Accuracy vs. Precision
Accuracy vs. Precision in Detector Tubes
The new Sensidyne/Kitagawa detector tube handbook lists a standard deviation value for each of our detector tubes. These handbooks are now being distributed nationwide, so this is an excellent opportunity to discuss the meaning of standard deviation and its relationship to accuracy. Often these terms are confused. One competitor has standard deviation values listed in their handbook, and their sales people often erroneously quote these figures as accuracy. This is quite misleading
Since standard deviation values for detector tubes are often numerically less than the +/-25% that is the generally accepted accuracy level for detector tubes. End users may be misled into believing that they are obtaining accuracy levels that may not be correct for the system they are using. We do not want our customers to misinterpret the values listed in our handbook. The following is offered to prevent such misinterpretation.
First, let's take a look at accuracy. The accuracy of a detector tube system is the level of agreement between the system and a standard target gas of known concentration. The accuracy specification for detector tube systems is +/-25% of reading as established by NIOSH in their certification program conducted between 1973 and 1983. The American Industrial Hygiene association (AIHA) in their manual, Direct-Reading Colorimetric Indicator Manual (Second Edition, 1993) makes the following statement on detector tube accuracy:
The accuracy of detector tubes generally is considered to be +/-25% of the reading. This is based on the criteria for certification of detector tubes as originally defined by the National Institute for Occupational Safety and Health (NIOSH).
The NIOSH certification program Referenced above was discontinued in 1983, but it was picked up by the Safety Equipment Institute (SEI) in 1986 and Continues today. Under both programs, Tubes are tested by an independent laboratory and must meet an accuracy level of +/-25% at test levels of 1.2 and 5 times the TLV (threshold limit value) and +/-35% and 1/2 the TLV. Both programs are limited to tubes in the TLV range and to a handful of listed substances, so by design they cannot cover all detector tubes.
Next, let's look at standard deviation, which is a measurement of the tubes' precision. Standard deviation is an indication of how far a group of repetitive measurements strays from the average of all the measurements. This value is completely independent of accuracy. A box of detector tubes could display a very small standard deviation (i.e., very good precision) and still produce a large error,
For example, suppose we have a box of carbon monoxide detector tubes that ate date expired. We decide to test them, so we sample a standard gas mixture of 100 ppm CO in air. The tubes produce ten readings between 45 and 55 ppm with an average of 50 ppm. Our standard deviation would be quite low, because all of the readings are close to the average. However, our accuracy is very poor with the average reading showing a 50%error.
The classic illustration of the difference between accuracy and precision is depicted using targets on a rifle range. In this case the accuracy is based on the distance from the bull's eye.
A.) Good Accuracy and Good Standard Deviation
B.) Good Accuracy and Poor Standard Deviation
C.) Poor Accuracy and Good Standard Deviation
The accuracy specification for Sensidyne/Kitagawa detector tubes is stated in the tube instructions as +/-25% of reading. An accuracy specification is the maximum expected spread. Actual accuracy of a specific reading could easily be better. To assure optimum accuracy in using detector tubes, follow the tips below:
- Detector tubes are calibrated at 20 degrees C and 50% relative humidity. Accuracy will improve as these conditions are approached.
- Fresh detector tubes will generally be more accurate than tubes approaching their expiration date.
- Refrigeration storage will prolong the freshness of the tubes and improve accuracy. Be sure to warm tubes to ambient temperature prior to use.
- Detector tubes systems using rubber bulbs or "thumb pumps" generally have wider accuracy specification then systems with high precision hand pumps.
- Crossing certain brands of detector tubes and pumps can cause huge accuracy losses due to the differences in flow rate curve provided by the various pumps. Always consult the supplier and request data to back up compatibility.
- Regular leak checks of the pump will assure that the optimum accuracy is always attained.
- Accuracy improves with longer stain lengths. If several ranges are available, choose the tube that will provide a stain length on the upper half of the tube's range.
For further information on our detector tube system, please contact our customer service group.