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Dew Point Calculation

Discussion and questions about Cumulus weather station software version 1. This section and its subforums are the main place to get help with Cumulus. Anything which is not specific to the type of weather station goes in here; for anything specific to a type of weather station, please use the appropriate subforum. Use the 'website development' section for any questions relating to creating or running a web site for Cumulus data. Discussion of the stations themselves in these sections is fine.
northantsweather
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Dew Point Calculation

Postby northantsweather » Thu Aug 24, 2017 11:19 am

I am using Cumulus 1 with an Instromet weather station. The calculation for dew point always appears too low. The extra sensor has not been ticked so that Cumulus treats the second temperature sensor as a wet bulb. Relatively humidity reads well, as does the wet bulb sensor. What could be the problem? At the moment with a dry bulb reading of 18.6 and a relative humidity of 60% I am getting a dew point of 4.5C when it should be 10.5C.

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steve
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Re: Dew Point Calculation

Postby steve » Thu Aug 24, 2017 3:42 pm

From a quick look at the code, it looks like there might be a bug when the user has selected Fahrenheit, but it sounds like you're using Celsius. Still quite possibly a bug in the code, though, but it's difficult to say for certain as I no longer have the Cumulus 1 development environment installed, so looking at the code is tricky.

If it were a bug in the code, I'd expect the humidity to be wrong, though. The dewpoint calculation uses a third party library which uses the Davis dewpoint calculation.

Remind me whether a setup like yours supplies humidity direct from the station?
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northantsweather
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Re: Dew Point Calculation

Postby northantsweather » Sat Aug 26, 2017 11:36 am

Hi Steve

The system I have works with a wet and dry bulb sensor. I presume that the datalogger is doing the calculation for humidity as this is being displayed on the analogue display panel as well as Cumulus. However, without calibration the humidity value is always far too low. Once calibration has been achieved, I have to say the humidity records well. I am just wondering whether Cumulus is calculating the dew point from the uncalibrated readings (although when I hover the cursor over the dew point reading, I do see the corrected wet bulb reading.

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steve
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Re: Dew Point Calculation

Postby steve » Sat Aug 26, 2017 4:41 pm

I think it calculates the humidity from the wet bulb and dry bulb temperatures, but I can't remember whether it uses the uncalibrated figure for the dew point calculation, I suspect that may well be it.
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Re: Dew Point Calculation

Postby sfws » Sat Aug 26, 2017 5:13 pm

http://sandaysoft.com/forum/viewtopic.php?f=4&t=5430&p=46823&hilit=dew+point#p46823 suggests Cumulus 1 does use calibrated temperature for dew point?

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steve
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Re: Dew Point Calculation

Postby steve » Sat Aug 26, 2017 6:00 pm

I think it's different for Instromet stations, because of the unusual data they supply (compared to the other stations that Cumulus works with).
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northantsweather
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Re: Dew Point Calculation

Postby northantsweather » Sat Aug 26, 2017 7:47 pm

Is there a fix? I appreciate that you no longer work on Cumulus 1, and if it is time consuming I won't worry too much. There may be a way of calibrating the sensors outside of Cumulus - I'll contact Instromet. You could calibrate the analogue display on earlier versions of the model I have, but the new model would need calibrating in the junction box and I am not sure this is possible.

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steve
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Re: Dew Point Calculation

Postby steve » Sat Aug 26, 2017 8:42 pm

I think there's a calibration setting for dew point, so if you can work out settings for that, it may be a solution.

Edit: actually, I don't think there is.
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northantsweather
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Re: Dew Point Calculation

Postby northantsweather » Sun Aug 27, 2017 3:26 pm

I think the easiest solution would be to apply the calibration I give to relative humidity directly into the formula used to calculate the dew point. Could you tell me Steve the name of the folder/file where the dew point formula is located. I assume it's only in the one place - cumukus.exe file? I'm okay with the maths so should be able to sort something out.

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steve
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Re: Dew Point Calculation

Postby steve » Sun Aug 27, 2017 3:46 pm

It's in the Cumulus code (so yes, it's in Cumulus.exe), and hence you can't modify it.
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northantsweather
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Re: Dew Point Calculation

Postby northantsweather » Sun Aug 27, 2017 5:40 pm

Sorry Steve - it's much harder than I thought. I had a look through the cumulus.exe file and, I have to be honest, I didn't know where to start looking. The code clearly isn't written as a conventional mathematical formula as I thought it might be.

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steve
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Re: Dew Point Calculation

Postby steve » Sun Aug 27, 2017 6:12 pm

You would need to be quite a hacker to be able to modify any of the formulae in the Cumulus binary! It's intended to be readable by computers, not humans.
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northantsweather
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Re: Dew Point Calculation

Postby northantsweather » Tue Sep 19, 2017 10:06 pm

I had a good look at the dew point issue today. Decided to uncalibrate both dry and wet bulb sensors and humidity, and Cumulus returned accurate values for dew point and humidity, so clearly the code which calculates the dew point is using the uncalibrated values. Easiest solution seems to be to calibrate the datalogger so that clean values are received by cumulus.

Am I right in thinking I can use PuTTY to do this? If so, how do I obtain the configuration settings. All I know is com port and baud rate. Also, is it just a question of typing in the correct code in the terminal screen and pressing return, ie WRST5,2800 to raise the dry bulb temperature by 2.8C?

Any advice much appreciated.

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steve
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Re: Dew Point Calculation

Postby steve » Wed Sep 20, 2017 1:32 pm

I don't have an Instromet station now, and I'm not sure I have a copy of the protocol spec, so I'm afraid I can't be of much help.
Steve
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Please read the posts in the Announcements section about the current status of Cumulus development now that I have retired

northantsweather
Posts: 24
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Re: Dew Point Calculation

Postby northantsweather » Sat Sep 23, 2017 12:16 am

Further to the problems linked to calculating accurate values for dew point on my Instromet station, is there a way of removing dew point from the graph shown on the Trends page? Removing dew point from the other pages appears straightforward, but it would be good to remove dew point from the outside temperature graph (see attached). I've tried recalibrating via the datalogger using HyperTerminal as recommended by Instromet, but I have not been able to get the logger to respond.

Furthermore, am I right in thinking that the dew point bug for Instromet stations will be inherited in Cumulus MX? I was just wondering if upgrading MX might solve the problem.
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