Reed Switch Into PLC AC Input Problem

Ok guys this is scaring me. I have a pneumatic cylinder with reed switches on it triggered by a magnet on the piston.

The reed switches are Greenco GRS-1004 NO SPST Reed/LED.
It is rated for AC or DC up to 1A.

I am running the reed switch contact into an AutomationDirect
DL-06AR 120vac plc input. The reed switches close when the piston is over them, but remain closed when the piston is moved away. If I tap on the reed switch it opens up.

We then ran the reed switch contact to energize a 120vac coil on a relay instead of to the plc input. The reed switch behaves correctly when in this configuration. What the heck is going on here. We use the DL-06AR in a lot of our machines using a variety of input devices without any problems, although this is the first time we have used it with reed switches.

Our company switched to hall-effect magnetic switches after number of similar issues. No 100% explanation was found, only speculations, and we decided not to look for troubles.

I believe it has to do with current draw. I have seen similar behavior with solid state relays where there needs to be sufficient current draw on the load contacts to close/open them. I would follow Sergei's advice regarding the Hall Effect sensors.

Jeff

Thanks guys. I think you are correct in that the current draw is the problem. The DL-06 input draws 10ma of current. I suspect when I check the specs on the reed switch that this current is below the threshold. The relay coil on the other hand provided enough current draw for the reed switch to operate correctly.

Valuable lesson to tuck away!

I doubdt that lower current draw can explain sticky reed switch.
I would even judge opposite.
My speculation is that DC, being more prone to spark on mechanical contacts, microwelds them causing occasional sticky contacts.
I also noticed that projects with electrostatic high-voltages are the worst for the issue.