Ensure Accurate Humidity & Dew Point Even In Areas With High Concentrations Of Chemicals With Chemical Purge
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Ensure Accurate Humidity & Dew Point Even In Areas With High Concentrations Of Chemicals With Chemical Purge

By 03/08/2022No Comments

Dpstar have been the industry leader in the field of moisture and humidity measurement for over 30 years and is able to create solutions even for very demanding measurement conditions. Dpstar provides a wide range of transmitters, instruments and system solutions capable of measuring trace moisture, humidity and dew point in a vast range of applications and industries.

The purging process involves rapid heating of the sensor to remove possible contamination. Vaisala HUMICAP® humidity and DRYCAP® dew point sensors both feature good chemical tolerance. However, sometimes the chemicals present in the measured gas can gradually change their performance. For these demanding applications Vaisala has developed chemical purge function, which helps to maintain measurement performance under chemical exposure. Reliable measuring and monitoring of the ambient conditions are a prerequisite for successful operations in any environment. Vaisala measurement devices provide stable real time measurements that extend equipment lifetime, and improve productivity, energy efficiency, and end-product quality, as well as help you fulfill regulatory compliance.

What is Chemical Purge?

Chemical Purge Minimizes Effects of Contaminant

In environments with high concentrations of chemicals and cleaning agents, the chemical purge option helps to maintain measurement accuracy between calibration intervals. The chemical purge involves heating the sensor to remove harmful chemicals. The function can be initiated manually or programmed to occur at set intervals. Chemical purges use hydrocarbon-based solvents, cleaning chemicals, and sterilizing agents to enhance the purge as well as to clean hard-to-reach/negative flow areas. Chemical compounds call for a chemical reaction to take place inside the machine, reducing the viscosity of the material in the barrel and making it easier to move out. A chemical purging compound creates a reaction within the contamination, breaking the molecular bonds that hold it to the metal surface. Once the bonds are broken, the contamination can be easily removed from the system.

Chemical Purge Minimizes Effects of Contaminant

In environments with high concentrations of chemicals and cleaning agents, the chemical purge option helps to maintain measurement accuracy between calibration intervals. The chemical purge involves heating the sensor to remove harmful chemicals. The function can be initiated manually or programmed to occur at set intervals. Chemical purges use hydrocarbon-based solvents, cleaning chemicals, and sterilizing agents to enhance the purge as well as to clean hard-to-reach/negative flow areas. Chemical compounds call for a chemical reaction to take place inside the machine, reducing the viscosity of the material in the barrel and making it easier to move out. A chemical purging compound creates a reaction within the contamination, breaking the molecular bonds that hold it to the metal surface. Once the bonds are broken, the contamination can be easily removed from the system.

What Is The Purpose Of Purging?

What Is The Purpose Of Purging?

Solving Quality Issue

Effectively reducing the amount of waste generated. This improves your factory productivity and efficiency.

Remove Contamination

Cleaning the machine properly after a material/color change or long shutdown will prevent product rejections by your quality team or awkward orders returned by your customer.

Production Planning

Decrease cost by reducing downtime and scrap during color and material changes. When evaluating this solution, all factors must be considered: material, machine, labor, scrap, customer satisfaction, lost production, and purging compound price.

Solving Quality Issue

Effectively reducing the amount of waste generated. This improves your factory productivity and efficiency.

Remove Contamination

Cleaning the machine properly after a material/color change or long shutdown will prevent product rejections by your quality team or awkward orders returned by your customer.

Production Planning

Decrease cost by reducing downtime and scrap during color and material changes. When evaluating this solution, all factors must be considered: material, machine, labor, scrap, customer satisfaction, lost production, and purging compound price.

How It Works?

HUMICAP and DRYCAP sensors are thin-film polymer sensors consisting of a substrate on which a thin polymer film is deposited between two electrodes. The polymer film absorbs or releases water vapor according to humidity changes in the environment. As humidity changes, the dielectric properties of the polymer film change, and so does the capacitance of the sensor then convert into a humidity reading.  Typically, small molecules such as hydrocarbons, slowly penetrate into the sensor polymer. Absorbed chemicals lower the capability of the polymer to absorb water molecules and thus reduce the sensitivity of the sensor. The change in the measurement performance is usually interpreted as drift as sensor gain decreases. The changes can be verified by measuring the response of the sensor in constant known humidities. Figure 1 shows sensor drift due to chemical exposure, measured in 0% and 75%RH.

Figure 1. Sensor performance is influenced by absorbed chemicals.

During a chemical purge, the sensor is rapidly heated to 160-180°C by forcing a current through the temperature element of the sensor. Heating results in rapid evaporation of the chemical contaminants that have been absorbed into the polymer. The Chemical purge thus cleans the sensor internally, improving its stability and accuracy, see Figure 2. The chemical purge cycle lasts about 6 minutes and includes heating and a settling stage. When sensor temperature has been re-stabilized to the condition prior to the purge, the sensor continues in its normal measurement mode. The output value of the transmitter is locked during the chemical purge. Figure 3 shows an example of the chemical purge cycle in conditions where the sensor is under chemical exposure of ethyl acetate (concentration 700 ppm). Two purge intervals (720 and 120 min) are applied. The deteriorating sensor performance is restored by the chemical purge. In this case, the shorter purge interval (120 min) better maintains the measurement performance.

How It Works?

HUMICAP and DRYCAP sensors are thin-film polymer sensors consisting of a substrate on which a thin polymer film is deposited between two electrodes. The polymer film absorbs or releases water vapor according to humidity changes in the environment. As humidity changes, the dielectric properties of the polymer film change, and so does the capacitance of the sensor then convert into a humidity reading.  Typically, small molecules such as hydrocarbons, slowly penetrate into the sensor polymer. Absorbed chemicals lower the capability of the polymer to absorb water molecules and thus reduce the sensitivity of the sensor. The change in the measurement performance is usually interpreted as drift as sensor gain decreases. The changes can be verified by measuring the response of the sensor in constant known humidities. Figure 1 shows sensor drift due to chemical exposure, measured in 0% and 75%RH.

Figure 1. Sensor performance is influenced by absorbed chemicals.

During a chemical purge, the sensor is rapidly heated to 160-180°C by forcing a current through the temperature element of the sensor. Heating results in rapid evaporation of the chemical contaminants that have been absorbed into the polymer. The Chemical purge thus cleans the sensor internally, improving its stability and accuracy, see Figure 2. The chemical purge cycle lasts about 6 minutes and includes heating and a settling stage. When sensor temperature has been re-stabilized to the condition prior to the purge, the sensor continues in its normal measurement mode. The output value of the transmitter is locked during the chemical purge. Figure 3 shows an example of the chemical purge cycle in conditions where the sensor is under chemical exposure of ethyl acetate (concentration 700 ppm). Two purge intervals (720 and 120 min) are applied. The deteriorating sensor performance is restored by the chemical purge. In this case, the shorter purge interval (120 min) better maintains the measurement performance.

Figure 2. Sensor performance is restored by chemical purge

Figure 3. Chemical purge cleans the sensor and restores the humidity reading to its real value. Conditions: Ethyl acetate exposure (700 ppm), purge intervals 720 and 120 min.

Example Of Applications That Benefit from Chemical Purge

Example Of Applications That Benefit from Chemical Purge

Lithium Battery Manufacturing

Lithium batteries are manufactured in dry conditions as the raw materials have a low tolerance for moisture. The solvents (e.g. dimethyl carbonate, ethylene carbonate, or ethyl methyl carbonate) evaporating from the electrolyte solutions can potentially contaminate the dew point sensor. Typical process dew point is around -50…-40°C. To ensures trouble-free operation of the sensor, the chemical purge interval needs to be optimized to a certain level. Typical purge interval in the application is 1 hour.

Lithium Battery Manufacturing

Lithium batteries are manufactured in dry conditions as the raw materials have a low tolerance for moisture. The solvents (e.g. dimethyl carbonate, ethylene carbonate, or ethyl methyl carbonate) evaporating from the electrolyte solutions can potentially contaminate the dew point sensor. Typical process dew point is around -50…-40°C. To ensures trouble-free operation of the sensor, the chemical purge interval needs to be optimized to a certain level. Typical purge interval in the application is 1 hour.

Wood Drying

Wood drying is a demanding application where the sensor is exposed to a variety of chemical compounds that evaporate from wood during the drying process. Volatile compounds include alcohols, aldehydes, esters, ketones, terpenes, phenols, aliphatic, organic acids, and aromatic hydrocarbons and more, amount and exact composition varying according to wood species. The temperature in wood drying is typically around 85°C and the humidity level in the initial stage is close to 100%RH. The sensor performance data from a test in a wood dryer shows the measurement data extends over a three-year test period. It can be seen that the sensors featuring chemical purge keep performing extremely well over the whole time period, whereas the sensors without chemical purge suffer from drift.

Wood Drying

Wood drying is a demanding application where the sensor is exposed to a variety of chemical compounds that evaporate from wood during the drying process. Volatile compounds include alcohols, aldehydes, esters, ketones, terpenes, phenols, aliphatic, organic acids, and aromatic hydrocarbons and more, amount and exact composition varying according to wood species. The temperature in wood drying is typically around 85°C and the humidity level in the initial stage is close to 100%RH. The sensor performance data from a test in a wood dryer shows the measurement data extends over a three-year test period. It can be seen that the sensors featuring chemical purge keep performing extremely well over the whole time period, whereas the sensors without chemical purge suffer from drift.

Cleanroom

Relative humidity is a fickle issue in cleanroom. Both outdoor and indoor factors affect it. Too much humidity will increase particulates and provide a breeding ground for biological contaminants. You need the equipment to perform over an extended period of time. Chemical vapors in cleanroom can keep water molecules from the sensor, creating low readings when the relative humidity is high. This same issue can cause inaccurately high readings in low humidity. Should consider monitors with a heat-driven chemical purge function or other similar features. Sensor purge provides superior chemical resistance.

Cleanroom

Relative humidity is a fickle issue in cleanroom. Both outdoor and indoor factors affect it. Too much humidity will increase particulates and provide a breeding ground for biological contaminants. You need the equipment to perform over an extended period of time. Chemical vapors in cleanroom can keep water molecules from the sensor, creating low readings when the relative humidity is high. This same issue can cause inaccurately high readings in low humidity. Should consider monitors with a heat-driven chemical purge function or other similar features. Sensor purge provides superior chemical resistance.

HUMICAP & DRYCAP’s Unique Benefits

HUMICAP & DRYCAP’s Unique Benefits

Excellent long-term stability 

2-year calibration interval

Rapid response time

Full recovery from condensation

Resistant to particulate contamination, oil vapor, and most chemicals

Sensor heating for measurements even in condensing environments

Chemical purge option for stable measurements in environments with high concentrations of chemicals

Products Solutions

Relative Humidity (RH), Temperature (T) & Dew Point (DP) Measurement

The chemical purge option is available in all Vaisala DRYCAP® dew point instruments and is selectable in the following Vaisala HUMICAP® humidity instruments: HTM330 and HMT310 series transmitters; HM70 hand-held meter. When a new Vaisala humidity transmitter is ordered with the chemical purge option, the purge is set to start at regular intervals. This interval can be modified using a serial line command or from the product display/keypad. Chemical purge can be set to run on startup. It can also be initiated manually or be turned off in case it is not needed.

Browse our selection of measuring and monitoring instruments designed and manufactured for optimum reliability, accuracy, and durability.

Vaisala Dew Point and Temperature Meter Series DMT340

Vaisala DMT143 Miniature Dew Point Transmitters

Vaisala DMP5 Dew Point & Temperature Probe

Vaisala DMP7 Dew Point & Temperature Probe

Vaisala DMP8 Dew Point & Temperature Probe

Vaisala DMT132 Dew Point Transmitter

Vaisala DMT143L (Long) Miniature Dew Point Transmitters

Vaisala DMT152 Dew Point Transmitter

Vaisala DMP6 Dew Point Probe

Vaisala DM70 Handheld Dewpoint Meter

Vaisala HM70 Handheld Humidity & Temperature Meter

Vaisala HMT310 Series Humidity & Temperature Transmitter

Vaisala HMT330 Series Humidity & Temperature Meter

Your Partner For Accurate & Reliable Measurements

The choice of the correct measuring device is decisive for correct dew point measurement and humidity measurement in the chemical purge. Experts around the world trust the reliable dew point measuring devices from Dpstar. Our measurement technology experts will also be happy to help you find the right measuring device to measure and monitor your system’s dream point. Contact us now, we look forward to advising you!

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Off Jalan Puchong, 47160 Puchong,
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