First, I was happy to see a real-time Penman PET equation addded to Campbell's routines. I hope user understand how important the Rso value is - most of you probably don't calculate this value to verify proper functioning on your pyrameter!
However, after some time playing with it and generating a lot of bad data, it is not as intuitive of a function as Shortcut makes it seem, nor is the documentation very thorough. The ETsz command needs solar radiation in MJ; the others are pretty straightforward. However, all our solar radiation sensors (LP02 in my case) have sensitivity factors to convert volts to watts/m^2. Here are some odd things i've noticed, all with a scan interval of 5s and an hourly output table;
1. By default, Shortcut took the raw voltage multiplied it by 0.001 and called this SlrMJ, then used my sensitivity coeffecient to calculate SlrW. This seems in error for a number of reasons - but I don't see how you can get to SlrMJ from raw voltage! My bad for not analyzing the code first.
2. Let's correct #1 and produce w/m^2 then convert to MJ by SlrW*0.001*3.6. Now we're in MJ m-2 h^-1. If I send this to ETsz I get huge values of PET (60-80 mm/h yesterday). So that can't be correct.
3. A search on the forum turns up some mention of needing to use the scan interval in this conversion.
So my question: what is the required solar radiation (MJ) conversion to use in ETsz?
One more note, I am using an older CR1000 OS, std.27. The sites are remote and I haven't been able to visit them an update the OS to std.29 yet.
You want MJ/scan. Short Cut does the proper calculation.
I don't see how the Short Cut method can be correct:
'LP02 Pyranometer measurements 'SlrMJ' and 'SlrW'
VoltDiff(SlrW,1,mV25,1,True,0,_60Hz,1,0)
If SlrW<0 Then SlrW=0
SlrMJ=SlrW*0.001
SlrW=SlrW*20.0
The cumulative annual ETo totaled 3998mm using the ETsz value. I ran the same input data through refET and got 1200mm - which is much more reasonable for one year in central Texas, no? My class is using this PET value for input into a vadose zone model and having nothing but trouble!
What is the sensitivity from your calibration sheet?
Sorry, i accidentally unscribed to the topic last week. Ok, my actual S = 17.12 on this LP02 - the '20' above is a dummy value used in CRBasic. At this sensitivity (and as a multiplier), it seems that my max values seldom get above 300 w/m2. In comparison to the Rso returned from ETsz - it's consistently 1/3 of the potential solar radiation. Odd? However, ETo would only go up if I had more solar radiation in the Penman equation.
VoltDiff with a multiplier of 1 gives a result in mV.
The sensitivity is in uV/(W/m2). To convert to mV/(W/m2), you must divide by 1000. Then, to have (W/m2)/mV, you take the inverse. So, your multiplier to get watts would be 1000/17.12 . That value is 58.4112, which is what Short Cut uses for a multiplier when you input a sensitivity of 17.12 .
To get MJ, you need to factor in your scan rate in seconds. A Joule is a Watt for 1 second. Mega Joules is a bigger by a factor of 1 million (mega).
For a 5 second scan rate:
5 * 58.4112 * 0.000001 = 0.0002920561 MJ per mV per scan
That is what Short Cut is using. Pay attention to the order Short Cut does the calculations. It puts the raw mV into the SlrW variable. It then does SlrMJ=SlrW*0.0002920561 SlrW is actually millivolts at this point.
Afterwards, Short Cut does SlrW=SlrW*58.41121 After this, SlrW is in Watts.
Well, now I see how the 'S' vaulie is actually 1000/S. That should incease hourly w/m2 and I assume the ETsz will likely be more correct now, considering the 'S' was inherently in the conversion to MJ/m2. This is a little tricky to see in Shortcut - particularly, when you're only using shortcut for one station then applying it to many others (as I was doing!).
Hi,
I have a Relative Humidity sensor HS2C3 and a CR1000, as I do for the relative humidity, as the Short Cut program only habiliata me ETo and Sample.
Thanks by device
Average of relative humidity is a bad number mathematically. That is why we disable it. If you want to do average of relative humidity, click with the right mouse button on the measurement name on the 'Outputs' screen.