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What Is the Density of Air at STP?

At MAC Instruments, we are a leading manufacturer of industrial moisture analyzers. These critical components measure the amount of moisture mixed into the air to help our customers keep their facilities in compliance with industry and environmental standards.

Air density plays a significant role in this measurement process. A popular method of moisture measurement is based on water vapor density in the air at various standard conditions of temperature and pressure (STP). Knowing these standard measures can help customers use monitoring equipment more precisely.

Density of Air at Standard Temperature and Pressure

STP—standard temperature and pressure—represents conditions that people and systems use to calculate gaseous factors. Commonly accepted STP values are 32° F (0° C or 273 K) and 1 atm pressure. However, while STP is a relatively common term, it can mean different things based on the agency applying the standard. For example, the National Institute of Standards and Technology (NIST) sets its standard at 68° F and 101.325 kPa. On the other hand, the International Standard Metric Conditions are 59° F and 101.325 kPa for natural gas.

As different industries and applications use different STP values, the best practice is to describe the standard and list out the exact conditions for each measurement. Stating the temperature and pressure conditions for every readout, inspection, and process can keep records clear.

According to the International Standard Atmosphere (ISA) values—15° C at sea level—the density of dry air is at:

  • In Metric units: 1.225 kg/m^3
  • In Imperial units: 0.0765 lb/ft^3

Although pure water vapor cannot exist at STP, these density values for dry air can be used to calculate a theoretical value for water vapor density at STP. As the density of pure water vapor is 62.19% of the density of dry air, theoretically, 100% water vapor would have a density of 0.0762 kg/m^3 or 0.048 lb/ft^3 at STP.

Uses of STP in Moisture Analyzer Applications

Knowing these measurements can help facilitate a better understanding of moisture analyzers. At MAC, we offer several high-performance moisture analyzers, including:

  • MAC 116 Moisture Analyzer controls the flow of steam in food industry applications to maintain proper moisture levels for optimal production yield and cooking time.
  • MAC 125 Moisture Analyzer can measure absolute humidity and total pressure to calculate the area’s moisture level. This continuous monitoring helps keep facilities compliant with environmental and industrial regulatory standards.
  • MAC 155 Moisture Analyzer monitors emission stack gases. The equipment can help ensure that the stack gas’s moisture or humidity levels meet regulatory requirements.

Contact MAC Instruments for Quality Moisture Analyzers

Continuous monitoring is the first step to keeping your facility compliant and safe. By using our moisture analyzers, customers benefit from:

  • MACEasier installation and operation
  • Higher sensitivity to water vapor and moisture content
  • Faster and more accurate results
  • A more user-friendly interface
  • Greater performance stability

If you are interested in learning more about our moisture analyzers, contact us at 419-621-2322 or request a quote.

What Does a Moisture Analyzer Do?

Since 1990, MAC Instruments has built high-quality, heavy-duty industrial moisture analyzers. Our instruments help facility managers measure an environment’s absolute humidity in challenging high-temperature conditions.

MAC products use patented, proprietary sensors that consistently provide accurate readings even in highly variable conditions. Clients across the industrial spectrum rely on our moisture analyzers to help them fine tune the efficiency of their processes.

Our clients include those in the following industries:

  • Power generation
  • Cement mixing
  • Oil and gas refining
  • Pulp and paper processing
  • Food and beverage processing
  • Horticulture and agriculture
  • Metalworking

How Is Moisture Analyzed?

Water is certainly essential to life, however, too much of it can corrode or short circuit your equipment, and too little of it can negatively affect the quality of perishable items and dried product. Moisture analyzers operate by taking holistic measurements of the following physical or chemical properties:

  • water vapor pressure

The moisture analyzer uses data from these readings to calculate the climate’s absolute humidity, or the measure of water vapor in the air regardless of temperature or other conditions. Maintaining a consistent absolute humidity means that you can create an optimal internal environment for whatever process you are conducting, be it drying product, refining oil, or creating sensitive pharmaceutical products.

Moisture content analyzers can be used to take direct measurements of the air, but our products also include features that allow you to analyze fluctuations in absolute humidity over time.

The FDA and other governmental and regulatory bodies stipulate a wide range of humidity standards that facility owners must follow in order to optimize efficiency and maintain customer health. An inconsistently monitored or controlled relative humidity can result in moldy or stale food items as well as corroded equipment.

How Do our Moisture Analyzers Work?

A sampling pump within the analyzer draws a sample of the atmosphere being monitored through the analyzer. The sample first passes through a 10 micron filter to remove any particulates from the sample stream. The sample travels through the barrel of the instrument and past the sensor. The sensor and the barrel are electrically heated to prevent condensation of any moisture in the sample. There are two pressure sensors in the analyzer. An absolute pressure sensor measures the total pressure of the process atmosphere. The partial pressure of the water vapor is measured by a proprietary high temperature capacitive sensor that responds only to water vapor partial pressure. The signals from these sensors are used to compute the % moisture by volume of the sample.

% water vapor by volume = (water vapor pressure / total pressure) X 100

The total pressure varies with altitude above sea level, local barometric pressure, and process induced pressure. In some applications the humidity ratio is preferred instead of % moisture by volume. The humidity ratio is defined as the mass of water vapor per unit mass of dry air. A jumper in the MAC125 allows the user to select the desired moisture scale.Some industries that require the use of moisture analyzers include:

  • Stack emission monitoring
  • Product drying
  • Metal processing
  • Cooking and baking
  • General industrial applications

MAC Instruments offers a number of different moisture analyzers intended to perform specific functions within a variety of industries. Our products include:

  • MAC 155: This moisture/humidity analyzer operates via user-friendly components that measure humidity in exhaust gases at temperatures of up to 1,200°F (650°C). The MAC 155 helps you to augment your safety process and comply with EPA regulations.
  • MAC 125: This high-temperature absolute humidity sensor operates without the use of chemicals, compressed air, wet bulb techniques, optics, or mirrors. Its moisture sensor and humidity meter rely on a patented, proprietary solid-state process for measuring humidity, making this a straightforward system that experiences few breakdowns.
  • MAC 116: This analyzer monitors humidity differences in the flow of steam in a cooking atmosphere, which will enable you to cook products as effectively and quickly as possible. It can be used to monitor the drying and pasteurization of food products, and facility owners can install it on continuous conveyor belts used to cook meat and poultry.

Contact MAC Instruments

At MAC Instruments we have been building high-quality moisture analyzers out of our Sandusky, OH, plant since 1990. If you’re looking for an efficient way to monitor your facility’s absolute humidity, we’ve got you covered. Be sure to contact us today to learn more about our moisture analyzers or any other industrial products that we deliver.

What’s Wrong with Relative Humidity Above 100°C?

Water vapor above a temperature of 100°C at atmospheric pressure is in the super heated state. The characteristics of the relative humidity scale change drastically in the super heat area. It is somewhat like going through the sound barrier, the physics take on new characteristics in the super heat area just as they do in the super sonic area. This can be confusing if not prepared for the change.

Below 100°C it is possible to achieve 100% relative humidity at any temperature. Above 100°C (in the super heat area) the maximum possible relative humidity plunges rapidly as the temperature increases. Pure steam (100°C dew point) will register only 20% on the relative humidity scale at 150°C. At 175°C the maximum possible RH is only 10%. At 200°C, maximum RH is only 5.9% and at 370°C, maximum RH is a mere .48%.

An RH instrument with a published accuracy of +/-1% will only indicate true moisture level with an accuracy of +/-5% at 150°C (+/-1% out of 20%), +/-10% at 175°C, +/-17% at 200°C, and will be useless at 370°C, even if the sensor is not destroyed by the high temperature.

Most RH instruments have a reduced accuracy specification above some specific temperature. An instrument with a published accuracy of +/-1% may be +/-2% above 90°C. This is sometimes hard to find on the data sheet, but is usually there somewhere. If the RH accuracy drops to +/-2% at elevated temperatures the true moisture level will have an accuracy of +/-10% at 150°C, +/-20% at 175°C, and +/-34% at 200°C.

Even if we forget about the absolute accuracy of an RH instrument above 100°C, the RH scale itself has problems in the super heat area. An RH instrument that is totally accurate may indicate 10% RH at a temperature of 150°C. This point is equivalent to 50% moisture by volume, or a dew point of 82°C or a humidity ratio (lb water vapor/lb dry air) of .622. This same instrument will display 10% RH at 175°C at 100% moisture by volume, or a dew point of 100°C, or a humidity ratio of infinity.

The above shows that the relative humidity scale is essentially useless or even misleading at temperatures above 100°C.

Some relative humidity instruments can indicate dew point or another absolute humidity scale. This is done by measuring RH and temperature and then calculating absolute humidity. This method is not accurate above 100°C because of the limited maximum indication on the RH scale.

For more in depth information on this subject refer to pages 7 & 8 of the
MAC Humidity/Moisture Handbook.”

The Mac Humidity / Moisture handbook can be viewed online.

What’s Wrong with Relative Humidity Above 212°F?

Water vapor above a temperature of 212°F at atmospheric pressure is in the super heated state. The characteristics of the relative humidity scale change drastically in the super heat area. It is somewhat like going through the sound barrier, the physics take on new characteristics in the super heat area just as they do in the super sonic area. This can be confusing if not prepared for the change.

Below 212°F it is possible to achieve 100% relative humidity at any temperature. Above 212°F (in the super heat area) the maximum possible relative humidity plunges rapidly as the temperature increases. Pure steam (212°F dew point) will register only 20% on the relative humidity scale at 300°F. At 350°F the maximum possible RH is only 10%. At 400°F, maximum RH is only 5.9% and at 700° F, maximum RH is a mere .48%.

An RH instrument with a published accuracy of +/-1% will only indicate true moisture level with an accuracy of +/-5% at 300°F (+/-1% out of 20%), +/-10% at 350°F, +/-17% at 400°F, and will be useless at 700°F, even if the sensor is not destroyed by the high temperature.

Most RH instruments have a reduced accuracy specification above some specific temperature. An instrument with a published accuracy of +/-1% may be +/-2% above 200°F. This is sometimes hard to find on the data sheet, but is usually there somewhere. If the RH accuracy drops to +/-2% at elevated temperatures the true moisture level will have an accuracy of +/-10% at 300°F, +/-20% at 350°F, and +/-34% at 400°F.

Even if we forget about the absolute accuracy of an RH instrument above 212°F, the RH scale itself has problems in the super heat area. An RH instrument that is totally accurate may indicate 10% RH at a temperature of 300°F. This point is equivalent to 50% moisture by volume, or a dew point of 179°F or a humidity ratio (lb water vapor/lb dry air) of .622. This same instrument will display 10% RH at 350°F at 100% moisture by volume, or a dew point of 212°F, or a humidity ratio of infinity.

The above shows that the relative humidity scale is essentially useless or even misleading at temperatures above 212°F.

Some relative humidity instruments can indicate dew point or another absolute humidity scale. This is done by measuring RH and temperature and then calculating absolute humidity. This method is not accurate above 212°F because of the limited maximum indication on the RH scale.

For more in depth information on this subject refer to pages 7 & 8 of the
MAC Humidity/Moisture Handbook.”

The Mac Humidity / Moisture handbook can be viewed online.

Accurate and Responsive Humidity Probes by MAC Instruments

Since 1990, MAC Instruments have been manufacturing industrial moisture measurement instruments. With a MAC Instruments moisture analyzer one can instantly measure absolute humidity in the most demanding high-temperature conditions. MAC’s moisture analyzer range renders all the necessary data through a simple, clear interface. All our products are used in several high-temperature humidity measurement applications at different installations across the world.

The Efficient Features of MAC 155 Humidity Probe

MAC155 is implemented in worldwide industrial operations for 24/7 monitoring of stack emission gases. It is designed and built for measuring humidity found in exhaust gases in temperature ranges up to 1200°F. Our MAC155 moisture/humidity probe is core-compliant with US EPA regulations. This device strengthens your process safety and makes it user-friendly as it consists of several high-end components like humidity transmitter and probe. Moreover, it is also protected by a sintered filter element made up of a 10 micron ceramic element that can be conveniently cleaned and replaced without any hassle.

MAC155 has plenty of features and characteristics. You can find more information on our website.

Furthermore, we also provide a range of special accessories to extend the use of our probes in temperature ranges up to 2400°F. All the probes we offer are exceedingly sensitive to water humidity/vapor. They do not depend on any optics, chemicals, wet bulb, mirrors, or compressed air to calculate readings. All our products are highly responsive to the partial pressure of water humidity/vapor enabling instantaneous readings. Please visit us at www.macinstruments.com for any further concerns or assistance. Also drop us an email atmoc.stnemurtsnicam@ofni or speak with our MAC representative at 419.621.2322.

MAC Instruments Offers Industrial Compliant Moisture Measurement Equipments

MAC Instruments is the leading manufacturer of steam flow meters, transmitters, and industrial moisture analyzers in the industry. We mainly manufacture these high-end products to serve high-temperature industrial applications. Our entire range of analyzers provides all the data you need through a simple and clear interface. Our moisture measurement instruments offer NIST-traceable direct moisture level measurement that helps you meet with emission monitoring needs.

Moreover, MAC moisture analyzersintroduce high performance that helps you to take an advanced scientific and result-driven approach towards understanding humidity levels in your industrial process.Since 1990, we have been in this industry, and are producing moisture measurement devices to serve every industrial sector with our moisture controlling products. All of our devices easily enable you to immediately measure absolute humidity in the most unfavorable conditions.

Pivotal Features of MAC Moisture Instruments

All MAC moisture measurement instruments and products that we offer are unlike any other products available on the market. And the aspects that distinguishus from the rest are the techniques and methods we use in order to manufacture moisture control equipments. Wedo notuse chemicals, optics, or compressed air to measure moisture, or any wet bulb techniques. Alternately, MAC products integrate a proprietary sensor that offers consistent accurate readings and performance even during adverse climatic conditions or highly variable humidity conditions. MAC Instruments uses high-quality materials in all of our products to ensure that our customers receive error-free results on every use.

The presence of moisture is inevitable in our surroundings, and this particular aspect can build immense negative impact on various process efficiencies. In that case, MAC Instruments has emerged as the most trusted source for moisture control activities.

You may drop us a line at moc.stnemurtsnicam@ofni or simply visit our website www.macinstruments.com for more detailed information. Also, if you want to directly get in touch with our MAC representative, call us at 419.621.2322.