Tech Talk – The Great Spacer Debate

There is sometimes confusion over the performance of RFID tags placed on wet objects. This is not a problem for low frequency passive tags used by our HDD System, but for the newer UHF passive tags this can present challenges. Unless race timers use Battery Assisted Passive (BAP) tags, they will need to make allowances for proximity of a passive UHF tag directly next to the liquid-filled, moist human body.

There is no ‘black magic’ here. Even when high performing, industry leading Monza 4 or Higgs 3 ICs are used with the very best dipole designs, passive UHF tags suffer performance problems when directly placed on the human skin. Simply put, it is just physics and the result of detuning and signal loss due to water.

The solution is to space the tag away from the skin with the cheapest electrically inert substance: closed cell foam. Our testing has shown that the spacer needs to be only 2-3 mm thick to guarantee excellent read rates, even when the runner’s race bib is stretched tight against the body – and the singlet dripping wet with sweat.

Some of our competitors claim they don’t need a spacer, yet they have to employ two tags per bib. Even a 1 mm thin PET spacer will help things, but we believe that to get near 100% read rates, some type of thin foam spacer is required to guarantee performance in all racing conditions. Using two tags is also a good insurance policy given that each tag costs less than $0.15 each*. Yes, that’s 15 US pennies, not the exorbitant $1.00 per tag that our competitors are charging!

* Approximate cost of the passive tags that we recommend: Alien Squiggle and UPM Raflatac Dogbone. Adding a spacer adds about $0.04 per tag.

Comments

  1. I’ve contacted a few vendors, but none of them seem to know anything about these foam spacers. They offer spacers that go between the tag and the surface that the label is mounted to, but the entire thing costs about 50 cents a whack. They are designed for metal mount situations.

    Do you have any suppliers who can provide us with rolls of spacers that we can print on (preferably with the tag under the spacer so we dont need to manually apply the spacer on top of the tag on the back of the bib.

    And as a poor mans solution, is it possible to just put a standard wet mount tag on the front of the bib rather than the back?

    • Hi John

      We would not recommend putting tags straight onto the bib with no spacing. Sure it is a poor mans way of timing but the day it rains hard, or the athletes wear tight fitting vests in hot conditions, will be the day you start missing reads. Think of the spacer as an insurance policy. Having said that you would still expect something like 96% of tags to be read without a spacer most of the time. With twin tags that might be 99%. To miss one in 100 is not really good enough for an important race!

      There are a number of ways to prepare a foam spacer. The most popular is to just get adhesive backed EVA foam sheet cut up at an establishment like Kinkos (USA). You are looking at a few cents per spacer. Otherwise other timers have used single sided adhesive foam on roll and precut these strips to match the length of the tag.

      RFID Race Timing Systems is also working on a precut foam spacer to match the width of the most popular tags. More on this in the future.

      Finally we have been working with a company to produce a high quality fully finished tag on foam spacer (printed and encoded as an option) and can get the pricing below 40c each if bought in minimum qantities of 20,000 tags. Prices will be further reduced at higher quantities.

      Please email us at info@rfidtiming.com for more details.

Speak Your Mind