Randy.
You can now purchase cheap gamma meters. Buy one and place it in the middle of the radios. They are slow to respond but will give a reading. If there is an elevatrd reading find a radio with highest gamma and test the flux.

Flux measurement with charcoal might give you a yes or no answer on the tile. You need two. One exposed on a cardboard box and one under double layers of 6 mil poly that is taped to the tile floor.
A much better method is to purchase a radon eye and a large metal mixing bowl. Seal radon eye under the metal bowl with poly tape. Leave on the floor for 24 hrs. You can also put a radio on the tile floor with radon eye and seal poly over it.

Bigger and bigger fans to fix problem houses is a rampant misconception among mitigators especially those who dont own or use a micromonometer. Of course there are cases where higher airflow is needed because sealing can't be done or soil is too porous. A micromonometer is your most critical instrument. We have to teach mitigators over and over, for gods sake please buy a micromonometer. With it you can measure the edge of the pressure field which is -0.001" or -0.1 Pa. I would guess each fan change you made changed u tube 20% greater. That means your previous vacuum readings are now -0.0012" or -0.12 Pa. That ain't much change.

What does work is adding another suction where micromonometer says there is no vacuum. That is always a better choice than the clueless mitigator simply installing a bigger fan and hoping it works.

My suspicion is your elevated readings are from a slab that butts up to the basement like a garage, front porch or family room slab. These adjoining slabs are often trapping radon that is bleeding into the basement.

Test the water, just to eliminate that from the equation, but for gods sake, get a manometer, you can’t quantify smoke!

I agree with Rob.... if the house has a well, try testing the water supply to eliminate that as a possible source.... simply to do and effective...especially if there is a source of water usage such as a bathroom with a bath or shower near that bedroom in question...

Next question... were there any renovations done in the last few years where there were openings made in the concrete slab.... such as a new bathroom.... plumbers like to leave a nice big hole under the bathtub or shower to install his P-trap and rarely seals up that hole after his work...

I also agree with Bill and Rob..... get a good manometer to qualify your numbers.... you can get good used ones on ebay for around $250 - $350...

Great advice- Thanks guys.

QUESTION: I can get a Corentium Home tomorrow, or a Radon Eye mid week. Will the Corentium Home do the same job on the flux test as the Radon Eye?

BIG FANS I agree that a big fan should be the last resort. We tried suction at 6 points in the small furnace room before doing a 2nd suction point at the best one. When the retest showed no change in the radon level, we returned with the Rn4. Any additional suction points in the slab will have to be in finished rooms- probably a 2nd system in a finished room.

GARAGE SLAB I was planning to (and probably will) do suction at the garage, but am hesitating because the basement adjacent to the garage has 7.4 pCi/L, while bedroom 1 is away from the garage and has 8.9.

WELL WATER Although I've never known Minnesota well water to be a problem and the basement bath is unused and we had 2.3 pCi/L 17 years ago, I will test the well water.

MICROMONOMETER My guys prefer smoke to the micromonometer and are pretty good at using smoke to estimate Pa, but we used both here. With 2 suction points and the Rn4, we had communication across the entire slab, with -1 to -2 Pa at 8 test holes and -.4 at one.

ODD STUFF The crew did spend several hours working through our check list looking for things like an opening at the tub trap and rusted out furnace floor over open dirt, and found nothing that appeared meaningful.

Thanks again, and if you think of anything else...

I have seen quite a few old radios in the past with radium dials. But, hard to believe the dials would be the source. Please let us know what you find. It would be interesting to know if the homeowner uses ground water.

Interesting reading below, but can not attest the accuracy of the articles.

http://www.antiqueradio.com/Apr07_Auyer_Glow.html

https://www.hodinkee.com/articles/new-report-shows-radium-dials-might-pose-serious-danger#:~:text=The%20study%20found%20that%2030,the%20recommended%20maximum%20safe%20level.

https://www.orau.org/ptp/collection/consumer%20products/electrontubes.htm

Randy
For the homeowner wanting to know the average radon level airthings and safety siren are fine. But for diagnostic measurements the Radon Eye RE200 is a great tool for mitigators and costs about the same. The RE200 sees 30 cph/pci/l. Compare that to the wave having about 1 cph/pci/l. Its sensitivity is so high it can give the 1st radon result in 13 minutes after plugging in and a new result every 10 minutes. The 1st result is about 75% of the actual level. In 33 minutes it is very close to actual level. You can buy 4 rd200s for less than than the cheapest professional monitor. On Amazon buy a few usb 5 volt to 12 volt dc 5.5mm plug cables. They are cheap. Now you can use any cell phone powerbank battery to make rd200 portable for 10 hours or more.
When you have a problem house. Put the 4 rd200s in all the suspect locations. 15 minutes later you have 4 simultanous results to compare. Only downside where I live is they only read to 100 pci/l so you blow them out at houses i work on.

If you leave them at the house you get hr by hr radon once downloaded to you phone.

All great comments. While soil-gas is the likely suspect, if the home uses WELL water, then that could be the problem (measure a shower/bath room). Lastly, slate is metamorphized from shale, which is known to contain much more radium (Ra-226) than granite, and they have equivalent emanating factors. If you use an inverted bowl/box to measure flux from the slate, also measure something like ceramic tiles or maybe concrete for a "background" since some radon WILL be coming off the slate and you have to have context.

1) Are there floor drains or plumbing drains that terminate below ground and have no traps, or traps that are dry from disuse? They may great soil gas entry points even when you have a negative relative subslab pressure.
2) Look at the garage or any other large slabs whose foundation walls are also basement walls, particularly any slabs that have been added or enclosed in recent years. We just "fixed" a house that had an existing 3-suction point system in the basement by mitigating the garage on the main level, which was an open carport when the original work was done. (There was tight clay with no PFE under the basement slab, which is why I looked at the garage).
3) One can also look for hotspots (such as radio rooms) by simply closing doors between rooms and placing test devices in each room.
4) I assume you have ruled this out, but will mention it anyway: Was the testing conducted with a properly calibrated monitor in a proper test location, by someone who knows how to use it? Electronic interference and defective monitors are always possibilities. With a Corentium Pro (and maybe others) it is possible to print the wrong dataset from the several it stores: printing the same dataset will always yield the same result!
5) Are the HVAC supply vents closed in the basement?
6) Is there an HRV, ERV, or ventilation unit in the house that has a plugged intake or dirty filters, or is turned off? That may be what was actually controlling the radon previously when it was operating properly.

Randy, I not sure what the soil conditions are like at this house but I've had couple of difficult houses that had great PFE and still high radon. All had really pourous rocky soil and the answer was "flow" Swapping 3" pipe to 4" was the answer.

Mike Kitto mentioned thoron. Sime CRMs are sensitive to thoron and will include a percentage of the ambient thoron as radon. The Radon Eye RD200 has the shortest open path at the bottom to its chamber and thus includes a high percentage of thoron as radon in its result. If you use an RD200 for flux you must cover the open slots on the RD200 with poly tape. Thus reduces thoron response to about 5% but does not change the hour by hour radon result.

I was part of an effort to decontaminate and release a 30’ by 60’ building use to store old, mostly military, airplane gauges and radios. In addition to radon gas there was a lot of particulate radium contamination, mostly from leaking gauges, although some of the radios also leaked flakes of paint containing radium. You did not mention if the radios at your site were civilian or military. Military radios would be of more concern. Some of the radios we saw had toggle switches with a small glass ball at the tip. Break one of those and you would need to hire a consultant for the clean up. Once you buy an inexpensive radiation meter, if you see dose-rates over a few times background, you might want to contact your states Radiation Control office for an evaluation.
Minnesota Department of Health Radiation Control
625 Robert Street North
PO Box 64975
St. Paul, MN 55164-0975
Phone: 651-201-4545
Fax: 651-201-4606
Email: healthdotramatstatedotmndotus (Radioactive Materials Unit)

Nice thread. A good example of the high quality of the advice this listserv can give ... and the amazing stuff one might never have known!

I am enjoying this thread. Fantastic advice.

I had experience with antique radios, clocks, etc. so I am going to comment to this component only as everyone has already provided priceless advice for the other part of this.

I had a customer that had a room filled with around 200 items that did register on the meter. So, to test this theory in more detail we, covered all of them and sealed the covers to the floors and took more readings. Bingo! This was a source. We continued to allow the multiple CRMS to collect data before items were covered and while covered. We forced the radon out of the home with ventilation, tested with radon system on and off. We performed every scenario the homeowner and I could think of to pinpoint and collect as much data as possible. Did I mention the homeowner was a little bit of a science geek too who wanted to learn as much as I did?

We were without a doubt able to attribute an increase in radon levels to those items.

Quick update:

The radios were removed from the room with the slate tile and put in the garage. A Corentium under a stainless steel salad bowl taped to the slate floor (covering the corners of 4 tiles) and another on a table both read about 8 pCi/L. The radios and slate are not the problem.

The well water tested at 400 pCi/L, meaning it is contributing about .04 to the room air.

We did suction at the adjacent garage slab.

Radon is still about 8. More to come. And thank you for all the suggestions!

I remember Bill Brodhead had a situation in his high homes where a tiny doorway entry pad (only a couple? of square yards in area) needed to be addressed as the stubborn source. Hadn't seen that ruled out yet. And I like Henri Boyea's checklist. Really hoping that you can get some fast-responding grab results to give more "texture" to the airspace you're working with--for example, are there any gradients at all that might at least point you to the right wall to focus on?

One more weird one:
Years ago at Symposium someone (Bill Brodhead?) told me a tale of a problem house where a major radon entry point was the large conduit to the bottom of the electric breaker panel, the other end of which terminated underground. Sealing the upper end of that conduit dramatically reduced radon concentrations in the basement.
Fast forward several years: In the basement of a difficult house I was working on, I noticed an electric wire in a 1" pipe that went through near the top of the exterior wall. I went outside and found that the 1" pipe was inside a 2" pipe buried in the ground: the pie was about 2 feet long and open on the end which was buried and the end which was in the basement.

When you get a tough one you can spend a month going back to a house multiple times trying to fine tune your existing system and or expand its reach when the main source is something overlooked like a pipe leading to soil. I had a house fail several years later after it initially passed. One finished room was slightly higher and one of the paneled walls seemed to also be higher. I finally removed the paneling and there was the old sewer pipe leading to the old septic field in the back yard. The owner then told me he had city sewer installed a few years ago. The guy who redid the plumbing and the guy who installed the paneling never bothered to stick a tennis ball or rag in the old sewer line! I don't know why it didn't smell.
One shot of expanding foam and system was fixed. It only took me 2 months to figure that out.

It took us a couple months too to figure out an old farmhouse. It had city sewer, so we didn't worry about the floor drains- just made sure they had water in the traps. Then we found an old floor drain that apparently went out to a dry well. A wad of paper and a blob of concrete and we were done. We had the same problem with a cistern drain on an old house.

I'd heard about the electric conduit house. As I recall, 100's of pCi/L were flowing in through the conduit. It's on my "Problem House Checklist". I hope every mitigator who doesn't have such a checklist is building one, and including all the suggestions in this string.

As a young builder, Dad taught me to go to conventions and listen quietly as others shared their secrets. I'm going to go against his advice and share my checklist. We generally bring it out after we have communication across the entire slab, including suction at adjacent slabs, where need is indicated. See attached. Feel free to point out what it is missing.

400 pCi/l is still can be a significant source depending what is going on with water usage i would look very hard at a 7 day electronic test and see if you are seeing hourly variance around water usage it typically takes a few hrs to show results after water usage on the device.

Thanks for sharing!

Running away from water issue we have had two tough houses starting at over 10,000 Bq/m3 or 270 pCi/l that refused to come down to below 100 because of compacted damp sand under slab we had 11 draw points on the system in one of the houses. To get the last i had to tap the block walls in 5 places (already had drop points right there ) we only discovered this when we drilled the block wall it blew the grab sampler at above 39,000 Bq/m3. We did connect restrictor plates inside piping to control amount of air drawn from block wall.

Very generous! You have certainly become one of those folks your Dad advised you to go to conferences to listen to. :-)