Test and Measurement Forum

PH probe STILL wrong

Post any questions about our parallel port products

PH probe STILL wrong

Postby cornz » Wed Oct 06, 2010 8:49 pm

OK, angry mode.

I've just decided to give my DrDaq another go, as, after the fiasco i had last year with the Ph probe i gave up and bought a 10 quid ph probe.

Well, its packed in so i decided to get the Drdaq out as my fish are important to me.

the (replacement) ph probe was/is still in its little bottle and still wet. So i set it up. Well, the reading are STILL way off the mark.
Ph4 calibration solution reads at 7.29 apx and Ph7 calibration solution read about 6.93.

If i short the pins out, Ph7.
This is a NEW probe. Folks, twice ive had this problem and its not good for a 130 quid bit of kit to fail like this, especially when a 10 quid prob reads the calibration solutions at 3.96 and 7.04 respectively.
Please dont tell me the probe has dried out, it hasnt and has never been out of its bottle.
I recall a post where another user had had ther same issue and had altered some driver from an earlier version which solved the issue.
Does anyone have any ideas??? Else, i have a useless DrDaq for sale.
cornz
User
User
 
Posts: 4
Joined: Mon Mar 16, 2009 7:51 am

Re: PH probe STILL wrong

Postby Chris » Wed Oct 13, 2010 7:42 am

We have looked into this problem and found that the DrDaq & Probe are actually OK, it is a bug in PicoLog itself! The shift in pH away from neutral pH of 7.0 is in error by a division factor of eight. Thus a pH of 7 reads correctly at 7.0 but a true pH of 14.0 would read an indicated 7.875. We will be fixing this in a new release of PicoLog but in the meantime you can correct for this error by using the "Scaling" option in PicoLog for the pH channel.
Start PicoLog and set-up your sample rates as required.
When you get to setting up the pH channel ("Edit DrDaq measurement") click on

"Options" to go to the next dialogue.
In the "Parameter Options" dialogue box
click on "Scaling" to go to the next dialogue.
In the "Parameter scaling" dialogue box,
select "Table Lookup" as the Scaling method
and in the visible text box, type:-

7 7
7.875 14

(note the space in both lines)

This tells the program to equate pH7.0 to pH7.0 on the graph and pH7.875 to pH14.0 on the graph.
Go "OK" to go back through the open boxes and the pH channel should now be scaled correctly.

You can also choose "Formula" instead of "Table Look-up", in which case you can enter into the text box the following equation:-

(X-7)*8-7

Please contact again if you still have problems.

Sorry for the inconvenience.
Chris
Site Admin
Site Admin
 
Posts: 169
Joined: Tue Aug 17, 2010 9:00 am
Location: St. Neots

Re: PH probe STILL wrong

Postby cornz » Wed Oct 13, 2010 8:03 pm

Sadly, doesnt seem to work, here are some results, using ph4 and ph7 test fluid.
V5.16.2

Ok, without the scaling mod..
test ph7 reads 7.00 (SPOT on)
test ph4 reads 7.23
tap water reads 7.06

WITH scaling mod..
test ph7 reads 7.00 (SPOT on)
test ph4 reads 8.84
tap water reads 7.48

Seems incapable of reading a PH less than 7.
What next???? Getting quite urgent this..
cornz
User
User
 
Posts: 4
Joined: Mon Mar 16, 2009 7:51 am

Re: PH probe STILL wrong

Postby cornz » Fri Oct 15, 2010 4:34 pm

I'ts really quiet important i get this working so decided to try a few other things. One of which was to run picoscope and lo-and-behold the Ph reading is now something like accurate.
Its not perfect but Ph4 solution reads at 3.82 which is close enough. However, i dont really want to use pico scope as its total overkill. Are there any shared files that might fix the issue of Pico log, or could you guys possibly offer a solution as to why this is. Is there any logging i can do to assist you in nailing the bug???
cornz
User
User
 
Posts: 4
Joined: Mon Mar 16, 2009 7:51 am

Re: PH probe STILL wrong

Postby PeterF » Mon Oct 18, 2010 10:19 am

Hi,
I think I have found the problem. When using PicoLog with the pH probe and DrDaq, you MUST have the external temperature sensor plugged-in to EXT channel 1 to allow temperature compensation. The readings are invalid and near 7.0 if you do not.
If you use PicoScope to read pH the external sensor is not used for compensation so readings are not affected if the temp sensor is removed. Please try with the temp sensor if you wish to use PicoLog.
Regards,
PeterF.
PeterF
Zen Master
Zen Master
 
Posts: 435
Joined: Thu Jun 07, 2007 10:53 am
Location: Cambridgeshire

Re: PH probe STILL wrong

Postby cornz » Mon Oct 18, 2010 11:49 am

Ah, thats fair point. I shall attempt this later...

Thanks..
cornz
User
User
 
Posts: 4
Joined: Mon Mar 16, 2009 7:51 am

Re: PH probe STILL wrong

Postby cornz » Tue Nov 02, 2010 12:55 pm

OK, I'm in the process of testing this as we speak..
Only the Ph probe and a temp sensor next to the Ph probe plugged into ext1.

Right, using PLW recorder, the Ph is all over the place, until i set the program "recording" then the Ph reads between 6.9-7.1
in tap water. Using my el-cheapo meter, it reads the same liquid at the same temp as Ph 6.6. - Ph 6.8
Using the calibration solutions, the results are as below.
In Ph 4 calibration liquid, it (Dr Daq) reads 5.71 - 5.77
My El-cheapo reads 4.1 - 4.3
In Ph 7 calibration fluid it reads 7.21 (better but still almost 25% innacuracy.

I think it's fair to say that there is something wrong with my Dr Daq and has been from Day one, which, in retrospect, accounts for the sudden increase I had in the number of my fish dying when I first started to use the device. It also explains why changing the probe also never made any difference.
What is my next step folks please...
cornz
User
User
 
Posts: 4
Joined: Mon Mar 16, 2009 7:51 am

Re: PH probe STILL wrong

Postby PeterF » Wed Nov 03, 2010 8:40 am

Hi,
I am sorry you are still having problems with your pH probe. Could you please now come over to Help-desk so I can sort out a replacement for you:-
support@picotech.com
Just put "Attn-Peter" at the start of your message and I will tell you what we need to have.
Regards,
PeterF.
PeterF
Zen Master
Zen Master
 
Posts: 435
Joined: Thu Jun 07, 2007 10:53 am
Location: Cambridgeshire


Return to Parallel Port Products

Who is online

Users browsing this forum: No registered users and 0 guests