It is correct that the Kelvin method is a relative one, however if the tip is calibrated against another 'reference' surface then the tip work function is known too. However this value is obviously only as good as the value taken for the reference surface.
Our ambient KP Technology Kelvin probe systems are shipped with a 'reference' sample composed of two metals, Aluminium (Al) and Gold (Au). The objective of this sample is for you to measure on the gold surface, and taking the gold reference as 5.10 eV absolute, calculate the tip work function. The objective of the Al surface is as the work function of Al is about 1V less than Au, measurement on this surface tells you whether the tip is below or above Au (5.10eV). Our experience is that the tip is between 50 and 200 mV below gold. The range depends upon tip handling during mounting and tip treatment in operation (i.e. history of tip to sample collisions).
For instance if the tip to gold contact potential difference (CPD) is 200 mV and the tip to aluminium CPD is -800 mV then the tip work function is 200 mV below gold, i.e. approximately 4.90 eV. The Aluminium work function is then 4.90 - 0.80 eV, i.e. approximately 4.10 eV. The stated value for Aluminium of about 4.2 - 4.3 eV and the reason your Al surface is initially lower is that we mirror polish the Al, the surface changes and the polishing media tend to lower the bare Al surface work function slightly.
You will probably find that the Al surface slowly oxidises (over a period of months and years). This will cause the mirror finish to become tarnished and the work function to rise to 4.5 - 4.6 eV. However the gold surface, if held pristine, will maintain its work function. One must remember when using gold in this way that it is sensitive to humidity and if the humidity in your laboratory changes the measured CPD will too, however generally these changes are quite small 10-30 mV for the typical humidity changes in a laboratory, some of which are climate controlled in any case. When using the reference sample one must also remember that the gold surface is fragile and can be damaged by tip contact, fingerprints etc. We recommend that you consider changing the reference sample every 3-6 months (depending upon its use and condition). It cannot be mechanically or ultrasonically cleaned without damaging the gold. Note - It should be possible to extend the life of your sample with careful handling and storage in a separate enclosed box.
If you find that the data you get for your reference sample on gold is -200 mV and for Al is 800 mV then there are two possibilities (i) you have applied the backing potential to the sample rather than the tip, or (ii) you have changed the trigger position of the incoming waveform so that it now triggers at a point 180 degrees advanced in time than when we set the default parameter. Both of these operations would result in the CPD data being multiplied by -1, neither will however change the magnitude of the CPD.
Here is another important point to consider. As a PhD student Prof. Baikie found that the CPD between two surfaces changes with probe parameter (amplitude of oscillation, mean spacing between tip and sample, Vb applied to tip or sample). These results are published in both his thesis and in the journal "Review of Scientific Instruments". They showed the importance of using exactly the same set-up to compare CPD measurements on Al, Au and other samples. In his system this involved keeping the gradient approximately constant (for example 300 with an allowed error of 20). If you do not maintain this consistency (either manually or automatically and using the same parameter file) then, in general, the CPD data will differ each time you take a new measurement. It is this property of accurate, repeatable data, that distinguishes KP Technology equipment from other vendors.