Doubt Regarding Chlorine Analyser

What are the two readings?

Both readings suggest that, though biologically protected, the water is unpalatable. Those readings would be obtained from a municipal swimming bath, for example.

2.5 is 3, to 1 significant digit!

why the chlorine levels showing by online chlorine analyser and handheld photometer are different?

They are the same to 1 significant digit, as already indicated.

What are the specifications of the two instruments?

It is to do with the numbers of significant digits in the displays. One of them has only 1 significant digit. The other has two. Rounding-up convention makes 3 from 2.5. If it were 2.4 then rounding down would make 2.

The on line analyzer has no sample tube or sample preparation involved. It is a direct read and therefore, if calibrated properly, will give a far more accurate reading of chlorine levels.

The photometer, most probable utilizing the same principles of analysis, still requires a sample tube, vial....., this will give refraction of the sample valve, or dust collected or air bubbles introduced in the sample gathering...........This accounts for the offset of readings one from the other

cyoung and others have given you many possibilities.

Direct reading versus sample testing.

Have you made necessary adjustments for temperature (inside pipe versus in your hand held device), agitation in the sampling process, exposure to atmospheric conditions and every other aspect of taking the sample.

Plus the requests for detail of equipment resolution.

Alternatively it could be that one of your instruments is actually incorrect (or improperly calibrated).

Your question begs knowledge of methodology of testing. Your hand held photometer should use only the DPD method and not orthotolidine method. Orthotolidine will give you a false low reading if the chemical reagent orthotolidine is old (> 6 months) or has been exposed to light. I am not sure if that is happening in your case.

DPD chemistry is well understood and the absorbance curves with respect to chlorine is established and consistent. Hach industry claims; "By measuring the sample blank absorbance with each measurement to provide an automatic zero reference, the accuracy of the analyzer's calibration is maintained".

There is one area where human error is introduced and that is in zeroing the reference. I have watched custodians of schools that would use a cuvette with a lot of humidity on the surface and take zero checks. That would false the zero setting. In another case, the operator would not clear the cuvette under testing and get a false high reading. That is a very dangerous scenario as the reading may be construed as a good residual. Thankfully, anyone now taking tests for drinking water needs to undergo basic schooling on chlorine procedures and be licensed in NA. Your cuvette for zeroing or testing must be wiped clean with lint free paper or cloth. I throw this information out as I do not know the details of your testing but can be as source of errors.

You should contact your supplier of both the hand held photometer and the in-line tester for methods of calibration. I am sure they will have some downloads available on line. Most suppliers will be more than willing to help and I would contact them for advice and direction.

Your residual does sound a tad high. However, free chlorine can vary from a minimum 0.2 mg/L to 5 mg/L as a maximum and is allowed for drinking water. The recommended guideline maximum is set at 2.0 to prevent taste and odour problems. Hope this info is of some help.

If you are using a calibrated instrument why are you cross checking it?

It is quite unlikely that two different instruments measuring the same sample will give exactly the same reading.

As others have said one instrument has 1 significant digit and the other has 2 significant digits. If you measured the sample with an instrument with 3 significant digits then you will get a different reading.

Whether or not this is acceptable depends on the application.

Insufficient information. First, you should define what you mean by "free chlorine", some think of this as only the HOCl in the water. Some think of this as "uncombined chlorine", i.e. not chloramine, iodine, or bromine. If both instruments are photometric, and both are using the DPD method, the online instrument may in fact read higher due to longer delay time in reading, during which some of the chloramines may react. Free chlorine is meant to be as near an instant reading (say within the first 30 seconds) as possible. Some electrochemical instruments (such as Hach) utilize a totally different methodology and will not produce exactly the same results, but the results you have are too disparate to be accounted for by this.

I might start by double checking the reagents.

It is possible your online analyzer is measuring TOTAL chlorine and your hand held FREE.

First question should be this: what is your online chlorine monitoring method and instrumentation? Is it DPD or amperometric?

Next question would be - in case it's DPD - did you calibrate the analyzer? If yes - go to default calibration and then cross check the readings again.

In case it's amperometric - calibrate the analyzer against DPD instrument (online, handheld, or bench) and then cross check. By definition, the amperometric analyzers read only what they are told to read by the reference method, such as DPD colorimetric. At the same time, DPD-based instrumentation is normally accurate without additional calibration.

If these procedures helped - live in peace, if not - check your sampling timing (the online analyzer reading at the very time you took the grab sample), reagents and handheld device (verification/calibration records, etc.) and if nothing suspicious found - contact the manufacturers of your instrumentation, both online (start with that) and handheld units.

I'm going to have to ask this again, because until it's answered everything else is *issing in the wind:

What are the specifications of the two instruments?

Normally, this type of instruments can easily differentiate between 2.5 and 3.0, therefore I believe the specs is not an issue, however, one should always make sure.

Agreed - but (until we know better) they could be antiques dragged out of a storage room, not having been calibrated for that last 15 years.

surendra1024, manufacturer & type number for each would do (you could make us do the looking-up if it's too much effort to supply links). Would be handy to know whether they're in calibration.