Standard aspiration (sampling) volume
Most detector tubes are calibrated for a sampling volume of 100mL. However, depending on the type of target gas, or the concentration, the volume should be changed to 50mL, 200mL, 300mL, or 500mL, which necessitates the number of pump strokes to be changed accordingly. With 100mL being the standard sampling volume, a 500mL volume would require 5 strokes to obtain the correct volume. The detector tube label includes a chart that shows how many strokes are necessary for various sampling volumes.
Extension of the measuring range
For most GASTEC detector tubes, if the colour change layer exceeds or does not reach the calibration scale during measurements with standard sample volumes, the concentrations outside of the calibration scale can be measured by changing the sample volume accordingly.
| When the colour change layer does not reach the calibration scale |
Sampling is repeated until the discolouration stain reaches the minimum scale. Then the indicated value is read off the scale and divided by the number of strokes (or in some cases, multiplied by the correction factor to obtain the true concentration. *Although in principle, the lowest concentration can be measured with a maximum of 10 full pump strokes, this number needs to be reduced in some cases to ensure accuracy. Please check the instruction manual "number of pump strokes [n]" before measuring. |
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| When the colour change exceeds the calibration scale |
Replace the tube with a fresh one, and sample with half of the standard volume. When the discolouration stain stays within the calibrated scale, the tube reading should be doubled (or in some cases, multiplied by the correction factor) to determine the true concentration. *Although in principle, the highest concentration can be measured with a minimum of 0.5 pump strokes, this number needs to be increased in some cases to ensure accuracy. Please check the instruction manual "number of pump strokes [n]" before measuring. |
Accuracy tolerance when using detector tubes
There can be slight deviations in values obtained even under the most ideal circumstances. There are random errors that the detection pipe itself has in this variation, and recurring (systematic) errors attributable to the manufacturer or the operator conducting the measurement. Their respective characteristics are described below.
Random errors
Even when a sample of a definite concentration is measured with highly precise detector tubes, the results will fluctuate to some extent on both sides of the mean. This type of error is referred to as random. To evaluate random errors, the relative standard deviation is used, which shows (in percentages) how the reading deviates from the mean value. This value Is also called the coefficient of variation (CV).
Random errors can essentially be attributed to :
- fluctuations in the inner diameters of detector tubes, densities of filling reagents, or sensitivities of reagents
- the operators who read the tubes
Systematic errors
Systematic errors can generally be attributed to the manufacture and/or the operator and they generally involve one or more of the following factors:
- leakage of sampling pump
- incorrectly calibrated detector tubes
- incorrect sampling time, inappropriate storage or usage of detector tubes
- presence of interferents
Reaction principles
Detector tubes generally employ one of the following three types of reaction principle:
| Direct reaction | The target substance (to be measured) directly reacts with a detecting reagent |
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| Compound reaction | The target substance instantaneously reacts with several detecting reagents within the detector tube |
| Two-step reaction | The substance is oxidised in the pretreatment layer before reacting with the detecting reagent itself |
Effects of temperature
Among the many types of detector tubes, some are more susceptible to the effects of temperature than others. This section introduces the effects of temperature variances as well as the correct response procedures.
Correction for temperature
All GASTEC detector tubes are calibrated based on a tube temperature (NOT sample temperature) of 20ºC (68ºF). Some tubes, however, are very sensitive and may not provide correct indications at temperatures other than 20ºC. If indications are affected more than+/-10%, tube readings should be corrected. The appropriate temperature correction values (or any other adjustment that may be required) are provided in the instruction sheet that comes with each detector tube package.
Temperature correction factors for 1,1,1-Trichloroethane detector tube(No.135)
| Temperature | 0ºC (32ºF) |
10ºC (50ºF) |
20ºC (68ºF) |
30ºC (86ºF) |
40ºC (104ºF) |
|---|---|---|---|---|---|
| Correction factor | 1.4 | 1.2 | 1.0 | 0.8 | 0.65 |
Temperature correction for Tetrachloeoethylene detector tube(No.133M)
| Tube Reading | True concentration(ppm) | ||||
|---|---|---|---|---|---|
| (ppm) | 0ºC (32ºF) |
10ºC (50ºF) |
20ºC (68ºF) |
30ºC (86ºF) |
40ºC (104ºF) |
| 100 | 410 | 155 | 100 | 80 | 65 |
| 80 | 310 | 125 | 80 | 65 | 50 |
| 60 | 210 | 95 | 60 | 50 | 40 |
| 40 | 130 | 60 | 40 | 35 | 25 |
| 20 | 55 | 30 | 20 | 17 | 15 |
| 10 | 20 | 13 | 10 | 8 | 7 |
| 5 | 8 | 6 | 5 | 4 | 3 |
Effects of temperature variances
There are two basic types of phenomenon involving the effects of temperature:
| Effects on reaction rate |
Generally chemical reaction rates are proportional to the temperature. As a rule of thumb: the lower the temperature falls, the slower the reaction rate becomes; and the higher the temperature, the quicker the reaction rate. |
|---|---|
| Effects on physical adsorption | Generally the quantity of physical adsorption of a certain substance to its reagent is inversely proportional to the temperature. At lower temperatures, some of the substance will be physically adsorbed to the reagent that has already reacted with the preceding substance, and will not reach the usual reaction zone. As a result, only a short stain of colour change is produced, giving a lower indication. On the contrary, at higher temperatures, higher indications will be obtained. Some of the GASTEC tubes thus affected are 1La, 100B, 171. |
When air is sampled, its temperature instantly adapts to that of the detector tube. The term "temperature" when used for correction of tube readings refers to the tube temperature, NOT the sample temperature. If detector tubes are kept within a certain ambient temperature for a while, the tube temperature will adapt to the ambient temperature. Therefore, a detector tube just removed from cool storage (e.g. a refrigerator) is the same temperature as the storage itself, but it will gradually acclimatise to the ambient temperature.
Effects of humidity
Although the vast majority of GASTEC detector tubes is not affected by a relative humidity in the range of 0 to 90%, some are susceptible to humidity. In the following section, both types of detector tubes will be discussed.
Moreover, when the relative humidity exceeds 100% (super saturation), water vapour in the air condenses, which dilutes concentrations of water-soluble gases (for instance, hydrogen chloride) and accurate results will not be obtained.
GASTEC detector tubes unaffected by humidity
There are two main reasons why detector tubes remain unaffected by humidity
| When the detector tube uses a water-based reactant |
The reactant in the reagent matrix in the detecting tube is generally in the form of a water-based solution. In this case, the target gas and the reactant interact in an "air-liquid reaction" and although the resultant water vapour may dilute the concentration of the reagent, there is no adverse effect by humidity on the value indicated by the detector tube because the reactant remains in its liquid state and the absolute mass is not affected either. Typical water-based reactant detector tubes are 1LA, 2L, 4LL, 5LB, 8LA, 9LA. |
|---|---|
| When the detector tube uses concentrated sulphuric acid or a desiccant |
When the detector tube uses concentrated sulphuric acid as an oxidant, or when it contains a desiccant, water vapour tends to be absorbed so there is no adverse effect by humidity on the value indicated by the detector tube. Some of these detector tubes are 1H, 13, 14M, 71, 91, 122, 132L. |
GASTEC detector tubes affected by humidity
Only five detector tubes are actually affected by humidity. Detector tubes No. 15L, 17, and 185 are calibrated for a relative humidity of 50%; No. 137 and 138 are calibrated for an absolute humidity of 10mg/L. The aforementioned tube types require a correction for humidity, similar to the correction for temperature. Humidity correction coefficients are provided in the instruction sheets for these detector tubes 15L, 17, 137, 138, 185.
Effects of atmospheric pressure
Generally the gas concentration is proportional to the pressure. All GASTEC detector tubes are calibrated based on normal atmospheric pressure (1013hPa or 760 mmHg) and their indications will not be affected over the range of +/- 10% of normal pressure, that is 912 to 1114hPa or 684 to 836 mmHg. When the pressure at the time of measurement is not in this range, the tube reading should be corrected as follows:
Factors that cause atmospheric pressure to have an effect
Generally the gas concentration is proportional to the pressure. All GASTEC detector tubes are calibrated based on normal atmospheric pressure (1013hPa or 760 mmHg) and their indications will not be affected over the range of +/- 10% of normal pressure, that is 912 to 1114hPa or 684 to 836 mmHg. When the pressure at the time of measurement is not in this range, the tube reading should be corrected as follows:
Measures to correct/offset effects of atmospheric pressure
For instance, the indicated value needs to be corrected when the atmospheric pressure at 1000 meters ASL (above sea level) and the civil engineering work (pneumatic method) at high atmospheric pressure diverge. The correction formula is as follows:
= Actual tube reading x 1013 (hPa) / Measured atmospheric pressure (hPa)
Interference gases (interferents)
When a gas with similar characteristics to the target object gas is present during sampling, it will affect the measurement. These gases which are referred to as interferents affect the accuracy of any tube reading. Some of the common interferents are described below.
Interference in direct-reading detector tubes
- The reagent(s) will also react to interferents, giving a higher indication. An example of such an interferent is hydrogen sulphide to the hydrogen cyanide detector tube (No.12L).
- If a pH indicator is contained in the tube, acids and bases will react as interferents, giving a higher indication. An example of such an interferent is hydrogen chloride to the hydrogen cyanide detector tube (No.12L).
Compound reaction / Interference in two-step reaction type detector tubes
- When the substances that result from the chemical reaction are identical to the target gas, a higher value (than the actual concentration) will be indicated.
An example of such an interferent is tetrachloroethylene to detector tube 132L. - Because the oxidant reacts with the interferent, the inherent oxidation rate is insufficient, and the indicated value will be lower.
An example of such an interferent is aromatic hydrocarbons to detector tube 132L.
Safekeeping and Storage of GASTEC gas detector tubes
In principle, the accuracy of detector tubes can be maintained for a longer time at low temperature; we recommend dark refrigerated storage at 0 to 10ºC (32 to 50ºF) and direct sunlight should be avoided at all times. Be sure to always keep them in a safe place out of the reach of children's eyes and hands.
Disposal of detector tubes
Detector tubes contain chemical reagents that may include substances subject to laws and regulations requiring specific disposal procedures.
Therefore, a detector tube that has been used (or whose expiration date has passed) should be disposed of in accordance with the relevant disposal regulations valid in your area or jurisdiction. For further information contact your local distributor or GASTEC representative.
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About gas detector tubes
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General information on gas measurement
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Reference
