Will the badge scanner wreck my Bart pass?
May 12, 2007 6:19 PM   Subscribe

Will the badge scanner at my work demagnitize my Bart pass? I've been carrying my Muni month pass with my badge in the little plastic case that clips onto my belt. To get onto my floor, I need to scan the badge. Yesterday, it occurred to me that I could be demagnitizing the pass. Will I still be able to go through the Bart turnstiles? (I usually take Muni, but not always.)
posted by sfkiddo to Technology (10 answers total)
I carry my fastpass in my wallet with my magnetic office key card, and I've never had a problem with the Bart turnstiles.
posted by trip and a half at 6:26 PM on May 12, 2007

posted by caddis at 6:27 PM on May 12, 2007

The little window at the Powell St. station where you exchange demagnetized tickets has a sign saying that key cards will damage BART cards (as will cell phones, purse clasps, and a whole bunch of other things). In my experience, BART tickets are more easily demagnetized than FastPasses, though.

Clearly, that's not true for everyone. It may be worth testing it at some point for your own peace of mind. Now if someone can tell me how to keep my pass from getting demagnetized in the first place (OTHER than not carrying it in my purse with my cell phone -- a girl only has so many places to put things) I am all ears!
posted by gingerbeer at 6:45 PM on May 12, 2007

Your badge scanner at work reads your card by listening for the radio frequency it emits. The card is energized by the low-power electromagnetic field set up by the card reader.

This is unrelated to how the BART card works. They are semipermanent magnetic strips that are read by physical contact with a reader head. The information on them is stored by an even stronger electromagnetic field in direct contact with the card. Your work badge reader won't damage it.
posted by odinsdream at 7:47 PM on May 12, 2007

You should be fine. I'm no expert in this stuff and a real EE will hopefully come along and correct me, but here's my take.

My impression is that there are two-ish types of badges. Both use magnetic fields to induce voltage in the electronics on your badge. Different cards do different things with the voltage. Some actively transmit the data stored on the card back to the reader. Some just alter the magnetic field in a particular way that's sensable by the reader. Either way, we're talking about relatively small fields.

On cards like your bart pass, you have to pass them through the reader because they use much stronger magnets that forcibly align the bits on the strip to emit a very faint magnetic field when read. The magnitude of the magnetic fields involved are much higher here, so I think it's very unlikely that you have to worry about the card reader causing a problem.
posted by heresiarch at 8:00 PM on May 12, 2007

Here's the list of things that gingerbeer references that will demagnetize a BART pass. It includes "Building Access Cards."

It's unclear whether these items will also demagnetize a Muni Fast Pass (regardless of whether you use it in the BART system or not), but no sign warning of the danger of demagnetized Muni passes exists anywhere in the Muni system. That said, if your Muni pass does become demagnetized or otherwise damaged, BART booth agents won't manually let you through the way Muni booth agents will. BART has a sign about that at Millbrae.
posted by Jeff Howard at 8:14 PM on May 12, 2007

In more than 30 years of living (on and off) here in the Bay Area, I have never, once, had a BART card de-magnetize.

But, when I have had a card fail, usually because I sent it through the wash and it got all wrinkled and fucked up, I've simply taken it to the booth and exchanged it. No problem.
posted by serazin at 10:02 PM on May 12, 2007

Thanks to everyone for their feedback and an special shout out to odinsdream and heresiarch for expanding my knowledge of magnetic fields.
posted by sfkiddo at 11:14 PM on May 12, 2007

FWIW: I'm in NYC. I have to carry 2 RFID cards in my wallet for work access. I also carry a MetroCard for the subway (and often a spare) and have never had any indication that the smart cards are damaging the MetroCards.
posted by Artifice_Eternity at 11:25 PM on May 12, 2007

Odinsdream, those aren't wholly unrelated situations. Waving your BART card near an RFID reader is a lot like waving it at a degaussing coil, except that the RFID reader's at an unusually high frequency (125kHz for the common-as-dirt building entry systems) and not very powerful. On the other hand, a reader that uses a higher power signal, like maybe for reading cargo crates or car-based tags at a longer distance, might well demagnetize older, lower-coercivity magstripe cards, especially over time.
posted by hattifattener at 12:52 AM on May 13, 2007

« Older Hard-boiled Egg Filter: Eat 'em or Toss 'em   |   I need to find a religious studies graduate... Newer »
This thread is closed to new comments.