Conversion factor

The following table shows how different people use different conversion factors for the same tube. Who is right ?

Geiger Tube Isotope CPM * conversion factor = μSv/h Used by
LND 712 60Co

137Cs

1/107 = 0.00926

1/120 = 0.00833

blackcatsystems
GM10
LND 712 60Co

137Cs

1/123 = 0.00812

1/111 = 0.009

Nuova Elettronica
Kit LX.1710
LND 712 1/100 = 0.01 cooking-hacks
Radiation Sensor Board for Arduino
LND 712 1/429 = 0.00233 libelium
Wireless Sensor Networks to Control Radiation Levels
LND 712 1/429 = 0.00233 (up to June 9, 2011)

1/120 = 0.00833 (since June 9, 2011)

pachube
Geiger Counter Feeds from Asao Kawasaki
LND 712 1/429 = 0.00233 geigermaps
Create a new radiation feed to an open database
LND 712 137Cs 1/123 = 0.00812 Kansan
簡易なカウント数からシーベルトへの換算
J305ß 1/360 = 0.0027 Youtube
First tests with Arduino based Geiger sensor
J305ß 1/123 = 0.00812 cooking-hacks
Radiation Sensor Board for Arduino
J305ß 60Co 1/123 = 0.00812 Libelium (private email correspondence)
SBM-20 60Co

226Ra

1/150 = 0.00664

1/198 = 0.00504

Nuova Elettronica
Kit LX.1710
SBM-20 60Co

226Ra

1/150 = 0.00664

1/198 = 0.00504

Kansan
簡易なカウント数からシーベルトへの換算
SBM-20 60Co

137Cs

1/150 = 0.00664

1/171 = 0.00584

Libelium (private email correspondence)
SBM-20 1/175 = 0.0057 cooking-hacks
Radiation Sensor Board for Arduino
SBM-20 1/360 = 0.00277 libelium
Wireless Sensor Networks to Control Radiation Levels
SBM-20 1/360 = 0.00277 youtube
Enotria1 con Libelium primo start
Unknown 1/360 = 0.00277 Safecast
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Lesson Learnt: EMI

For the whole month of July I had been concerned with my measurements being too high, about 3 times what everybody else seems to measure.

I never had 100% confidence that my instruments was telling me the truth, probably because I assembled it myself from a kit. On top of that, every time I switched on or off an electric fan or a neon light in my apartment, I was getting a spike in the measured values. I learnt to live with that, and my data post processing procedure takes care of that and removes those spurious spikes. A few weeks back I also added ferrite chokes to the cables plugged into the unit, and the spikes induced have become a little smaller.

Yesterday finally I decided to wrap all the cables and the instrument itself in aluminum foil, and voilà, today the reading was down by a big step. Now my reading are only about twice what other stations report. Big improvement !

I have a feeling that I am getting close to doing accurate measurements, even though I think there is still some residual EMI to be shielded off. In the next few days I will replace some internal wiring with coaxial cables too.

Now that I am getting closer to a more realistic ball park range of measured data, I am also starting to think that the formula that I am using from converting from CPM to μSv/h is correct (the tube is an LND 712)

123 CPM = 1 μSv / h

but I would still appreciate if anybody could add any comment.

In light of this, I will also have to update and replace the picture in the “Histogram” page, but I’ll get around that in a few more days.

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High Resolution Histogram

After looking at the picture in the Histogram page, I was getting concerned about the first bin on the left, which was basically saying that 8% of the readings had a value less than 0.049 μSv/h.

I wanted to achieve a better characterization of the histogram in that region, so I pre-processed (decimated) the original data, which was given as one sample every 10 seconds, to obtain one sample every minute, leading to a 6-fold improvement in the bin width (narrower is more precise). I fed that data to my plotting procedure and the results is the following picture:

One comment about this picture is that it could possibly explain why my readings are about 3 times higher than what is reported by the official sources: I am reporting the average value of the distribution, and maybe the other sources are reporting the minimum value, which in my picture lies at about 0.049 μSv/h.

Another interpretation could be given by observing that the general envelope shape is not entirely like a bell shape, but it has some small “bumps”, specifically one on the left at 0.049 μSv/h, and a couple on the right above 0.211 μSv/h. Assuming that each partial bell shape is due to a separate stochastic process, we can identify three contributes to the picture:

  1. I can explain the bumps on the right as caused by electrical interference in the measuring instrument (I know for sure that I need to provide better electrical shielding to my measuring instrument).
  2. Arguably the little bump at 0.049 μSv/h is what everybody else is measuring and reporting.
  3. Now the question is: what is the cause for the main bell shape ? For the time being I will assume that it is indeed the gamma radiation that I am intending to measure.
Posted in Histogram | 2 Comments

Extraordinary radiation ?

A big dilemma now is knowing whether the current readings are normal or they are affected by the Fukushima accident. There is a way finding out, regardless of the fact that my graph shows higher values than the official sources.

If you keep watching this site and let’s say for example that in the next few months you notice a progressive attenuation in the curves plotted in the Home page, that will be a clear indication that the older readings were affected by extraordinary radiation.

So let’s keep on eye on those diagrams and find out !

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Safe levels ?

Is the level of radiation indicated by the time plot in the Home page safe ?

According to http://en.wikipedia.org/wiki/Background_radiation the world’s annual average is 1.26 mSv/h, which divided by 365 days and 24 hours gives 0.143 μSv/h, so it appears that my graph is in line with those values.

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Discrepancy of Results

The values in my Home page graph seem to be about four times higher than what is reported in the official documents. Compare it for example to http://mextrad.blob.core.windows.net/page/01_Hokkaido_en.html

I am really anxious to find out why there is such a discrepancy with the other web sites. I have been contacting various “experts” in the field, but so far nobody has replied.

Posted in Time Plot | 2 Comments