Humidity Sensing


      The humidity sensor used on our DataScribe RH-52 is a monolithic IC using a capacitive polymer sensing element. With an operating range of 0 to 100% RH, -40 to 185 F, this sensor offers reliable long-term performance over a broad range of applications. It successfully resists contaminant vapors such as organic solvents, chlorine, and ammonia. It is unaffected by water condensation. The engineers at Avatel evaluated many different types of RH sensors from all the major manufacturers before deciding on this sensor. It costs us 5 to 10 times as much as some of our competitor's choices, but it's quite simply the best sensor available.
        Specifications:
             Total Accuracy:                  +/- 2%, 0 to 100% RH @ 25C
             Operating Temperature:      -40 to +85C (-40 to +185F)
             Hysteresis:                          +/- 0.8% of span maximum
             Linearity:                            +/- 0.5% RH
             Repeatability:                      +/- 0.5% RH
             Time Constant:                   15 seconds in slow moving air
             Stability:                             +/-1% RH typical at 50% RH in 5 years
 
        Checking Calibration:
 
      Sensor calibration should be checked at 2 points. We use 33% and 75% at the factory. Never use 100% as a calibration point. One of our competitors suggests testing their calibration at 100% using saturated air. This is not accurate for the simple reason that most all humidity recorders limit the top reading to 100% If your instrument is reading 10% high, it will still look good at 100% due to the instrument's ceiling.

      The only practical way to accurately check your RH calibration is to use known humidity standards. You can create your own standards using saturated salt solutions in a closed container. Different types of salt can be mixed with distilled water to form saturated solutions having a fixed RH at a given temperature. The following table shows the RH of various salts solutions at room temperature (68 F).

             Lithium Chloride           11%
             Magnesium Chloride     33%
             Magnesium Nitrate        54%
             Sodium Chloride           75%
             Potassium Chloride       85%
 
      Accurate results are possible with a little care. Please follow these guidelines:

      a.) Use only pure water and salts. Do Not use common iodized salt for sodium chloride.
      b.) The calibration must be done in an airtight container. If you're calibrating an RH-52 with an internal sensor you must make provisions to suspend the instrument above the solution. The sensor should be close to the solution, but avoid getting the solution on the sensor or instrument. Watch out for splashing if moving the container!
      c.) The sealed, airtight container needs plenty of time to equilibrate. You should allow at least 8 hours, 24 is better. The key to faster equilibration is to have a small air volume and a large solution surface area. Some Tupperware containers work well, but be sure they're truly airtight.
      d.) The RH values assume a constant temperature.

      When making up the solution, slowly mix the distilled water and salt until a saturated solution is obtained. Make sure there's an excess of salt. Handle all these salts and solutions with care!



Back to Support