  The in-plane magnetic penetration depth, $\lambda_m(T)$, was measured in
single crystals of SrPd$_2$Ge$_2$ superconductor in a dilution refrigerator
down to T=60 mK and in magnetic fields up to $H_{dc} = 1$ T by using a tunnel
diode resonator. The London penetration depth, $\lambda$, saturates
exponentially approaching $T\rightarrow 0$ indicating fully gapped
superconductivity. The thermodynamic Rutgers formula was used to estimate
$\lambda(0) = 426$ nm which was used to calculate the superfluid density,
$\rho_s(T)=\lambda^2(0)/\lambda^2(T)$. Analysis of $\rho_s(T)$ in the full
temperature range shows that it is best described by a single - gap behavior,
perhaps with somewhat stronger coupling. In a magnetic field, the measured
penetration depth is given by the Campbell penetration depth which was used to
calculate the theoretical critical current density $j_c$. For $H \le 0.45$ T,
the strongest pinning is achieved not at the lowest, but at some intermediate
temperature, probably due to matching effect between temperature - dependent
coherence length and relevant pinning lengthscale. Finally, we find a
compelling evidence for surface superconductivity. Combining all measurements,
the entire $H$-$T$ phase diagram of SrPd$_2$Ge$_2$ is constructed with an
estimated $H_{c2}(0)=0.4817$ T.
