One problem that I've been having with burying the Smartrac Dogbone tags is that soil is that they break after some use. After taking a close look at some of the broken tags it appears that this is caused by the abrasive soil eventually cutting through the outer lamination of the Dogbone tags and then damaging the IC that controls the tag, causing them to become unresponsive. To counter this, I designed a two part protective enclosure for the tag that will hopefully protect them from damage. The design is outlined below:
This "Dogbone Sandwich Shield" is made up of two identical 3-D printed parts. The sandwich ends were designed to be slightly larger than the Dogbone's footprint with two tabs on the long ends. The tabs were placed like this to minimize interference with the moisture sensitive area of the tag which sits at its center.
Assembling the product was a fairly simple process. First, the Dogbone tag was stickered onto one of the sandwich ends. Then a small amount of acetone was brushed onto all of the sandwich end's tabs. They were then pressed together to create a sandwich with the Dogbone in the middle. The sandwich was then treated in an acetone air bath to seal the edges. After 20 seconds in the air bath, the part was removed and put in a vice under a small amount of even pressure to make it even until it hardened. It takes around a day to fully harden, but it is tough enough after an hour. Below is a picture of the final shield.
The presence of the shield surrounding the Dogbone tag made it slightly less sensitive to moisture, however, the variance from readings with the shield was no greater than it was without them. Therefore, using the shield will not influence the accuracy of the sensor once, but it will require some re-calibration.
Thinking ahead, creating shields like this one with varying thickness could allow for changing the sensitivity of the tags. Currently, without a shield, the tags read their lowest value before the soil has gotten completely saturated and are able to read soil that is more than completely dry. Adding a shield that is a certain depth should shift the reading range to be centered and allow the tags to read over a larger range of soil conditions than they currently do.