Mastering Measurement Uncertainties

Measurement outcomes are impacted by:

  • Diurnal Cycles
  • Background Radiation
  • Thoron
  • Protocols & Methods
  • Device Considerations
  • Decay Factor
  • Equilibrium Ratio

Thermal Effects of Diurnal Cycles

 A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth. During this diurnal cycle in which day and night are created, the Earth experiences a thermal effect as it rotates on its axis. Thermal effects of diurnal cycles are temperature lag (thermal response), lunar/solar response, irradiation and tidal response. These lags can have an effect of three or four hours.

Your best practice is to conduct radon measurements in full diurnal (24 hour) cycles when possible. Adding half of a diurnal cycle can skew results high or low depending on the thermal response.

Background Radiation

Naturally occurring low level ambient radiation can interfere with accuracy. CRMs and passive devices analyzed at a lab will have a background determination/factor at the lab/chamber from a previous sample or electronic noise within the measuring device. Passive devices analyzed by the practitioner, such as Electret Ion Chambers, will have a background correction factor built into the software


Thoron is an isotope of radon that can interfere with accurate radon measurements, especially in lower efficiency equipment. Sometimes a measurement specialist can predict a potential thoron interference such as measurements conducted near a dentist office or in a hospital where thoron can be found in higher levels.

Protocols & Methods

Strict adherence to protocols and methods can help prevent variation in your results, such as:

  • Compliance With Closed-building-Conditions (CBCs)
    • To achieve Dynamic Equilibrium in tests lasting 96 hours or less
  • Observing & Detecting Interference
    • Mechanical, weather disturbances
  • Device Placement & Retrieval
  • Good communication with all parties involved.

Device Considerations

Counting Efficiency of Devices

The counting efficiency of a device directly correlates with accuracy. A fraction of the total radioactive decay events are detected and recorded. Comparing counts per minute detected to actual dpm (disintegrations per minute) emitted; usually reported in counts per hour (cph).  Using high sensitivity and high efficiency equipment will improve the accuracy of results.

Low Efficiency vs. High Efficiency

  • High Efficiency
    • Detectors that have the ability to integrate hourly data.  i.e. Continuous Radon monitors.
    • Detectors that have a high “counts per hour”. 
      • i.e. 16 – 24 cph vs. 2.5 cph
      • This difference can translate into a 2.6 pCi/l difference.
        • A 3.9 pCi/l on a high E monitor can be as high as a 6.5 or as low as a 1.3 on a low-E monitor.
    • Monitors with tamper detection features.
      • Monitors that detect environmental data. 
        • i.e. temperature, relative humidity, barometric pressure.
      • Monitors that detect power disruptions or physical tampering.

Decay Factor

Decay Factor is especially important in passive devices. Some passive charcoal devices must be sent to a lab for analysis.  During this “transit” time (from home to lab), the radon that has adsorbed onto the charcoal continues to decay. Therefore, Start/Stop times and prompt shipping is very important. Start/Stop times are important in all radon measurements because remember, 1.0 pCi/L of radon decays at 2.22 disintegrations per minute. If the start/stop times are incorrect, the overall average will be incorrect.

Equilibrium Ratio