Detector Tube Questions and Answers
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I was using tube 147S for carbon tetrachloride not realizing that
it is a dual tube. I failed to connect the pre tube onto the analyzer tube. The
tube stained. Is this a positive response?
It is a positive response, but not for carbon tetrachloride. Tube 147S
uses a two-stage reaction in which carbon tetrachloride is converted to phosgene
in the front tube. The back tube actually responds to phosgene. In fact it is
identical in function to phosgene tube 146S. If you received a positive response
to tube 147S without the front tube in place, the tube either saw phosgene or
one of the interfering substances listed for tube 146S.
I need to sample a pressurized system. Is it okay to connect the
tube and pump directly to the system’s sample port using a short section of
flexible tubing?
No. This method will cause a loss of accuracy. The detector tubes are
calibrated at the factory using the flow rate curve provided by the pump as it
works against atmospheric pressure. In your scenario the pump is acting against
an elevated pressure, and the sample will be drawn at a flow rate faster than
that at which the tube was calibrated. Under those conditions, the system is
likely to read high by producing a pale stain that is too long. An easy remedy
is to first collect the sample in a gas sampling bag, then draw the detector
tube sample from the bag.
I notice that several of the new Sensidyne tubes do not have
printed scales, but require comparison to a calibration card. Is there any
advantage to that method?
Yes there is. The calibration card is actually an older technology that
pre dates printed tubes. There is an additional step in using them, but they do
hold an advantage. The printing process used on a flat card allows more lines to
be printed in a given distance than the process for printing on tubes. Where a
printed tube may have seven or eight calibration marks, a printed card for the
same tube may have ten to twelve lines. More reference lines provide improved
resolution. For example, it would be easier to interpret a 12ppm reading on a
tube with surrounding marks at 5, 10 and 20ppm than it would for a tube that
only had marks at 5 and 20.
I need to have a record of the tube readings. How long will the
stain last on a tube?
The stability of a stain on a given tube is a function of the stability of
the chemicals used in that particular tube. Some stains last for weeks with
little deterioration, while others fade or crawl within a few hours. Some
customers have tried freezing the tubes with some success, and others have used
photos or photocopies. We still believe that the best record is a nicely
detailed field notebook with complete data signed by two readers. Don’t forget
to record the catalog number, lot number and expiration date along with a
description of the sample and surrounding conditions.
If I obtain a zero response on a tube, can it be used
again?
In general, this is not a good practice, and we don’t recommend it. Many
tubes have a pre layer that removes moisture as the air sample passes through.
Once it becomes saturated, the analyzer layer can also become saturated with
moisture and stop working. This can happen if a tube is used for more pump
strokes than it was designed to handle, or it can result from moisture diffusing
into an opened tube. Many times detector tubes are used in worker exposure
sampling, and it is just not worth the risk. A non working tube will always
report zero response. It is much safer to use fresh tubes, and be confident that
the tube will respond to the target gas.
When I use the
Sensidyne water vapor tube, it sometimes produces two colors, purple and green.
Which one do I read?
The
chemistry used in our water vapor tubes produces a two-color stain as
follows. The original color of the analyzer layer is
yellow. When water vapor contacts the layer, it produces a green
stain. As the layer becomes saturated with moisture, it will go from green to
purple. For this reason, you can sometimes observe a stain that is purple
at the lower part of the scale and green at the upper part. The upper part
of the stain has not gotten wet enough to produce the purple color. However,
both colors represent a moisture stain, and the correct procedure
is to read both colors as one.
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