Tft100 vS hach pocket colorimeter II

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lindsaygib

lindsaygib

Active member
Jun 28, 2022
35
Las Vegas
Pool Size
14500
Surface
Plaster
Chlorine
Liquid Chlorine
I was testing my water with the Hach colorimeter until my test kit came in the mail. The tft is testing the free chlorine at less then half of the hachs results. My husband works for the water district, uses colorimeters, and just calibrated it to make sure it was correct. He says there’s no way it’s off. What could I be doing wrong?? I got my test water, added the dpd while on the speed stir, dropped 8 drops of the r-0871 in and then it became clear. So that’s 4ppm, right?? My Hach says it’s 8.8ppm.
 
I would ask this first - how was the Hach colorimeter “calibrated”? Most of the residential pool colorimeters don’t do calibrations … you have to send them back to the manufacturer for that. So what exactly did your husband use to calibrate the unit and what procedure did he use to adjust the units output?
 
JoyfulNoise said:
I would ask this first - how was the Hach colorimeter “calibrated”? Most of the residential pool colorimeters don’t do calibrations … you have to send them back to the manufacturer for that. So what exactly did your husband use to calibrate the unit and what procedure did he use to adjust the units output?
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It’s not a residential pool colorimeter, it’s a professional grade colorimeter. I shouldn’t say it was calibrated, it was tested to see if it was within range using the manufactures gel color samples. He tested all the highs and lows and they were accurate.
 
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duraleigh said:
Welcome to the forum :wave:

The FAS/DPD test is about the most precise and dependable chlorine test there is for our purposes. I assume you are using a 10 ml water sample, right?
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And thank you for the welcome!!! I’ve been reading on here non stop for the past couple weeks and it’s been so helpful!!
 
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You certainly are doing nothing wrong in your test method.

There is another test in the TFTestkit that may give you some insight. It is the R-0600 reagent and a comparator block. Run that test and see what it tells you.

That test is not particularly accurate but it is really dependable so let's see what it says.
 

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duraleigh said:
You certainly are doing nothing wrong in your test method.

There is another test in the TFTestkit that may give you some insight. It is the R-0600 reagent and a comparator block. Run that test and see what it tells you.

That test is not particularly accurate but it is really dependable so let's see what it says.
Click to expand...

I checked that and it looked like it was in the 5-10 range. So the fc read a 4, that was 5-10, hach was 8.8, and my test strip looked to be about a 4. I’m all over the place! And I’ve been having my husband test too so I know it’s not just me.
 
duraleigh said:
You certainly are doing nothing wrong in your test method.

There is another test in the TFTestkit that may give you some insight. It is the R-0600 reagent and a comparator block. Run that test and see what it tells you.

That test is not particularly accurate but it is really dependable so let's see what it says.
Click to expand...

And I got the tf-pro if that makes any difference. I know I read at some point on here that one of the tests was off in the beginning with the pro but I didn’t think it was the Fc.
 
lindsaygib said:
It’s not a residential pool colorimeter, it’s a professional grade colorimeter. I shouldn’t say it was calibrated, it was tested to see if it was within range using the manufactures gel color samples. He tested all the highs and lows and they were accurate.
Click to expand...

That particular Hach colorimeter is an OBSOLETE item according to the Hach website. Not that I don’t trust your husband but I don’t trust colorimeters that’s have not been been serviced in a while. It’s all well and good to use standards to check the unit is working but these devices can age with time and typically need annual servicing to keep them functioning properly.

A difference of 2X is pretty far off. Make sure you are being very diligent when dispensing drops of titrant (R-0871). Make sure the bottle is completely vertical when dispensing, that the droplets are well formed and falling off the tip and that the droplet rate of dispensing is about 1 drop per second.

Another thing you can test is your tap water. Municipal water supplies typically have less than 1ppm FC and some measurable CCs. I know my tap water is about 0.6-0.8 FC and around 1-1.2 CC. Use a 25mL water sample and you should see something similar (assuming you’re not on well water).
 
One other thing to try and this applies to ALL your drops-based testing. Make sure you get to the end point. Continue to add drops until there is NO FURThER color change. Then subtract that last drop and that is your end point.

It won't make it match up to the Hach but it might get it .5 closer or so.

Time and time again for 14 years (more for some of us!!) the FAS/DPD has proven to be the standard. I would trust it over all the others.
 
JoyfulNoise said:
That particular Hach colorimeter is an OBSOLETE item according to the Hach website. Not that I don’t trust your husband but I don’t trust colorimeters that’s have not been been serviced in a while. It’s all well and good to use standards to check the unit is working but these devices can age with time and typically need annual servicing to keep them functioning properly.

A difference of 2X is pretty far off. Make sure you are being very diligent when dispensing drops of titrant (R-0871). Make sure the bottle is completely vertical when dispensing, that the droplets are well formed and falling off the tip and that the droplet rate of dispensing is about 1 drop per second.

Another thing you can test is your tap water. Municipal water supplies typically have less than 1ppm FC and some measurable CCs. I know my tap water is about 0.6-0.8 FC and around 1-1.2 CC. Use a 25mL water sample and you should see something similar (assuming you’re not on well water).
Click to expand...

That was a good idea to test the tap water. He says the range here is usually .7. He’s been doing water quality sampling here for the last 12 years and tests it daily at work. The hach read a .62, the tfpro read a .5. So at least that was accurate. I don’t know what to think about the higher ranges then.
 
duraleigh said:
One other thing to try and this applies to ALL your drops-based testing. Make sure you get to the end point. Continue to add drops until there is NO FURThER color change. Then subtract that last drop and that is your end point.

It won't make it match up to the Hach but it might get it .5 closer or so.

Time and time again for 14 years (more for some of us!!) the FAS/DPD has proven to be the standard. I would trust it over all the others.
Click to expand...

Actually, I put in 2 drops and then it was clear. So would that be a .5 or less since you’re supposed to subtract the last drop?? If it’s .25 then it would be testing really low for what it should be.
 
lindsaygib said:
Sorry, that was when I was testing the tap water. Hach said .62 and I got 2 drops in the tf pro before it turned clear.
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0.4ppm. It’s a low end measurement so you’re not that far off. And you need to be really accurate with your water sample size.

I would be more inclined to believe the colorimeter is not acting linear on the higher end. Colorimeters are usually only linear over a fixed range. We have seen no end of problems with digital testers and water analysis. Trust the chemistry and not the colors …
 
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