of 0.6, in order to avoid high excitation PNe for which the iron correction scheme adopted here may not apply [@Rodriguez_05]. The final sample of 56 PNe is presented in Table \[tab:1\].

[lllll]{} & $4200^{+1800}_{-1200}$ & $7300\pm200$ & $8800\pm100$ & 1\
& $7200^{+1800}_{-1200}$ & $7800\pm200$ & $7800\pm400$ & 2\
& $5300^{+3400}_{-1800}$ & $13100^{+600}_{-800}$ & $12300\pm300$ & 2\
& $1600^{+900}_{-500}$ & $9800^{+800}_{-900}$ & $9800\pm200$ & 1\
& $8700^{+2600}_{-1700}$ & $11500\pm1200$ & $10200\pm200$ & 1\
& $11200^{+2300}_{-1600}$ & $11000^{+400}_{-500}$ & $11000\pm200$ & 1\
& $1800\pm300$ & $11500^{+400}_{-300}$ & $12000\pm300$ & 2\
& $9000^{+7500}_{-2900}$ & $12300^{+700}_{-1300}$ & $11000^{+200}_{-300}$ & 2\
& $10300^{+1700}_{-1400}$ & $9400\pm300$ & $8900\pm200$ & 3\
& $3800^{+800}_{-600}$ & $12400^{+400}_{-500}$ & $10500\pm200$ & 2\
& $4500^{+600}_{-500}$ & $13100\pm500$ & $14600\pm400$ & 4\
& $2100^{+700}_{-500}$ & $18800^{+1200}_{-2900}$ & $11500\pm300$ & 5\
& $14800^{+2400}_{-2300}$ & $11300\pm500$ & $10300\pm200$ & 6\
& $2400^{+400}_{-300}$ & $10200\pm300$ & $10000\pm200$ & 6\
& $1700^{+700}_{-500}$ & $9800^{+900}_{-1000}$ & $8500^{+200}_{-300}$ & 7\
& $2500^{+1700}_{-900}$ & $19500^{+500}_{-4600}$ & $11800\pm300$ & 1\
& $10100^{+4000}_{-2400}$ & $12100^{+2000}_{-2100}$ & $10600\pm400$ & 8\
& $5100^{+1300}_{-1000}$ & $13700^{+3400}_{-2800}$ & $10800\pm400$ & 9\
& $400^{+600}_{-300}$ & $10300^{+900}_{-800}$ & $12700^{+2000}_{-1000}$ & 7\
& $12100^{+4900}_{-3200}$ & $11000^{+600}_{-700}$ & $9900\pm200$ & 1\
& $1400^{+300}_{-200}$ & $9100\pm200$ & $10000\pm200$ & 