counting events and miss first count

[Alice Dobie-Galuska]

This is a problem I recently sent to NI support, but did not get much help.
Do any of you have any suggestions?


Problem Description :

I am trying to count events from a geiger muller (GM) tube over a period
of 15
seconds. THe GM tube gives a 20 microsec TTL signal when a decay is detected.
It also will beep when a decay is detected. I modified the example vi "count
events" so that it will stop running after 15 seconds. I am using the get
date/time function to count seconds. The PROBLEM is that sometimes the first
event is not shown on the count indicator. I hear a beep but the indicator
says zero. When and if there is a second event (a second beep) the count
indicator says 2 (indicator goes from zero to two upon the second event).

This is an intermittant problem and only occurs on the first event. It is
not
a big deal when I am measuring a sample that has many events in 15 seconds.
However it is a problem when I am measuring background radiation when it
is
possible to get only one count in 15 seconds. When this problem occurs and
there has been only one event, the indicator will show zero counts.

If I run the count events example several times (about 10) I usually observe
this problem at least once. I noticed that the problem is solved if the
first
while loop from the left in the count events example is removed. But then
that
causes the indicator to show the count from the previous run until there
is a
count. THis is a problem if there are zero events in 15 seconds (which can
occur when reading background radiation).

I am stuck. Any suggestions?



NI Software : LabVIEW
Version : 5.1
OS : Windows NT
NI Hardware : labPCplus
Drivers : Ni-Daq 6.5.1
Other Hardware/Instruments : N/A
CPU : Pentium
RAM : 128
Vendor : Dell

Customer Information :

Alice Dobie-Galuska
Indiana University
US
adobiega@indiana.edu
Ph: (812) 855-4442

[markwysong@my-deja.com]

In article <393fb577@newsgroups.ni.com>,
"Alice Dobie-Galuska" wrote:
>
> This is a problem I recently sent to NI support, but did not get much help.
> Do any of you have any suggestions?
>
> Problem Description :
>
> I am trying to count events from a geiger muller (GM) tube over a period
> of 15
> seconds. THe GM tube gives a 20 microsec TTL signal when a decay is detected.
> It also will beep when a decay is detected. I modified the example vi "count
> events" so that it will stop running after 15 seconds. I am using the get
> date/time function to count seconds. The PROBLEM is that sometimes the first
> event is not shown on the count indicator. I hear a beep but the indicator
> says zero. When and if there is a second event (a second beep) the count
> indicator says 2 (indicator goes from zero to two upon the second event).
>
> This is an intermittant problem and only occurs on the first event. It is
> not
> a big deal when I am measuring a sample that has many events in 15 seconds.
> However it is a problem when I am measuring background radiation when it
> is
> possible to get only one count in 15 seconds. When this problem occurs and
> there has been only one event, the indicator will show zero counts.
>
> If I run the count events example several times (about 10) I usually observe
> this problem at least once. I noticed that the problem is solved if the
> first
> while loop from the left in the count events example is removed. But then
> that
> causes the indicator to show the count from the previous run until there
> is a
> count. THis is a problem if there are zero events in 15 seconds (which can
> occur when reading background radiation).
>
> I am stuck. Any suggestions?
>
> NI Software : LabVIEW
> Version : 5.1
> OS : Windows NT
> NI Hardware : labPCplus
> Drivers : Ni-Daq 6.5.1
> Other Hardware/Instruments : N/A
> CPU : Pentium
> RAM : 128
> Vendor : Dell
>
> Customer Information :
>
> Alice Dobie-Galuska
> Indiana University
> US
> adobiega@indiana.edu
> Ph: (812) 855-4442
>
>
Alice,

I've looked at the counting events VI that you mention. The only thing that
I can see upon my first view is exactly when your first event occurs. The VI
first loads the counter with a value; then it reads the counter to get its
last value. Then it enters the first while loop, where the value of the
counter is read and compared to the first value it read. According to the
VI, the counter retains its last value until a TTL pulse occurs on its clk,
where it then loads the value that was passed to it at the beginning.
Therefore, if the TTL pulse occurs between the first Counter Control call
that loads the count and the first counter read, then the first loop has
missed that edge because the value it is comparing the current read to has
already seen the first edge.

If this is the case, my suggestion would be to put an indicator on the front
panel so you know when it has entered that first loop. That way, you know if
a trigger occurs AFTER that point it WILL be counted correctly.

Mark


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