The RAD8 and RAD7 are able to measure both radon (222-Rn) and thoron (220-Rn) concentrations simultaneously and independently. The standard radon calibration is performed with a precision of better than 1% and we guarantee the absolute accuracy for radon calibration to be better than 5%. However thoron has a very short half life (about one minute) so that the concentration in the air will change during sample acquisition. For every minute the sample gas is in transit from the sampling point to the measurement chamber, the concentration of thoron in the sample will drop by half.

The RAD8 and RAD7 manuals describe the standard setup for thoron sniffing. It is designed to minimize the transition time of the air sample from sampling point to measurement chamber. The pump runs continuously and the desiccant is in a small tube. In those conditions, the acquisition time is about one minute, so the default thoron sensitivity of the instrument is set at approximately half the radon sniffing sensitivity. This setting is sufficient for most applications.

Occasionally a user requires more accurate thoron readings. For these applications Durridge can provide, as an option, a specific thoron calibration for the setup used by the user. The detailed conditions of the calibration, including the air flow rate, may be chosen by the user and are clearly specified in the thoron calibration certificate issued by Durridge. For any other setup, with a different flow rate or a different volume in the sample air path, that thoron calibration is no longer valid. For some projects, Durridge has provided multiple thoron calibrations, each for a different setup of the instrument and air sample acquisition path.

Upon completing a RAD8 or RAD7 thoron calibration, Durridge will provide a signed document specifying the operating conditions of the thoron calibration procedure, plus the calculated thoron sensitivity and B to A spill factor values.

The price of Thoron Calibration for RAD8 and RAD7 is $208.

Shipping Information

1. Before you send a RAD8 or RAD7 unit back to us, please complete the Instrument Return Form and email it to Durridge at . Include a printed copy of the form in the box with the device. You can send your shipment to either of our facilities (USA or UK), listed below (see important, specific instructions for each below).

2. We typically need the RAD8 or RAD7 in house for 10 business days, provided it has no serious problems. Once it has been serviced, the instrument will be shipped back to you after payment is received, so the total time the instrument will be away from your facility is typically 10 business days, plus the time it takes the receive the payment, plus the time the instrument spends in transit. If additional service or repair is required, this may delay the return of the instrument.

3. When sending your RAD8 or RAD7, please send the instrument only, without the cables and accessories. Pack the instrument upright in a box with one-inch (2.2cm) padding all around. A 14x14x14in (or 36x36x36cm) box is suitable. Pack the box well, and seal it carefully.

4. International customers must be very careful, to avoid substantial extra shipping charges and delays, both for shipping to the nearest calibration facility, and for its subsequent return.

5. Commercial Invoice check-list (for international shipping only):

  • For your courier or freight forwarder, you will need to provide a Commercial Invoice.
  • Please write only in English on the commercial invoice.
  • Use the following description for a RAD7: “Used RAD7 Electronic Radon Detector”. For a RAD8, use “Used RAD8 Electronic Radon Detector”.
  • Insured value US$1,500.
  • Please state on the invoice that the instrument is MADE IN USA and that it is being “returned to the manufacturer temporarily for repair and recalibration“.
  • Please use the HS (Commodity) code 9030.10.00.
  • Please mark the box with the serial number of the instrument (as the RMA – Return Merchandise Authorization).
  • Please add the phone number of the Calibration facility you choose, in case the Customs Office wants to call us.
  • Next, you must ensure that your courier (i.e. UPS, FedEX) or freight forwarder will deliver the package to our door. The formal “INCOTERM” for this is “DDP”, which means “Delivered Duty Paid.”
  • We will not pay or be responsible for USA or UK import duties, or Customs clearance charges. Your shipper must be told this when you arrange your shipment.
  • It is important to make sure you prepare all the documentation you need to reimport the goods back into your country without being charged.
  • For the Return to you: unless you instruct us otherwise, we will return your Durridge instrument via the same shipping method you used to deliver it to us, Freight Collect. DURRIDGE can pay for the return shipment CPT (“Carriage Paid To”) to you, but our cost will be invoiced to you before shipment.

If we can help further, please email us at

Specific info for SHIPPING TO OUR USA FACILITY (from outside the USA)

DURRIDGE Company, Inc.
900 Technology Park Drive
Billerica, MA 01821-2812, USA
Phone: +1 (978) 667-9556
Fax: +1 (978) 667-9557

Declaration Form

  • There needs to be documentation proving that the instrument was previously imported into your country, otherwise you may be charged import duties when the instrument is returned to your country.
  • Please fill out the declaration form provided below. Attach the completed declaration form to the commercial invoice. If the invoice is scanned and submitted electronically, please scan the declaration form and submit it electronically as well.

Foreign Shippers Declaration of US Goods Returned (PDF)


Sheffield Technology Park
Cooper Buildings
Arundel Street
Sheffield S1 2NS, UK
Phone: +44 (0)114 221 2003

Please complete the checklist in the above section. It is very important to include the HS (Commodity) code 9030.10.00, otherwise import duty may be charged at the UK border, for which you will be liable. Please also include our EORI number GB219670885000.

Please ensure that you make the necessary arrangements with the Customs authority in your territory for the return of your instruments. There may be special procedures for temporary exports. For example, you may need to provide documentary evidence that the instrument was previously imported into your country, to avoid being charged import duty again.

Instrument Return Form

Fill out this form before returning a RAD8 or RAD7 to DURRIDGE for repair:
Instrument Return Form (PDF)

Related Article

Technical Note: Thoron Calibration (HTML)