[Halld-online] question about scalers in the EPICS archiver

[Hovanes Egiyan]

Richard,

The discriminator scalers are latched every second and the units are Hz. 
Sergey Furletov
runs the server that reads the values out into shared memory, so I do 
not know the details. I believe
that the value is non-integral because it gets normalized to a clock to 
get rates more precisely in Hz. So
the square root of the value in the DB for the scaler should be a good 
estimate of the statistical error.

Archiving of the scalers in MYA (or other PVs) has a deadband (for this 
particular variable it is
3*sqrt(value)+0.03*value), which means that if the next reading (which 
is indicated by a change in
the value of the EPICS variable) is within that deadband of the latest 
recorded value then it does not get
recorded. So, if you stripchart an EPICS variable live, you will see a 
lot more frequent value changes
compared to what you see when you look at the archive chart of the same 
variable.
This is done to reduce the update rate in the DB, otherwise it would 
cost a lot more money to keep all
variables in the DB without any deadbands. The deadbnads are adjustable 
though, I can request smaller
deadbands for a small subset of variables.

Hovanes



On 01/11/2018 12:41 PM, Richard Jones wrote:
> Hello Hovanes and all,
>
> I am accessing the GlueX scalers in the EPICS archive using the MYA 
> interface, and have run into a question. I am looking at a variable 
> like TAGM:T:8:scaler_t1 whose name suggests that it is the 
> discriminator scaler for TAGM column 8. When I look at the numbers in 
> that variable, they are obviously not scalers. They are non-integer 
> values in the range of 3000-8000 for the runs that I am looking at. 
> Are they rates? In what units? kHZ? What is the sampling interval? 
> This matters because of statistical error. I see entries in there 
> sometimes separated by tens of seconds during a run. In the time 
> between the samples, were the scalers free-running so that I have the 
> full statistical information in the one reading at the end of the 
> interval, or are all of the statistics during the seconds in between 
> the records in the DB lost?
>
> -Richard J.
>
>
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