Online Silica Analyzer Faulty

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Thread Starter

Sarmad

We are using a Hach Series 5000 Silica Analyzer to measure the silica concentration in boiler feed water.

The analyzer was taken in service in Feb, 2010. Recently we replenished the reagents and calibrated (Auto Calibration) the device, after calibration the analyzer started showing negative readings of Silica (-400 ug/l) although the concentration of silica in water is normal [2 to 5 ug/l] (as tested by a second analyzer installed in laboratory).

We have calibrated the device, checked the stir bar, checked the lamp and found them all to be alright (as they were before). The sample flow, sample pressure, SOV and sample tubing are all fine and no alarms appear but still the value is negative.

Any idea as to what might be the problem? I would welcome any diagnostic measures or advice.
 
If this problem has occurred immediately after replacing the reagents and subsequent calibration, then I suspect that your reagents may have been incorrectly connected to the Analyser. Recheck them. Are the correct reagents connected? Are they in date? Where the reagent lines adequately primed when reagents changed?Your Analyser may be "zeroing" on a silica standard, and "spanning" on a zero solution, leading to negative readings. Because of incorrect connection, the reagents may be being added to the sample cell in the wrong order, effecting the colour development, and hence Detector voltages.

Some HACH Silica Analysers deliver the reagents to the sample cell via a pressurized cabinet system. Check that the cabinet is at the correct pressure (usually a diagnostic alarm will be announciated on the Analyser if this is the case).

Also consider checking the Detetctor function, as well as the Lamp. Maybe run a calibration, and if there is a decent diagnostics facility with the Analyser, monitor Detetctor voltages at various stages of the calibration, particularly at each stage when each reagent is added. This may help to pinpoint at what stage of the calibration that you may have a fault. Check how clean the cell is too, taking care not to scratch it, as this could scatter light away from the Detector, affecting operation.
 
Thanks for the reply, it was very helpful. The reagents have been connected correctly and they are in date. Yes, the reagent lines were primed when the reagents were changed. The cabinet is pressurized and there are no alarms persisting on the analyzer. We are communicating with HACH and hope to resolve this issue in the coming days.
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One more thing, if we pour the Standard solution (500 ug per liter) in the measurement chamber the analyzer reads it as -500 ug per liter. Thanks again.
 
Problem Solved.
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An incorrect value of Reagent Blank was entered in the Analyzer. The Reagent Blank value is found on the Bottle of Molybdate Reagent and it is different for every other bottle. So whenever the reagents are replaced a new value of Reagent Blank needs to be entered.
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Thanks for the help and support.
 
E

Esoteric Stone

HACH 5000 Silica Analyzer Problem:

Correct Reagent Blank Value is entered
Checked all the associated hardware

Analyzer is giving correct value of silica standard i.e. 504 microgram/liter (autocalibration)

However analyzer is not giving the correct value of the sample. Sample value is less the 10 microgram/liter as per laboratory test but analyzer is giving 37 microgram/liter. This value fluctuates from 12-106 microgram/liter but never comes below 10 microgram/liter.

Any suggestions . . . ?
 
We have different issue with these analyzers. they calibrate nicely with 500 ppb standard, but read negative on demin water. we have 5 in a row. 3 read negative number and other 2 positive ( 0.2 - 0.9). the grab of demin with DR/4000 gives 4 - 6 ppb. any idea?
 
B

Bob Peterson

what is the lower limit of detection on your analyzer? I seem to recall 500 ppb as being the lower level of detection on some of them.

anything below whatever their lower limit of detection is and the reading is meaningless.

--
Bob
http://ilbob.blogspot.com/
 
I just had a similar problem. My 2008 5000 Silica Analyzer would not calibrate. It was reading about half (266 u/l) with the 500 Standard. I changed the bulb, cleaned all sample chamber, and the sensor. I was ready to order a new colorimeter sensor for it. (Around $1300) Taking into consideration that ours only runs when the plant is running. I ran De-Mineralized water through the sample chamber for over an hour and ran a good calibration.

Let me know if this helps.
 
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