The reflectance and discriminator setting for six Th and U standards were measured and found to fit a relation in which pulse height is proportional to (1 + reflectance). Errors of up to 20% can be incurred if this is not taken into account. The random delay between input pulse and gate output that is present in some timers is pointed out and its effect on the pair count rate evaluated. The optimum area and gate time for determining the ²³²Th pair count rate are evaluated and discussed. -- AATA