Measurement of MET3A Relative Humidity Sensor Response Time

by Tim Potter
  Summary
The relative humidity sensor of a MET3A system was tested to determine the speed of its response to a step change in humidity.  The results of this test demonstrate a relative humidity response time of nearly 70% in two minutes, even under condensing conditions.
  Background
The MET3A design incorporates aspiration, with the goal of minimizing the response time of the RH sensor.  This purpose of this test is to verify the rate of the MET3A RH sensor’s response to a step change in humidity. 
  Test Method

Refer to Figure 1.

The MET3A prototype was fitted with a 5” length of 1½” diameter plastic pipe fastened and sealed to the inner chamber of the MET3A.  The free end of the plastic pipe was suspended over a partially filled beaker of water, which was heated using a low-temperature hot plate.  An aluminum foil hood was attached to the free end of the plastic pipe, and was sealed tightly around the rim of the beaker in order to efficiently channel the humid air into the MET3A air intake.  The beaker was warmed from below by a small hot plate.  

The MET3A was connected via its serial port to a PC.  Digiquartz® Assistant logging software (DQA) was used to acquire and log RH, Temperature, and Pressure measurements from the MET3A.  Data were acquired and logged continuously throughout the test.

The test procedure was as follows:

1.      Begin logging data and note time.

2.      Turn on the hot plate.

3.      Wait for RH to reach 100%.

4.      Remove the plastic tube from the inner solar shield to interrupt humidity input. 

5.      Wait until RH stabilizes.

6.      Stop logging data.  

  Results

Figure 2 shows the results of the test. 

The test apparatus was capable of generating RH levels up to 100%.  However, since aspiration was essentially defeated as a result of the inner solar shield being tightly sealed to the beaker, significant condensation occurred within the inner solar shield.  Examination of the plot clearly shows a period where internal condensation is evaporating.  Once the condensation had evaporated, the RH indication quickly decreased to the ambient RH level.  It should be noted that in normal operation, the aspiration would never allow this level of condensation to occur within the inner solar shield, even in the most extreme humidity conditions.

  Conclusions
This test demonstrates the ability of the MET3A to quickly respond to step changes in humidity.  RH response rates up to approximately 60%/minute were observed, and the test data yields an RH recovery time constant of 0.75 minutes.

©2007 Paroscientific, Inc.