Debit Card Readers and Plastic Bags

Luke T.

Unregistered
L
Joined
May 2, 2003
Messages
14,716
On two separate occasions at two separate filling stations, my debit card wouldn't register on those ubiquitous debit card readers. I would slide my card through the slot and the readout kept telling me to slide it some more.

Both times, after several attempts, the store clerk whipped out a plastic bag like you see in most grocery stores these days, and put my debit card in the bag and then slid my card (inside the bag) throught the debit card reader and it worked the first time!

Anyone have any idea why this would be?
 
I don't know. I've seen the failure of magnetic card readers numerous times myself. I have, however, yet to get the plastic bag trick to work. However, we had plastic produce-type bags. It um, supposedly "realigns" the electrons so it can be read. To which I responded, "What? How?" and got a shrug and an "I dunno. S'what I heard."

Sometimes, simply wiping it off or cleaning it with a bit of Windex will do it. Dirty cards are hard to read. Likewise, dirty readers have trouble reading.

What supposedly kills the strip is a whole matter all together. One certainity seems to be this"

"Where do you keep your card?"
"On the fridge!"
"With a magnet?"
"Uh-huh!"
"Yeah, that's why it's dead."
"Oh."
(repeat a few days later.)

I've heard cell phones can do it, car radios, space aliens from Alpha Centuri, but sometimes they just die out.
 
My card is only a couple months old. And I don't think Windex on magnetic media is a good idea.

It could be that the readers are dirty and need cleaning, but I still don't see how a plastic bag works.
 
I've also seen this done with paper reciepts. It may have something to do with changing the distance to the scanner, or the speed of the swipe.
 
Having dealt with this phenomenon on a daily basis for a few years I can say that despite extensive Googling I cannot find a "why".

Best I can tell is the reader is worn friggin' out and needs to be replaced.
 
Luke T. said:
On two separate occasions at two separate filling stations, my debit card wouldn't register on those ubiquitous debit card readers. I would slide my card through the slot and the readout kept telling me to slide it some more.

Both times, after several attempts, the store clerk whipped out a plastic bag like you see in most grocery stores these days, and put my debit card in the bag and then slid my card (inside the bag) throught the debit card reader and it worked the first time!

Anyone have any idea why this would be?
Click to expand...

Just a theory, I could be wrong...

The magnetic strip consists of a collection of magnetic domains (think millions of little bar magnets and you'll be on the right lines). The data on the card is stored by particular alignment patterns of those domains. It's possible (probable even) that some of the domains will become randomly realigned - think of it as magnetic "noise". The magnetic field from alignments due to the real data will be stronger than the "noise bits" - but close up to the sensor the difference between the noise and data may be such that the "noise" tends to corrupt the data. But at a slightly longer distance away from the face of the sensor, the stronger data bits will dominate over the weaker noise - and so the difference between data and noise (the signal to noise ratio) will be greater. And so the sensor circuitry in the reader may find it easier to distinguish between the data and the "noise".

It's either that or aliens. Green scaly ones from Alpha Centauri! :D
 
Fidelio said:
Having dealt with this phenomenon on a daily basis for a few years I can say that despite extensive Googling I cannot find a "why".

Best I can tell is the reader is worn friggin' out and needs to be replaced.
Click to expand...

I Googled debit card reader plastic bag and found something on a Credit Union web page.

Merchants may place scotch tape on the magnetic strip of the card or place the card in a plastic bag, claiming it will allow the card to be read.

If there is a problem when the card is swiped, the merchant terminal is usually the cause of the problem.

As the terminal wears out, maintenance people often tighten the tolerance instead of replacing the read heads. The tolerance must be set to the proper thickness of the cards. Tape or plastic simulates tightening the tolerance so the magnetic strip can be read.
Click to expand...

So kudos to RussDill for what appears to be a correct guess!

There is a caveat at the end:

Tape or plastic can leave residue on a magnetic strip, causing future failures at other terminals or ATMs.
Click to expand...
 
Luke T. said:
...
So kudos to RussDill for what appears to be a correct guess!

...
Click to expand...

I'll be dagnabitted. The bags we used were thin. Like uber, uber thin. That would explain why they wouldn't have worked.

Pity I no longer work there nor do I care enough to pass that info on.
 
My understanding is that the bag trick it's more to smoothe out surface defects than anything electrical or magnetic.

I also hear that "cling film" or whatever it's called in various parts of the world ("gladwrap" to us Aussies) works well and it's pretty thin (but needs to be very carefully applied).

LA's failure may be more to do with tension/smoothing than bag thickness. I'd assume a very thin bag would be harder to get ALL of the wrinkles out of than a thicker bag.

I'd be interested to hear results of anyone doing some tests on this. I'll wait :book:
 
My guess is that some readers are not as fault tolerant as others. I have just replaced my card, as it was getting worn out. But some readers would still get it first time, others just wouldn't get it at all.

My guess is that some will read the whole 'width' of the strip, the cheaper ones only a thin region. I had wondered about the plastic too, and the thought just occured to me, what happens is that they usually wrap the card in the plastic. Maybe the card is just lifted higher up by the thickness of the bag, so the thin readers will now get a new look at the data strip. Just a guess.
 
my guess: Thickening the card so it fits tightly in the reader ensures that the magnetic strip remains at a constant distance from the read head as the card is swiped, so that any detected changes in field strength are due to the encoded pattern on the strip, as expected, rather than to random changes in the distance between the strip and the read head.

If this theory is correct, adding the extra thickness to the other side of the card should work just as well as, if not better than, covering up the magnetic strip.
 
The card readers are magneto resistive heads and they can saturate pretty easily. As the head throats wear, the field can increase and the output signal becomes increasingly non-linear. While adaptive DSP techniques can compensate for bit crowding/spacing variations, compensating for non-linear responses is near impossible.

A small film will increase bitcrowding (which can bve deconvolved) but will help reduce non-linear response a lot.

marty
 
My guess is that the readers themselves are bigoted and racist. When my card is slid through they read my name and immediately judge me based on their own ignorant, hateful assumptions and then refuse to approve my purchase. Placing the bag over the card obscures my name and then they can't tell it's me.
 
You must log in or register to reply here.

Back
Top Bottom