge – a lower pressure within the working range of ionization gauges is reached via expansion in several stages.
If the gas having volume V1 is expanded to a volume (V1 + V2), and from V2 to (V2 + V3), etc., one obtains, after n stages of expansion:
p1 = initial pressure measured directly in mbar
pn = calibration pressure
The volumes here must be known as precisely as possible (see Fig. 3.18) and the temperature has to remain constant. This method requires that the apparatus used be kept very clean and reaches its limit at pressures where the gas quantity can be altered by desorption or adsorption effects beyond the permissible limits of error. According to experience, this lower limit is around 5 · 10-7 mbar. This method is called the static expansion method because the pressure and volume of the gas at rest are the decisive variables.
c) Dynamic expansion method
- Volume 1
- Volume 2
- Inlet valve (conductance L1)
- Aperture with conductance L2
- to pump system
- to gas reservoir
- LN2 cold trap
- to pump system
- U-tube vacuum gauge
- McLeod vacuum gauge
- Calibrated ionization gauge tube
- to pump (pumping speed PSp)
- Gas inlet
- Mass spectrometer
- 19, 20 Gauges to be calibrated
- Nude gauge to be calibrated
- Bake-out furnace
According to this method, the calibration pressure p is produced by admitting gas at a constant throughput rate Q into a vacuum chamber while gas is simultaneously pumped out of the chamber by a pump unit with a constant pumping speed S. At equilibrium the following applies according to equation 1.10 a:
p = Q/S
Q is obtained either from the quantity of gas that flows into the calibration chamber from a supply vessel in which constant pressure prevails or from the quantity of gas flowing into the calibration chamber at a measured pressure through a known conductance. The pressure in front of the inlet valve must be high enough so that it can be measured with a reference gauge. The inlet apertures of the valve (small capillaries, sintered bodies) must be so small that the condition d << λ is met, i.e. a molecular flow and hence a constant conductance of the inlet valve are obtained. The quantity of gas is then defined by p1 · L1, where p1 = pressure in front of the inlet valve and L1 = conductance of the valve. The pumping system consists of a precisely measured aperture with the conductance L2 in a wall that is as thin as possible (screen conductance) and a pump with a pumping speed of PSp:
This method has the advantage that, after reaching a state of equilibrium, sorption effects can be ignored and this procedure can therefore be used for calibrating gauges at very low pressures.