Drager Toxic Gas Detector Extension Hoses Correction Factor

Drager Chemical Gas Detection Tubes - Correction Factors for Extension Hoses.


Recently, one of my friends posted a question to our alumni Group mail, in regards to correction factor required to be used for some of the Drager tubes when used with Extension Hoses. This is not really applicable to all tubes, but only to some of them.


The query was as follows:

I have recently came across documentation in a Drager instruction manual which stated that when tubes are used in conjunction with 10 or 15 metre length extension hoses, a correction factor requires to be applied for certain tubes. The reading obtained on the tube requires to be multiplied by this correction factor. As can be seen, the gases this applies to are those which are commonly encountered on board ships.

The problem now is that Drager does not manufacture hoses in lengths greater than 15 metres, but can and do supply these if they are specifically ordered. If a hose of greater length is used, there are no clearly defined correction factors available. You will note from the attachment to this message that the factors for a 15 metre length are greater than those for a 10 metre length. It therefore follows that as the length increases, the factor that requires to be applied should logically increase. I am not entirely sure for the reason for this correction factor, but it could have something to do with the atmospheric pressure differential between the pump and the tube. 

Information on the use of extension hoses with other Draeger Tubes is available upon request. 

Tel. 4511882808

Measurement by means of 1Om and 15m extension hoses

Single-stroke measurement

Draw the sampling air through the tube with 1 pump stroke. When the stroke is finished -the white indicator mark on the pump becomes visible - allow for a waiting period of 2 minutes before removing the tube from the sampling point. Evaluation is effected in accordance with the instructions for use of the tube in question. Correction of the measuring result is not required for the single-stroke  measurement. 

Several-stroke measurement

Draw the sampling air through the tube with the specified number of strokes. Always perform the next

stroke once the white indicator mark becomes visible. No waiting period is required after the final stroke. If several-stroke measurement is performed by means of 10 m or 15 m extension hoses, the tube

reading of some of the Drager Tubes must be multiplied by a correction factor (See attached pages on this blog).

Now the first question is why? Why is a correction required?

The reason is most trivial and dates back to high school physics.

With a 3 m extension hose, the error is immaterial and is not worthy of consideration. I would not be very wrong in saying the same for even 5m hose. In fact, if your measurements are based on single stroke measurement, no correction is required to be applied with 10m or 15m tube lengths. It is only required to be used, when you have multiple stroke measurements with a tube.

The reason can be found in the way the chemical reactions take place in the tube. A number of chemical reactions are used in such tubes and these include :

1. rapid reactions which are independent of the flow rate through the tube;
2. rapid reactions with secondary reactions which may depend on the flow rate through the tube; 
3. consecutive reactions in which the vapor reacts with one reagent to produce products that are detected by reaction with a second reagent; 
4. reactions where a large excess of reagent is necessary.

For tubes involving reactions (3) and (4), flow rate is often critical because the reaction rate with the reagent can be slow. In addition some tubes involve the use of a pre-cleansing layer of a reagent which is intended to remove possible interfering substances.

And the flow rate is definitely affected by the length of hose. Why? Bernoulli’s Theorem…guess simple physics. Weight of the tube, causes elongation, causes the tube dia to reduce, causes change in flow rate.

What correction to apply?

Now what correction factor to apply with a tube of greater length (>15m) will need to be confirmed from Drager themselves. However, I’ve not come across anywhere, where greater then 15m length tubes are used. So drager has not provided further guidance too.

The volume which is conveyed with each pump stroke is not changed when the extension hose is used, however, the flow characteristic which is typical of the Drager gas detector pump may be subject to changes. This in turn results in a change of the opening time of the gas detector pump. For some Drager Tubes in conjunction with 10 or 15-meter extension hoses, such change is liable to cause errors in the readings. The reading must therefore be corrected. Given a hose length of 3 m, however, the change in the opening time is so insignificant that the influence on the measuring result can be neglected in any case. 

However, my friend did come up with a possible calculation - 

Drager tubes are calibrated at an atmospheric pressure of 1013 hPa (i.e. 1013 mbar), to correct for the influence of pressure, the value read from the tube scale must be multiplied by following correction:

Correction Factor = 1013 hPa (14.692 psi) / actual atmospheric pressure in hPa

The atmospheric pressure can be calculated by following formula:
P = 101325 (1 - 2.25577 10-5 h)5.25588
Where P= air pressure (Pa) and h = altitude above/below sea level (m)

I do not agree with this correction methodology, since then it would affect all types of tubes, not only some of them. It has more to do with the flow rate. 

Should any one know more, please do let me know.