Before entering a vessel, the atmosphere is to be checked for:
- oxygen deficiency,
- flammable gases,
- hydrogen sulfide,
- carbon monoxide
.
The available multi-gas detector has suitable sensors.
Can the device simply be switched on at the vessel opening and held there for a few seconds?
In many applications, this is not sufficient.
The atmosphere inside a vessel can differ significantly from the atmosphere immediately at the opening.
For measurement from a safe distance, a:
gas detector with pump + sampling hose
is therefore often used.
For continuous personal monitoring during work, the same or another device is often worn in:
diffusion mode
in the breathing zone.
The two operating modes therefore perform different tasks.
A common mistake is to distinguish only between:
device with pump
and:
device without pump
.
For reliable measurement, the following must also be taken into account:
- measurement task,
- measurement location,
- type of gas,
- hose length,
- hose material,
- pump performance,
- sample transport time,
- sensor response time,
- filters and water traps,
- possible adsorption in the hose.
The decisive question is therefore not simply: “Do I need a pump?”, but rather: “Does the atmosphere need to be monitored directly where the person is located, or must a sample first be transported to the sensor from a remote or not yet safely accessible area?”
How Does a Gas Detector Work in Diffusion Mode?
In diffusion mode, the ambient air is not actively drawn in.
The gas molecules reach the sensor openings of the gas detector through:
- natural molecular movement,
- airflow,
- movement of the person wearing the device
.
The device therefore basically measures the atmosphere:
directly at the location of the device
.
This is precisely why this operating mode is particularly suitable for personal gas monitoring.
The gas detector is worn, for example:
- on work clothing,
- in the chest area,
- on a suitable carrying harness
within the breathing zone.
If the atmosphere changes where the person is located, that atmosphere reaches the sensors directly.
There is no long sampling hose whose internal volume first has to be exchanged.
Typical applications for diffusion mode:
- personal gas monitoring,
- continuous monitoring during maintenance work,
- work in plant areas where gas release is possible,
- work after a completed pre-entry test,
- portable monitoring of the breathing zone.
How Does Pump Mode Work?
In pump mode, the sample is actively drawn from a remote measuring point.
A typical measurement setup is:
sampling point → probe → sampling hose → filter/water trap → pump → sensors
.
This allows measurements to be taken from, for example, a:
- tank,
- shaft,
- silo,
- sewer,
- vessel,
- process chamber
without the person having to enter the area first.
This is particularly important for:
pre-entry testing
or:
atmospheric clearance testing before entry
.
The pump does not change the actual measuring principle of the sensor.
An electrochemical sensor, for example, remains an electrochemical sensor.
The pump merely performs the:
transport of the gas sample to the sensor
.
Pump or Diffusion in Direct Comparison
| Feature | Diffusion Mode | Pump Mode |
|---|---|---|
| Measurement location | directly at the device | remote sampling point possible |
| Sample transport | passive | active via pump |
| Sampling hose | normally not required | often required |
| Transport delay | no hose transport time | depends on hose and pump |
| Personal monitoring | very well suited | not automatically required |
| Pre-entry measurement | only directly accessible sampling point | targeted remote sampling possible |
| Measurement at several depths | difficult | easily possible with probe/hose |
| Influence of hose material | not applicable | can be significant |
| Risk of blockage | low | hose, filter or probe may become blocked |
This makes one point clear:
Pump and diffusion modes are not two competing solutions for the same task. In many applications, both are required one after the other.
Why Diffusion Is Suitable for Personal Monitoring
A personal gas detector should monitor the atmosphere that the person is actually breathing as closely as possible.
For this reason, the device is normally worn in the:
breathing zone
.
If a long sampling hose were used instead, the device would not measure the current atmosphere directly at the person’s body.
It would measure the atmosphere at the remote end of the hose.
The measured value would also be delayed.
For personal warning applications, a correctly positioned device operating in diffusion mode is therefore often the more appropriate solution.
It remains important that:
- the sensor inlets are not covered by clothing,
- the device is worn in accordance with the manufacturer’s instructions,
- audible, visual and vibration alarms remain noticeable.
Why a Pump Is Often Required for Pre-Entry Testing
Before entering a vessel, it is not yet known whether its atmosphere is safe.
The person should therefore not first place their head or upper body into the area in order to measure there with a diffusion device.
A pump makes it possible to draw the sample from a safe distance.
For this purpose, a sampling hose of:
several meters in length
can be used, for example.
The probe is placed at the intended sampling point.
The gas detector remains outside the hazardous area.
What matters is:
The sample must actually have reached the sensor before the measured value is assessed.
This becomes increasingly important as the sampling hose becomes longer.
Why Measurements Must Be Taken at Several Locations
The gas atmosphere inside a vessel does not necessarily have to be homogeneous.
Depending on:
- type of gas,
- temperature,
- air movement,
- vessel geometry,
- internal structures,
- source of gas release
very different local concentrations may occur.
It may therefore be necessary to measure, for example:
- at the top,
- in the middle,
- at the bottom,
- in depressions,
- behind internal structures
.
The specific measurement strategy depends on the risk assessment, the gases involved and the operational situation.
A single measurement directly at the manhole opening therefore does not automatically prove that the entire atmosphere inside the vessel is identical.
How Does Hose Length Affect the Measurement?
A sampling hose has an internal volume.
Before the atmosphere from a new sampling point reaches the sensor, the gas previously contained in the hose must first be displaced.
The longer the hose, the longer this process takes.
In simplified terms, the total waiting time consists of:
sample transport + sensor response + stabilization if required
.
It is therefore incorrect to immediately interpret the currently displayed value as the concentration at the new sampling point after positioning a long sampling hose.
The displayed value may still belong to the previous sample.
The following can additionally increase or influence the time:
- low pump flow rate,
- large hose inside diameter,
- filters,
- water traps,
- long probes,
- leaking connections,
- partial blockages,
- adsorption of the target gas.
Distinguishing Transport Time from Sensor Response Time
Two different times are often confused.
1. Transport time
This describes how long the sample takes to travel from the:
end of the hose
to:
the sensor
.
2. Sensor response time
Once the gas has reached the sensor, the sensor itself also requires time until the indicated value approaches the actual concentration.
A commonly used characteristic value is:
T90
.
The total waiting time must therefore not be determined solely by the pump performance.
Even a very powerful pump cannot make the actual gas sensor respond instantaneously.
Example: 20 m Sampling Hose
A multi-gas detector is used for a pre-entry test.
The sampling hose is:
20 m
long.
For the system used, the manufacturer specifies, for example:
at least 3 seconds per meter of hose
plus the normal sensor response time.
For the hose alone, at least:
20 m × 3 s/m = 60 s
must therefore be taken into account.
If the relevant sensor has a T90 of, for example:
30...40 s
the resulting order of magnitude is:
at least approximately 90...100 s
before a stabilized measured value can be assessed.
For certain gases, hose materials or process conditions, additional time may be required.
A 20 m hose and a waiting time of ten seconds therefore do not match.
What Role Does Sample Volume Play?
The internal volume of the complete sampling system is made up, for example, of:
- sampling hose,
- probe,
- filter housing,
- water trap,
- internal gas path inside the device.
In simplified terms, the theoretical exchange time can be estimated from:
volume / volumetric flow rate
.
However, this calculation is only an initial guide.
In practice, the following may also be relevant:
- flow profile,
- dead volumes,
- adsorption,
- desorption,
- sensor response.
For safety-related measurements, the waiting times and sampling requirements specified by the device manufacturer are therefore decisive.
Why Reactive Gases Can Be Lost in the Hose
Not every gas can be transported through every hose without loss.
Certain gases and vapors can be:
adsorbed
on the inner surface of the hose.
This means:
Some of the molecules initially adhere to the hose surface and reach the sensor later or at a lower concentration.
This can be particularly relevant, for example, with:
- ammonia,
- chlorine,
- hydrogen sulfide,
- ozone,
- hydrogen chloride,
- nitrogen oxides,
- certain VOCs
.
An unsuitable long hose can therefore cause:
an excessively low or delayed indication
.
This is particularly critical for safety-related measurements.
Selecting the Correct Hose Material
The hose must therefore not be selected solely according to:
length and connection diameter
.
The following must also be checked:
- target gas,
- hose material,
- chemical resistance,
- adsorption behavior,
- temperature,
- required length.
For gases that are critical in terms of adsorption or reactivity, manufacturers sometimes offer special:
low-adsorption or reactive-gas hoses
.
Nevertheless, the hose should only be as long as actually required for the measurement task.
A 30 m hose is not automatically better than a 10 m hose simply because the pump can technically handle 30 m.
Using Filters and Water Traps Correctly
During remote sampling, the hose may come into contact with:
- dust,
- particles,
- condensate,
- splash water
.
These substances can:
- damage sensors,
- block the pump,
- reduce the flow rate.
Depending on the device, the following may therefore be used:
- filters,
- water traps,
- float probes.
However, the filter itself must also be inspected.
A:
dirty or wet filter
can reduce the sample flow.
The presence of a pump therefore does not automatically mean that the intended flow rate is actually passing through the complete sampling system.
What Happens If the Hose Is Blocked?
A blocked sampling line can be caused by:
- a kinked hose,
- a contaminated filter,
- drawn-in water,
- a blocked probe,
- a crushed line.
In that case, either:
too little sample
or:
no sample at all
reaches the sensor.
On devices with pump monitoring, this may trigger a corresponding fault indication.
Before the actual measurement, the function of the:
pump + hose + probe
should therefore also be checked as a complete system.
A successful device test without the sampling hose connected does not automatically confirm that the complete sampling path is functioning correctly.
Sufficiently Purging the Hose Between Different Sampling Points
When changing the sampling point, the previous sample initially remains inside the hose.
For example, the measurement is first taken:
at the top of the vessel
and then:
at the bottom of the vessel
.
After moving the probe, the gas detector initially still indicates the atmosphere from the previous measuring area.
The hose must first be purged with the new sample.
Only then can the concentration at the new sampling point be assessed reliably.
This also applies when changing from a contaminated atmosphere back to clean air.
With adsorbing gases, the previous sample can even be released again from the hose surface for a longer period.
What Does T90 Mean for a Gas Sensor?
In simplified terms, T90 describes the time after which a sensor, following a sudden concentration change, has reached:
90% of the final indicated value
.
For example:
The actual test gas concentration is:
100 ppm
.
A T90 of:
30 s
means, in simplified terms, that under the specified conditions the sensor should reach at least approximately:
90 ppm
after around 30 seconds.
T90 is therefore not the same as:
time until the first visible response
.
A sensor may already begin to react after only a few seconds even though the measured value is still far from the stabilized final value.
Why Different Sensors Respond at Different Speeds
A multi-gas detector can contain several completely different sensor principles.
For example:
- electrochemical sensors,
- catalytic sensors,
- infrared sensors,
- PID sensors.
These sensors have different:
- response times,
- measuring ranges,
- cross-sensitivities,
- environmental influences.
With a multi-gas detector, it must therefore not automatically be assumed that all displayed gases have reached their stabilized final values at the same time.
For the waiting time, the:
slowest sensor relevant to the safety assessment
may be decisive.
Switch Back to Diffusion After Pre-Entry Testing?
A pre-entry test assesses the atmosphere at a specific point in time.
However, the atmosphere can change during the subsequent work.
For example, due to:
- welding work,
- cleaning agents,
- process leaks,
- renewed gas release from residues,
- changes in ventilation,
- oxygen consumption.
Therefore, after atmospheric clearance testing:
continuous personal monitoring
may be required.
With a device that supports both operating modes, diffusion mode can then be used again in accordance with the manufacturer’s instructions and the applicable procedure.
The pump therefore performs the task of:
drawing the sample before or from the space
.
Diffusion mode then performs the task of:
monitoring the atmosphere directly at the wearer
.
Practical Example: Pre-Entry Test on a 6 m Deep Vessel
A maintenance team needs to enter a process vessel.
The vessel is:
6 m deep
.
Before entry, the atmosphere is to be checked using a multi-gas detector.
Step 1: Prepare the measuring instrument
Before use, the following are checked, among other things, in accordance with operational and manufacturer-specific requirements:
- battery charge level,
- calibration status,
- sensor function,
- alarm function,
- pump.
Step 2: Connect the sampling hose
A hose suitable for the gases expected is connected together with a probe and the required filter or water trap.
Step 3: Measure the upper area of the vessel
The probe is first inserted into the upper area.
The required transport and response time is allowed to elapse.
Only then are the measured values documented.
Step 4: Measure the middle area
The probe is lowered further.
It must again be taken into account that the previous sample is initially still present inside the hose.
The required purging and measurement time is observed again.
Step 5: Measure the lower area
The sample is taken from the area close to the bottom.
Here too, the measurement is only assessed after a sufficient waiting period.
Step 6: Check other critical areas
Depending on the vessel, the following may also be relevant:
- depressions,
- dead spaces,
- areas behind internal structures.
Step 7: Assess and document the results
The measured values are assessed and documented in accordance with the atmospheric clearance procedure applicable to the plant.
Step 8: Monitoring during entry
If continuous personal gas monitoring is required according to the risk assessment, the suitable gas detector is used in the specified wearing position during the work.
This clearly separates two different tasks: pump mode for safe sampling before entry and personal monitoring of the actual breathing zone during the work.
Systematic Selection and Procedure
- Check the risk assessment and possible target gases.
- Ensure that suitable sensor principles are available for all relevant hazards.
- Determine whether measurement is to take place directly at the device or at a remote sampling point.
- For remote sampling, select a suitable device with a pump or suitable sampling equipment.
- Check the hose material for suitability for the target gases.
- Select the sampling hose only as long as actually required.
- Use a suitable filter and water trap when moisture or particles are expected.
- Check the hose for kinks, damage and contamination.
- Check pump operation with the complete sampling system connected.
- Take the transport time into account according to the manufacturer’s specifications.
- Also take T90 or sensor response time into account.
- For reactive gases, consider possible adsorption losses.
- Take samples at representative locations.
- After each change of sampling point, allow sufficient time for purging and measurement.
- Assess measured values only after sufficient stabilization.
- Document sampling points and results.
- After atmospheric clearance testing, assess whether continuous monitoring is required.
- For personal monitoring, wear the device in the breathing zone in accordance with the manufacturer’s instructions.
- Do not cover the sensor openings with clothing or personal protective equipment.
- Regularly review and document measurement and maintenance procedures.
Common Mistakes
- Holding a diffusion detector only at the vessel opening: The atmosphere deeper inside the vessel may be completely different.
- Putting your head into the vessel to measure: This already exposes the breathing zone to the unknown atmosphere.
- Reading the value immediately after connecting a long hose: The new sample may not yet have reached the sensor.
- Considering only the hose transport time: The sensor still requires its own response time afterwards.
- Considering only T90: With remote sampling, the transport time through the hose and accessories must also be added.
- Always using the maximum possible hose length: Unnecessary length increases delay and possible adsorption.
- Using any plastic hose: Certain gases can adsorb strongly on unsuitable materials.
- Ignoring reactive gases: An excessively low indication can result from losses in the hose.
- Never checking the filter: Dirty or wet filters can reduce the sample flow.
- Drawing water into the sampling line: This can impair pumps and sensors.
- Measuring at only one point: Vessel atmospheres may be stratified or vary locally.
- Not waiting again after changing the sampling point: The hose initially still contains the previous sample.
- Equating pump mode with a faster sensor: The pump does not shorten the intrinsic response time of the sensor.
- Equating a pre-entry test with permanent safety: The atmosphere can change after clearance has been given.
- Assuming that a pump-equipped device is automatically better throughout the entire job: Diffusion mode may be more suitable for personal breathing-zone monitoring.
- Performing the bump test only on the device: For pump measurements, the complete sampling path must also be functional.
- Selecting the correct device but expecting the wrong gas measurement: The sensor principle and target gas must match the actual hazard.
Suitable Multi-Gas Detector
For applications requiring both personal gas monitoring and remote sampling, the Crowcon Gas-Pro is one suitable option.
Depending on the configuration, the portable multi-gas detector can monitor up to:
5 gases
.
The optional integrated pump is particularly relevant to the topic discussed here.
It allows the Gas-Pro to be used for:
- diffusion monitoring,
- pump measurement,
- pre-entry testing,
- remote sampling with hose and probe.
In diffusion mode, the device can be worn in the specified position for personal monitoring.
For remote sampling, pump mode is activated using the appropriate flow plate or sampling adapter.
For pump operation, the following are particularly important:
- hose length,
- transport time,
- sensor T90,
- hose material,
- filter condition,
- target gas.
Suitable special sampling hoses are available for certain reactive gases or gases that are critical in terms of adsorption.
Additional accessories include:
- sampling hoses,
- probes,
- water traps,
- filters,
- manual aspirators.
Further portable and fixed gas detectors can be found under Gas Detectors and Gas Warning Devices at ICS Schneider.
Further information on use before entering vessels can be found in the article Gas Detector for Confined Spaces: How to Carry Out a Pre-Entry Test Correctly Before Entry.
The article Bump Test Passed, Calibration Failed: Correctly Interpreting Response Time and Sensor Deviation also explains why a successful function test does not automatically confirm measurement accuracy.
Conclusion
Diffusion and pump operation perform different tasks in portable gas detectors.
In diffusion mode, the device measures the atmosphere directly at its own location.
This is particularly suitable for:
continuous personal monitoring in the breathing zone
.
In pump mode, an atmosphere from a remote sampling point is transported to the sensor.
This is particularly relevant for:
pre-entry testing and remote sampling
.
However, the pump does not eliminate the time limitations of the measurement.
The sensor response time must still be added to the transport time through the sampling hose.
As hose length increases, the following therefore become increasingly important:
- pre-pumping time,
- purging time,
- adsorption,
- filter condition.
Particularly with reactive gases, it must not be assumed that any hose will transport the actual concentration to the sensor without loss.
When testing the atmosphere inside vessels, it must also be taken into account that the atmosphere can vary spatially. A single measurement at the opening is therefore not automatically sufficient.
After a successful pre-entry test, continuous personal gas monitoring may be required during the work.
The correct choice is therefore often not “pump or diffusion?”, but rather: “Pump for targeted sampling before access – diffusion for direct personal monitoring during the stay.”
FAQ: Gas Detector with Pump or Diffusion
What is the difference between a diffusion gas detector and a pumped gas detector?
In diffusion mode, ambient air reaches the sensors directly and passively. In pump mode, the sample is actively drawn to the device through a probe or sampling hose. This also allows measurements to be taken from remote or not yet accessible areas.
When do I need a gas detector with a pump?
A pump is particularly useful when a sample must be taken from a remote area, for example from a tank, shaft or vessel before entry. The specific measurement strategy depends on the risk assessment and operational requirements.
When is diffusion mode better?
For continuous personal monitoring, a gas detector is often worn in diffusion mode in the breathing zone. The device then directly monitors the atmosphere at the person’s location.
Can I use a diffusion device to test a vessel before entry?
Only if the required representative sampling points can be reached safely and directly. For deeper or not yet safely accessible areas, remote sampling with a pump and hose is often required in practice.
Why do I have to wait when using a sampling hose?
After moving the probe, the previous atmosphere initially remains inside the hose. It must first be displaced by the new sample. The sensor itself then also requires time to respond to the new concentration.
How long do I have to wait when using a sampling hose?
This depends on the device, pump, hose length, hose material, target gas and sensor. The manufacturer’s specification is decisive. For the Crowcon Gas-Pro, for example, at least 3 seconds per meter of hose plus the normal T90 sensor response time is specified.
How long does the measurement take with a 20 m hose?
With a requirement of at least 3 seconds per meter, the transport time alone is at least 60 seconds. If, for example, the sensor has a T90 of 30 to 40 seconds, the total time is in the order of at least approximately 90 to 100 seconds. Additional delays may occur with certain gases.
Does a more powerful pump make the sensor faster?
No. A pump can transport the sample to the sensor more quickly. However, the actual response time of the sensor remains unchanged.
What does T90 mean?
In simplified terms, T90 is the time a sensor requires to reach 90% of the final measured value after a change in concentration. The first visible response from the sensor may occur considerably earlier.
Can every sampling hose be used for every gas?
No. Certain reactive gases or gases that are critical in terms of adsorption can be partially retained by unsuitable hose materials. For these gases, hose materials specified by the device manufacturer should be used.
Why can H2S or NH3 be indicated too low with a long hose?
Certain gases can be adsorbed on the hose surface. As a result, part of the gas reaches the sensor later or initially at a lower concentration. Hose material and length are therefore particularly important for such gases.
What is a water trap used for?
It reduces the risk of liquid entering the pump or sensors through the sampling line. Depending on the measurement task, additional filters or suitable sampling probes may also be required.
Do I have to wait again every time I change the sampling point?
Yes. The atmosphere from the previous sampling point initially remains inside the hose. The hose must be purged with the new sample before the measured value at the new point can be assessed reliably.
Why should measurements be taken at several heights inside a vessel?
The atmosphere does not have to be homogeneous. Depending on substance properties, temperature, airflow and vessel geometry, gases can be distributed differently in different areas. The required representative sampling points are therefore defined on the basis of the risk assessment.
Is a pre-entry test sufficient for the entire time spent inside the vessel?
Not necessarily. Atmospheric clearance testing describes the atmosphere at the time of measurement. Conditions can change during the work. Depending on the risk assessment, continuous gas monitoring may therefore be required.
Can the Crowcon Gas-Pro be used both with a pump and in diffusion mode?
Yes, when configured with the optional integrated pump. For remote sampling, the pump is activated using the designated flow plate or sampling adapter. For personal monitoring, the device can be worn in diffusion mode.
Who is permitted to carry out atmospheric testing of a vessel?
For atmospheric testing in accordance with DGUV Rule 113-004, appropriate technical competence is required. This includes, among other things, knowledge of the measuring instruments used, the relevant hazardous substances, the measurement strategy and the operating conditions.