So you have metric papers, your graph will be possibly a european
product. We have a lot of those items running and in the ten years in
this museum I remember the change of the pens only a few times, they run
for years and years.
If you'll send me the name of the thermohygrograph, maker and so on and
perhaps the order-nr. of the papers I'll try to find out better pens.

regards
Gerhard Dangel-Reese
Augustinermuseum Freiburg
fax ++49-761-201-2597