Comparison between PID detection of PID gas detector and other detection methods
In other words, the LEL sensor detects explosiveness rather than toxicity.
(1)LEL sensor detected explosiveness, not toxicity.
The LEL sensor measures the percentage of the lower explosion limit. For example, if the lower explosive limit of gasoline is 1.4%, then 100% LEL is 14000 ppm gasoline. 10% LEL is 1, 400 ppm gasoline, 1% LEL is 140 ppm gasoline. 140 ppm is the minimum amount of gasoline vapor that can be detected by LEL sensor. The TWA value (time-weighted average) of gasoline is 300 ppm, and its STEL (short-term exposure level) is 500 ppm. These, together with the low resolution of LEL sensor, indicate that LEL is not suitable for detecting gasoline leakage. LEL sensors measure explosiveness rather than toxicity. In fact, many VOCs (organic compounds) are very toxic, even though their concentrations are far lower than the sensitivity of LEL sensors.
(2)LEL sensor is specially used to measure methane gas.
Initially, LEL sensor was specially used to solve the problem of measuring methane concentration in coal mines. Most LEL sensors use the principle of Wheatstone bridge to measure the heat generated by the combustion of combustible gas on the catalytic electrode. At this time, the temperature rise causes the change of resistance, which is measured by the instrument and converted into% LEL.
(3) Limitations of 3)LEL sensor
Two basic principles affect the performance of LEL sensor and its effective measurement of non-methane gas:
The heat output of gas is different during combustion: heavier hydrocarbon vapor is more difficult to diffuse to LEL sensor, so its heat output is lower.
Some gases burn to produce more heat, while others may be relatively less. These differences in physical properties lead to inconvenience when using LEL sensors. For example, 100%LEL methane (5% methane by volume) generates twice as much heat as 100% LEL propane (2.0% methane by volume).
Some "heavier" hydrocarbons may be difficult to diffuse through the fireproof metal mesh of LEL sensor. On the LEL sensor, the net is used to prevent the sensor itself from backfiring and igniting the environment, and to allow methane, propane and ethane to reach the electrode surface of the sensor Wheatstone bridge. However, the diffusion speed of gasoline, kerosene, solvents, etc. It is slower to pass through this network, so the quantity of the bridge is less, that is, the output is lower.
(4) The sensitivity of Wheatstone bridge LEL sensor is expressed by methane.
According to the table below, the heat generated by gasoline on the Wheatstone bridge is about half that of methane. Therefore, the signal it produces is also half that of methane. If LEL calibrated by methane is used to detect gasoline vapor, the reading displayed by the instrument is half of the actual concentration. Taking methane calibration as an example, if LEL shows a gasoline mixture of 50% LEL in the air, actually due to half the output, LEL is about 100% gas LEL(%vol) sensitivity (%) acetone 2.2 45 diesel oil 0.8 30 MEK 1.8 38 toluene 1.2 40 benzene 1.2 40 methane 5.0 100. After the instrument is calibrated with methane, the calibration coefficient can also be used to correct the gas to be measured, that is, the instrument can get the correct reading by software. However, even with proper correction coefficient, LEL sensor can't measure the toxicity of VOC because it lacks enough sensitivity to measure PPM.
PPM Measurement —— A New Contribution of Gas Sensors
At present, there are several methods to measure VOC at PPM level:
Colorimeter: lack of accuracy, and other shortcomings.
Metal oxide sensor: lack of accuracy and sensitivity.
Portable gas chromatography/mass spectrometry: good selectivity and precision, but it can not be determined continuously and is expensive.
FID (Flame Ionization Detector): Its limitation lies in its large volume and weight, and it needs bottled hydrogen.
PID: the most suitable. PID is the best choice in many emergency situations, which can provide reliable response.
Why not use a colorimetric tube?
In the past, colorimetric tube has always been the basic component of gas detection in emergency. They have been widely accepted and proved that many toxic and harmful gases can be measured at PPM level. Colorimetric tubes are not expensive, but they also have many limitations:
Colorimetric tube can only provide "point measurement", but can not provide quantitative analysis and continuous alarm detection. Only one detection tube can't warn the operator of a dangerous situation. The essence of "point measurement" is that it is easier to produce measurement errors. Because they have a small sampling amount, and there are factors such as air flow at the scene. Only by adopting continuous monitoring of 100-500 cc/min can we not be blinded by temporary high or low readings.
The reaction of colorimetric tubes is very slow, and they may take several minutes instead of seconds to give the results.
The best measurement accuracy of colorimetric tube is about 25%.
The reading of the colorimetric tube tends to be intermittent sampling.
Waste colorimetric tubes are easily polluted by glass and chemicals.
Users need to reserve a large number of colorimetric tubes for use, and the colorimetric tubes may be out of date.
Colorimetric tubes are limited to common compounds, and there are no special solutions for many specific compounds.
Why not use MOS sensor?
Semiconductor or MOS sensor is an early and cheap portable measuring instrument. It can also detect most chemicals. However, their limitations still limit their wide application in emergencies.
The sensitivity is very poor, and the general detection limit is about 10PPM.
The outputs are nonlinear, which will affect their accuracy. MOS is only a rough detector of various toxic gases and vapors.
Compared with PID, MOS has a slower response time.
MOS sensor is more sensitive to temperature and humidity.
Easy to poison, not easy to clean.
MOS sensor is a kind of "broadband" detector, which will react to various compounds.
Portable GC-MS
Gas chromatography/mass spectrometry (GC/MS) has high selectivity, but the measurement is discontinuous. It is also a kind of "point measurement" and cannot provide continuous alarm measurement. Because they have a small sampling amount, and there are factors such as air flow at the scene.
At the same time, there is no portable GC/MS instrument that staff can carry with them. At the same time, GC/MS is only an immediate rather than preventive means, and can only report what happened. Compared with continuous real-time images, chromatograms provide more "field test" photo results. Finally, GC/MS is more expensive in instrument price.
Flame ionization detector
Flame ionization detector (FID) is a broadband detector for organic compounds, which has no selectivity. They are very linear. The main limitation of FID in field detection is its large weight and volume, and it is necessary to configure hydrogen cylinders, so it is difficult to ensure the intrinsic safety of its instruments in dangerous environments. FID is relatively expensive and complicated to maintain, which also limits its application in the industrial field. PID and FID are both common organic compound detectors, which can effectively measure the same substance. But because PID is smaller, easier to use and safer, it is more widely used in industrial field than FID.
Photoionization detector
PID can be regarded as a gas chromatograph without separation column, so PID can provide excellent accuracy. Many people think that PID has good sensitivity and accuracy for many PPM toxic compounds, but it is of little use because of its lack of selectivity. In fact, most other methods, including colorimetric tubes, MOS sensors and FID detectors, are not very selective. The advantage of PID lies in its non-selectivity. It is a compact continuous measurement detector, which can provide real-time information feedback for workers. This kind of feedback can make the staff confirm that they are in a safe state without contact with dangerous chemicals, and better complete the task. Just like a video camera, PID is continuously measured, and its results can also be recorded (collecting data) or immediately "played back" (browsing data).
Why is PID not so common?
From 65438 to 0970, PID has been used in chemical pollution investigation from laboratory. But it is still very troublesome to use at this time, but PID can define the ability of pollutants without expensive and time-consuming laboratory tests, which makes PID an indispensable tool for many environmental cleaning industries. It is precisely because of its excellent detection ability that some emergency response teams also think that PID is very important to them. However, the disadvantages of PID at this time, such as high purchase and maintenance cost, poor tolerance, large volume and weight, sensitivity to humidity and radiation, limit the wider application of PID in emergency treatment.
PID has become the most advantageous tool for detecting organic compounds:
PID can measure 0- 1000 ppm organic matter with a resolution of 0. 1ppm, so it is the most suitable method to measure gasoline (and other toxic gases and vapors) that can cause cancer at very low concentration. PID provides the best protection for long-term poisoning. The breakthrough of PID technology overcomes the shortcomings of the original PID, thus providing the most powerful tool for emergency treatment.
The ability of PID to provide accurate measurement under various conditions can be found in the measurement of the following organic compounds.
Play an important role:
Preliminary personal protection decision
hunt a leak
Accident area confirmation
Leak confirmation
depollution
Initial personal protection confirmation
When approaching the place where an accident may occur, rescuers must first confirm the personal protective equipment. Some "possible" incidents may not be accidents without any personal protection; Some accidents have no signs of pollution at first, but they need special personal protection. No detector can provide all the answers for rescuers, but PID can provide a satisfactory solution for this. For many accidents, PID allows rescuers to determine whether there is toxic gas or steam around. A railway worker reported to the emergency rescue center that a tanker leaked in a hot and humid environment (35℃, 95%RH). According to the description, this tanker is loaded with liquid benzene. Due to the toxicity of benzene (personal exposure level is 1 ppm), rescuers decided to adopt Class A protection. However, due to the high temperature now, wearing such equipment will bring more harm to rescuers. Finally, after many efforts, it is confirmed that condensed water drops are dripping under the "leaking" tanker instead of leaking benzene. It turned out that the tank car was once stored in a warehouse at 20℃, and the liquid benzene inside was low in temperature, and the temperature outside was high and the humidity was high, which led to water condensation. In fact, using PID can help rescuers easily confirm whether there is "ionizable" steam. Because according to the records, the tanker is filled with benzene, which is very easy to "ionize". Rescuers can use PID to judge whether benzene vapor exists. This not only reduces the cost of determining the leakage, but also avoids the high heat injury caused by wearing class A protective clothing.
Leak detection using PID
Usually, the leakage is not easy to see, and the location of the leakage must be determined before it can be effectively prevented. In any case, any gas or vapor diffuses from its source and will be diluted by the surrounding air until the existence of the substance cannot be detected in some places. In this way, a concentration gradient is established, that is, when the gas diffuses completely, it will change from the source with the highest concentration to dilution to zero, that is, concentration.
As long as it can be detected, PID can be used to measure and "see" the concentration gradient of many gases and vapors. We use PID to "see" the concentration gradient just like Geiger counter, and find the source with the increase of concentration. The leak detection ability of PID can not only find the dangerous source quickly, but also save a lot of time and money.
Using PID to determine the danger range
When emergency personnel approach the scene of the accident, it is necessary to determine the danger range according to the toxicity, temperature and wind direction of gas or steam. However, the confirmation of the danger range is usually set manually by people without much experience. When the conditions change, it is impossible to adjust the danger range at any time because the peripheral personnel have no experience in identifying the conditions change. At this point, experienced emergency personnel are still concerned about the leak itself. In this way, the peripheral personnel may be in a dangerous state because of the change of conditions, because at this time, the scope of danger has required the peripheral personnel to retreat. For most accidents, using PID can change the definition of danger range at any time according to the change of the situation. PID can provide real-time alarm, so that peripheral personnel can leave the dangerous area at any time. The following picture is an explanation of an actual accident: in the early morning, due to the low temperature and weak wind, the leakage range of all overturned toxic liquid tankers was not very large. But at noon, due to the change of temperature and wind direction, this place, which was originally considered safe, is now in a very dangerous situation. And this change from time to time is easy to be detected by PID.
Data acquisition tools:
Using the data acquisition function of PID, emergency rescue personnel can obtain the record of exposure level and the basis for determining the cause of the accident. In the event of an accident, the staff can record it.
Piping instrument flow chart as leak confirmation
There may be various liquids at the scene of the accident, such as water, fuel, engine oil, fire fighting foam, etc. At this time, using PID can quickly judge the type of liquid and save a lot of time. PID can quickly reflect whether the leakage is dangerous or just water or other non-volatile substances.
Using PID to judge the pollution situation
The harm of dangerous substances to people is self-evident. After working at the scene of the accident, it is necessary to quickly confirm whether the workers are polluted by dangerous substances or whether the pollution has been completely eliminated. At the same time, the staff also need to quickly judge that those protective clothes are not polluted and can continue to be used. These problems can be solved quickly by PID. For polluted places, PID will immediately give a positive response, while for those clean or unpolluted places, there is no response. In fuel leakage accidents, firefighters often encounter a lot of gasoline contaminated by protective clothing, which is very dangerous for firefighters themselves. Using PID can quickly judge whether this danger exists.
Deal with the aftermath with PID
The ultimate goal of any emergency treatment is to control and eliminate leakage. Dangerous substances usually pollute the surrounding water and soil. Relevant units (communities, states and counties) should confirm the concentrations of these pollutants so as to decide whether to carry out further remediation work. If it is only oil leakage, which has been completely absorbed by the road surface, there is no need to deal with it again. However, if oil has been polluted and has polluted the surrounding soil and water, the situation will be different. Some authorities require that if TPH (total petroleum hydrocarbon) is higher than 100 ppm, further treatment is needed, while if it is lower than this value, no treatment is needed. At this time, PID has become the most effective tool for authorities and emergency personnel, so that they can quickly determine the soil and make decisions without losing better opportunities.