With all the purchases of Geiger counters and dose meters I wrote a post a while back about testing your food. It is not all that easy to take an off the shelf Geiger counter and accurately test stuff on your own. You can get an initial range or baseline to compare things, but accurate readings that can compare to another reading with another counter is difficult. There can be a wide range of counts per minute or second. The REM or Sievert readings are even more difficult to compare from one unit to another. So I recommended sticking to counts per second which is the same as Becquerels.
There are now radiation clubs posting results online. I found one group on Facebook thanks to Japan Probe. In the video, one Citizen Radiation Patrol member, measures a fairly high Sievert level in a street drain. Street drains should be higher than normal because radiation in the air can be rinsed out by rain and collect in the water run off. Remember that there is often natural Radon 222 that adds to the counts for a few days following rain.
The reading obtain is a little humorous. The little dp802i dosemeter sounds the alarm for high radiation. The Sievert reading hits 5.77 microsieverts per hour compared to the initial background reading of 0.11 Microsieverts per hour. So is this a danger signal that should be heeded?
First, isolated patches of higher levels are not uncommon. A 25% increase following rain is not uncommon, but should drop in four days if the extra radiation is due to Radon 222 washing out of the air. If you plan on living in the drain, that may be a indication that that is not a great idea.
Second, the Sievert readings on most dosemeters are very sensitive. It is after all suppose to warn you of potential danger. The counts per second readings are what the overall dose is based on, or at least that should be the plan.
In the video, the Sievert reading is in the middle of the display in the largest font. Below that is the counts in what appears to be per minute (could be counts per hour, hard to see the decimal placement). The 11.4 Counts per minute is pretty normal as I stated in my previous post.
A second video linked by the Japan Probe post shows a variety of counters or dose meters being compared when using a slightly radioactive lantern mantle or a test sample. One of those happens to be a model dp802i.
The dp802i is on the left.
In the test, you can see the Sievert reading fluctuate wildly and the counts gradually change up dating every 15 seconds or so.
The Japan Probe poster believes that the dp80i is either wildly inaccurate or requires a longer time to obtain a reliable reading. He(she) is right that it does take a longer time. As far as accuracy, the dp801i appears to be pretty good, it is just the sievert readings sensitivity is very sensitive. This is not a sign of inaccuracy, more of a safety feature. It warns of a increase, but it takes time to determine the energy which is needed to determine the actual potential harm. So I would say that the dp802i is not a bad dose meter, just that it is not the final word on radiation evaluation, which it is not designed to be.
The Tokyo Radiation Levels Facebook page is dedicated to locals learning about radiation detection. If you are interested, you can follow their efforts a learn along with them. There are and will be plenty of high sievert readings that seem scary, but once you learn that Sieverts are very inaccurate until properly calculated, you will be amused. Dose meters should read well on the high side for safety. Then the high quality equipment can be brought in to provide the needed accuracy. It is not a conspiracy, just the nature of the beast. The dp802I is on the overly sensitive side, but if you consider the counts, not too bad of an inexpensive dose meter.
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