Moisture Measurement Flashcards
True/False. The saturation vapor pressure of water increases as temperature increases.
True because warm air can hold more moisture
What is another term for % relative humidity?
% saturation
You take a sample of air in a closed container at 25 ⁰C and heat it up to 50 ⁰C. Does the % RH inside
of the container go up, go down, or stay the same?
%RH decreases because the hotter air has more capacity to hold moisture (saturation vapor pressure increases)
What is dew point?
A point at which a gas begins to condense
Describe a wet-dry bulb device (aka psychrometer) and how it is used to measure %RH
A dry temperature element and a wetted cotton sleeve temperature element are exposed to air flow. The wet bulb cools due to water vaporizing off it’s surface producing 2 different temperature readings. The dry bulb and the wet bulb measurements are used to find %RH on a psychometry chart.
Why would the wet bulb be cooler than the dry bulb in a wet/dry bulb device?
When water evaporates, it extracts heat from the material cooling it down.
In a capacitance sensor does the capacitance increase or decrease
when humidity increases?
Capacitance is proportional to %RH so the capacitance increases
How does the presence of moisture affect the output of a resistive moisture sensor?
The hygroscopic material absorbs water reducing the resistance of the surface.
What happens to impedance as the dew point increases?
Impedance decreases
What is a flashback arrestor? When should a flashback arrestor be used in dew point measurement?
A flashback arrestor allows gas into a probe but also cools gasses if they leave the probe. This ensures that the impedance probe will not act as an ignition source when in presence of flammable gasses.
How is dew point detected in the chilled surface analyzer (automatic type)?
Dew forms on a mirror due to a gas presence in the sample cell. As more dew begins to form the mirror reflects less light. Light detectors respond to this change by controlling the solid state cooler. A temperature sensor is mounted on the mirror’s surface to determine dew point.
What controls the temperature of the Peltier device in an automatic chilled mirror dew point
hygrometer?
The light sensors
What are some advantages of using the Bureau of Mines dew point tester? What are the
disadvantages?
Advantages:
- Less expensive
- Can measure dew point at high line pressures
Disadvantages:
- Not automatic or continuous
- Time consuming
- Accuracy depends on operator skill
Which type of dew point analyzer gives the truest measure of dew point?
Automatic chilled mirror
What happens to the dew point of an air mass as the moisture content increases?
Dew point increases to a warmer temperature
What happens to the dew point of an air mass as the humidity increases?
Dew point increases as there is more moisture present in the air mass
True/False. As pressure increases the dew point increases
True
What are the 3 ways that a dew point sensor can be calibrated?
1) Remove the sensor from the process and send to a lab to be calibrated with 1st and 2nd standards
2) Remove from process and calibrate on site using a secondary standard
3) Calibrate in the field using a reference standard or instrument
What are moisture standards made from?
Moisture standards are made of salt solutions
What sensors use hygroscopic material?
1) Resistive
2) Capacitive
3) Impedance
How can the dewpoint of a gas at a fixed pressure be lowered?
Remove moisture by drying the sample
What are some applications of monitoring moisture content?
1) Food industry quality control
2) IAS production
3) Meteorology
4) Agriculture, determining grain dryness
If you take a sample of air with a fixed humidity and heat it the dew point will _______________.
Stay the same
The __________ sensor can be used at high line pressures.
Impedance sensor
Which hygrometer measures dew point directly without inferring from a different device?
Chilled mirror hygrometer
The small size of the ______________ and ___________ types of electronic moisture sensors make them ideal for portable use.
Resistive, capacitance
What techniques may be used to measure the moisture content of liquid and solid samples?
1) Microwave absorption: measures the water content in foods and solids if the solids flow between the transmitter and receiver
2) IR absorption: measures gases and liquids. Can be used to measure solids by pointing the IR source at a moving solid. The solid reflects the IR intensity into an IR detector.
3) Neutron: measures moisture content on solids
