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Confined space entries need to be handled with caution. This includes testing the atmosphere within using your sampling accessories and pump equipment. You should always err on the side of safety when entering a confined space. Your life and the lives of others may depend on it.


The first step you should consider is the approach to the confined space. After you’ve checked and prepared your equipment, you should sample the atmosphere from a safe location. For example, you may want to stand upwind or off to the side of the entrance — you don’t want to be in the direct flow of any gas that could be escaping or evacuating from the space. Once you’ve safely approached the confined space and you’ve begun taking samples, you need to take things like stratification of gases into consideration.


Gases settle in a space based on their weight in relation to air. Toxic gases that you would commonly encounter in confined spaces have a typical stratification pattern.  For example, hydrogen sulfide (H2S) settles at the bottom— it’s a heavier-than-air gas. Carbone monoxide (CO), which is slightly lighter than air, tends to settle in the middle, while methane sits on top. However, these are not absolute rules. You also need to take into consideration things like air current or ventilation. You may find H2S at a different level due to these factors and when you see those readings on your gas detector, that doesn’t mean that your instrument is incorrect. It is likely detecting that gas at a different level. Remember, these are general guidelines, and for more specific information concerning confined space entry, consult your regulatory agencies for details.

You will sample each different level within the confined space using the four-foot rule—you should sample the levels or intervals approximately every four feet through the entire depth of your space. If your space happens to be smaller than 12 feet deep, sampling at the top, middle, and bottom will cover most of your areas.

Let’s take a look at the equipment that you might find in your confined space entry. These instructions cover the Ventis MX4, but the information also applies to the Ventis Pro5, the MX6 iBrid, and even the Ventis slide-on pump. For the sampling accessories, this guide uses the stainless-steel extendable probe, but different lengths and varieties of sampling tubing or even the polycarbonate probe are available.


When entering a confined space, filtration of your equipment is crucial. Filters MUST be installed in each of these pumped instruments, and that’s solely to protect the pump—the gas detector itself. You’ll also need filtration to protect the sampling path installed at the end of your extendable probe or polycarbonate probe. The end of your sampling lines can be protected using replaceable disc-style filters.

Let’s walk through an example. A worker is in the field and drops a line into a confined space vessel. The line has a filter on the end of it, and that filter pulls in a contaminant—so what does the worker do next? Removes the filter and continues sampling. Then, the sampling line becomes contaminated and it leads back to the filter in the equipment. What do they finally do? Remove the filter from the pump and it finally contaminates the inside of the pump. Filters are cheap, but pumps are expensive. Always use filtration when you are sampling. The importance of this cannot be stressed enough.


Before inspecting and testing your sampling accessories and pump, you must conduct the four steps of safe operation using a DSX Docking Station or similar. You can also perform the steps using your manual calibration and bump testing equipment. Let’s quickly review those four steps.

With a docking station:

  1. Dock the gas detector to charge it, bump test it, and calibrate it if needed. After it’s finished, the light on the dock will turn green.
  2. Turn on the instrument and check the battery.
  3. Conduct a zero in a clean and safe atmosphere.
  4. Clear the peaks.

Without a docking station:

  1. Power on the instrument and check the battery.
  2. Zero the gas detector in a clean and safe atmosphere.
  3. Conduct your bump test.
  4. Clear the peaks.

Once the four steps of operation are complete and your gas detector is ready for use, you can move on to the inspection and the test.

The first thing to do is check your gas detector and make sure that your filters are in good condition. Unscrew the cap to verify that the filter is in clean. The only time you need to replace the filter is when it’s either contaminated or damaged.  Next, inspect the probe you plan to use for your remote sampling.

Verify there are no breaks in your sampling path. With an extendable probe with a filter installed, extend the probe, cover the ends with your fingers, and collapse it to check that each of the seals are intact. When you collapse the probe and release one of your fingers, you should hear air escape. If you do, your probe is good.

In the next step, include the coiled tubing. The probe will attach to the tubing and the pump. You need to ensure that the tubing is also complete and in good condition. Connect the tubing to the end of the probe and conduct the same test you just completed. This time pinching the end of the tubing and collapsing the probe. If you hear air escaping when you release your finger from the end of the probe, your sampling path is intact.

In the final test, you need to ensure the pump goes into an overrun condition. This verifies the presence of blockage in your sampling path during normal operation.  The gas detector will notify you by shutting the pump off and alarming. To test this functionality, attach the pump to the end of your tubing, and when you finally block it, the pump should indicate a stoppage with visual, audible, and vibrating alarms. The instrument will pause for a few seconds, wait to see if the blockage has cleared and then restart. If the alarm condition clears, it your gas detector will return to normal operation and you can begin your confined space entry.  If the alarm returns, investigate the blockage and do NOT use your sampling equipment and gas detector until you can clear the alarm and verify proper operation.


The final step is knowing and applying the “two by two rule” for confined space sampling. The two-by-two rule states that for every foot of tubing, you allow two seconds for the gas to pass through every foot of length of tubing, plus two minutes at each location. For example, if you have 10 feet of tubing, you need to wait approximately 20 seconds for the gas to pass through the sampling tube to get to the instrument. At that point, the sensors will begin to respond and shortly thereafter you will begin to see those gas readings. Once they appear, you need give the detector two additional minutes at that location to get the full concentration of gas according to the two-by-two rule. This results in a sample time of approximately 2:20. Repeat this process every four feet within the confined space to ensure the atmosphere is clear of gas hazards and safe to enter. Additional checks should be conducted in the same manner as you move throughout the space entering different areas if it is a larger vessel with multiple areas.  When it comes to confined space entry and stratification of gasses, always err on the side of safety.

For additional information regarding confined space entry, download our guide for detecting gas hazards in confined spaces.