Unless you are Superman or Time Warp Man
But seriously, 1pf is not easy to measure. That's almost like trying to measure a resistance of 0.001 Ohms. Yeah, you can do it, but it takes special care.
My $100 meter doesnt even go that low, but it can measure 10pf so i measure the 10pf and get a reading, then parallel it with 2pf and get another reading, then use the formula for two caps in parallel:
Cp=C1+C2
where Cp is the total capacitance and C1 and C2 are the two actual capacitors.
C1 is the known capacitance which i take a reading first, and say i get 11pf, then C1=11pf, so then i parallel with unknown then get:
Cp=11pf+unknown_pf
and say this second reading is 13pf, then we have:
13pf=11pf+x_pf
and solving for x_pf we have:
x_pf=13pf-11pf
and we all know that 13-11 is equal to 2, so the unknown cap must be 2pf.
But what if the unknown was higher. Then i would have seen maybe:
20pf=11pf+x_pf
so here the x_pf (the unknown cap value) must be 9pf.
Note that i had to take a reading first with the known capacitor.
With a frequency oscillator like the 555 the frequency will be close to following the capacitor value in proportion to that capacitor value. So say we connect a 100pf cap with some resistance (possibly variable) and we see a frequency of 1000.0 Hz. If we connect a 10pf in parallel with that we have:
Cp=100+10 (in picofarads)=110
and 100/110=0.90909
so if we had a frequency of 1000Hz to begin with, the new frequency would be 1000*0.90909 which would be 909.09Hz which might show up as 909.1Hz.
This means if we measure a frequency of about 909Hz then the unknown cap is about 10 percent of the known cap. So we have the formula:
C_unknown=(F1/F2-1)*C_known
where F1 is the frequency with only the known capacitor connected, and F1 is the new frequency with both caps connected.
If the new frequency happened to come out to 500Hz, then we would have:
C_unknown=((1000/500)-1)*100=100
so the unknown cap would have to have been 100pf also.
Here is a list of factors for 'new' frequencies of 900Hz down to 100Hz:
900Hz, C/9
800Hz, C/4
700Hz, (3*C)/7
600Hz, (2*C)/3
500Hz, C
400Hz, (3*C)/2
300Hz, (7*C)/3
200Hz, 4*C
100Hz, 9*C
where C is the known capacitor here and the original frequency was 1000Hz.
If the original frequency was 10kHz instead, then just multiply all those frequencies by 10.
So for example if we measure 800Hz with a known cap of C=100pf, then the unknown cap must be C/4 which comes out to 25pf.
But if we measured 200Hz, then the unknown cap must be 4*C=400pf, but we usually dont have to measure larger caps this way.