This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

HDC1080EVM: Humidity measurment error during Low temperature (5C)

Part Number: HDC1080EVM
Other Parts Discussed in Thread: HDC1080,

Hi Teams,

 Recently I have topic for HDC 1080 humidity measurment performance.

 When I verify HDC1080 on normal temperature (25C) and set different humidity. the result could following target setup value. During I changed test temperature  to low temperature (5C), the measurement result cannot following target setup.

 Setup conditions: Temp 5C and target relative humidity 50%
 Measurement Result by HDC1080: 40.5%

 Now I'm try to rise ambient temperature up to 25C and setup relative humidity is 50%. As test, the humidity measurement result go back to normal and follow targe humidity, measurment result is 50.1%.

 Result: Look like the humidity sensor measurement circuit have some relationship with temperature (Espcially low temperature). Can you help to understand the root cause? I'm guess whether the air hydrone changed from gaseous to liquid water and effect humidity sensor. Am I right? How to fix this issue from root? Please suggestion and correct me if anywhere is wrong. Tks.

Regards,

Xiaobing (Eric)

  • Dear Eric -

    I think you have encountered a condensing environment. The test condition you describe could produce a damp environment - but here the interesting thing is you are reporting the humidity measurement reduced - I would have expected it to go up and i did same test here with HDC1080EVM and my chamber and exactly that occurred, as the temp reduced, the %RH of the chamber went up to about 81% and the sensor measured that. Then, in my chamber, after the temp was stable, it went to work on reducing the humidity back to where the setpoint was - @50%. So i cannot explain your results here - any chance you can repeat this test and watch the response of the unit as the chamber is in transition? I have attached the log output of my run for you to compare to. 

    xyz.csv

  • Hi Josh,

     Thanks for the quick reply and your log, it greatly help with me, so nice TI team and guy.

     I has arranged my guy for double confirm me result. Just have follwowing questions:

     1. Can you help to get more humidity data about 5C after temperature variation from 21C to 5C?

     2. A small request, Can you help to understand the form title, whhich are "evmSampleDelta(ms)", "logDelta MS", "termperature_CELCIUS" and  "Relative_Humidity_Percent"? It  will great help to understand this form very vell. 

    Regards,

    Xiaobing (Eric)

  • Dear Eric - 

    for #1, can you elaborate on more? the log I sent has is HDC1080EVM in a temp/humidity chamber, plugged into PC over USB extension cable, running the Sensing Solutions EVM GUI. 

    The log starts with chamber at ~21C and about 53%RH and can be enabled on your side by going to menu, data streaming

    then go to Select Log File, choose desired path and file name

    Then hit Data Streaming Start Button and the unit will be measuring and logging the data just like what i provided, so if it is possible for you to do same thing, you can collect all the data you need. If you want something else, let me know. 

    For #2: 

    These are the default timings for I section 3.6.2 of the EVM user manual goes over all the settings http://www.ti.com/lit/ug/snau189a/snau189a.pdf

    The logDeltaMs is controlled by sample rate settings of graph configuration - it is when new sample is added to the graph and is not cumulative.

    The next columns over are temp in Celsius, already calculated, relative humidity % calculated, and then the temp and humidity codes (in hexadecimal) that are supporting those calculations. 

     

  • Hi Josh,

     

     Received and tks  a lot, Josh.

     Refer following curve, during temperature change the humidity value become to not ture in some period.

     I would like to wirte codes and shield this periods humidity datas, any theory proof for this period, how long is better? Tks. 

    Regards,

    Xiaobing (Eric)

  • Eric - 

    I am not familiar with the word ture, did you mean something else by chance? when temp goes up, like that, it would be normal for humidity to go down - you can see same effect if you turn on the heater of the HDC. 

    what were conditions of this run of data? 

  • Josh,

     The curve is refer your above log data, attached please find Excel form for reference.

    Verificaiton.xlsx

     Learning the curve above, when temperature changes from 21C to 5C, the humidity value in some period is not accurate (see red box in the curve above).
     My questions are:
     1. How long could the humidity sensor get real data during temperature changing? Is there any formula?


     2. In your test log, the humidity value changed from 53% to 48.7% druing temperature changing from 21C to 5C, do you think 48.7% is the relative humidity value at 5C? Why is there humidity value difference between 21C and 5C in the same chamber? Is it necessary to improve or not?

    BR,

    Xiaobing (Eric)

  • Eric -

    Relative humidity compares the current ratio of absolute humidity to the maximum humidity for a given temperature and expresses this value as a percentage. The higher the percentage, the higher the humidity. It is affected by both temperature and pressure.

    Given the same amount of water vapor, there will be a higher relative humidity in cool air than there is in warmer air.

    I don't think there is an issue here, what is shown here is a chamber in transition and the sensor reacting to that, which also shows you how dew is formed, then the chamber settles after some time and the sensor is also detecting that as well. 

     

  • Josh,

     Tks for the explanation.

     The humidity value changed from 53% to 48.7% druing temperature changing from 21C to 5C, can I understand the humidity value chage caused by chamber vapour condensation? 

     Last questions:

     1. What instruments/meter do you used for sensor data calibration? Any suggestions. 

     2. If the vehicle is not only air, but includes volatile organic content (VOC), any necessary to improve for sensor/hardware/software? Tks. 

    BR,

    Xiaobing (Eric)

  • May I have the replies for above questions, Team? Tks, 

    BR,

    Xiaobing(Eric)

  • Eric - 

    1. Yes. 

    2. We use dew point mirror, oil bath and other similar calibrated lab equipment for the measurements. 

    3. it is well known that polymer based sensors can be affected by VoCs, you can see it in every datasheet from each vendor. Care should be taken to avoid them. 

  • Well noted and tks for your explanations, Josh.