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Accuracy of TDS meter


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#1
CoralWhisperer

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Been running my own rodi for a couple months now, itsa Kent 5-stage 70gpd, I bought it used off CL and looks to be about 1/3 used up with the color changing resin.

Only just today did I finally buy the dual TDS meter , shows input and output TDS and is connected directly into the quickconnect lines, Im sure youre all familiar.

Anyways, Im getting 0.00 on the output, and the input fluctuates between 100 and 112(havent seen it higher or lower in 8hrs).

Now my house is 94 yrs old, but I did replace all the plumbing with new copper pipes all the way to the main.

Im just wondering if this meter is accurate right out of the box, it says on the back of the unit to calibrate with NaCI.
Is that on an "as needed" basis, or do I gotta do it in the beginning.

Am I getting accurate readings?

god you cali guys suck so bad with your overabundance of awesome corals. damn you!! lol


#2
AZDesertRat

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The problem with the inlines is they are not really temperature compensated so can be significantly off. The reason is unlike the preferred handheld meters, the temperature probe on the inline is actually sensing air temperature and not water temperature so if the water and air temps are not exactly the same, the readings can be off like as much as 2 TDS for every degree C they differ is the numbers I have seen in literature.

I have seen the probes on the inlines disassembled and can tell you personally this is true, the temp probe is located in the fatter part of the probe close to where it sticks in the tee, right inside the little rectangular window you can see. Its in the air not the water and they are rarely the same temp.

On the other hand a good handheld such as the TDS-3 or TDS-4TM, both about the same $25 and from the same manufacturer, HM Digital, have the temp probe in the water so can be extremely accurate. I myself own two dual inline HM-1 meters along with the HM Digital COM-100 handheld and can tell you from experience they arrely if ever read the same. I never even turn the inlines on any more, I always use and depend on the COM-100 for accuracy, especially for the low end RO and RO/DI readings.

I never recommend the inlines for this reason.

Edited by AZDesertRat, 12 April 2012 - 06:01 AM.

16G Bowfront Nanoreef. 100G SPS dominated reef.

#3
C.I._Reefer

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The problem with the inlines is they are not really temperature compensated so can be significantly off. The reason is unlike the preferred handheld meters, the temperature probe on the inline is actually sensing air temperature and not water temperature so if the water and air temps are not exactly the same, the readings can be off like as much as 2 TDS for every degree C they differ is the numbers I have seen in literature.

I have seen the probes on the inlines disassembled and can tell you personally this is true, the temp probe is located in the fatter part of the probe close to where it sticks in the tee, right inside the little rectangular window you can see. Its in the air not the water and they are rarely the same temp.

On the other hand a good handheld such as the TDS-3 or TDS-4TM, both about the same $25 and from the same manufacturer, HM Digital, have the temp probe in the water so can be extremely accurate. I myself own two dual inline HM-1 meters along with the HM Digital COM-100 handheld and can tell you from experience they arrely if ever read the same. I never even turn the inlines on any more, I always use and depend on the COM-100 for accuracy, especially for the low end RO and RO/DI readings.

I never recommend the inlines for this reason.


I wanted to ask you, on those meters that you recommend, is the accuracy compromised when the batteries start to get low? I have a cheapo handheld that seems to be fairly accurate, except when the batteries are somewhat depleted. If i get one of these meters will i have the same issue?

#4
AZDesertRat

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I honestly don't know if batery condition has an effect on accuracy or not.
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#5
CoralWhisperer

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thanks AZ DR!

So, exact numbers aside, am i within an acceptable range?
I live in Sacramento Ca, and it has been said time and again that we have some of the best tap water in the state(if not the country).

WITHOUT BUYING MORE EQUIP, would you accept these readings as is, and go on with your life?

god you cali guys suck so bad with your overabundance of awesome corals. damn you!! lol


#6
nor_cal_nano

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thanks AZ DR!

So, exact numbers aside, am i within an acceptable range?
I live in Sacramento Ca, and it has been said time and again that we have some of the best tap water in the state(if not the country).

WITHOUT BUYING MORE EQUIP, would you accept these readings as is, and go on with your life?


When I tested my tap here in Natomas, I think I got something like 75tds. Not too shabby for tap I guess but it sure tastes like ####.

Anyways, if I was you, and was short on cash, I'd probably just move on for the time being. If you're having algae issues however, you might as well invest in a handheld and see if you can rule out your rodi as the problem.

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#7
AZDesertRat

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Place the HM-1 probes after the RO membrane and after the DI cartridge. You know your tap water TDS is around 100-120 and isn't going to change much and you have no contol over it. You do have control of the RO membrane and the DI resin so those are the readings you need to pay the most attention to. The RO only TDS should be 96-98% less tha the tap water, otherwise you are taxing your DI resin and spending money needlessly on DI replacements.

Say your tap water is 112 as you have seen, the RO only as measured on the IN probe of the HM-1 should be 2-5 TDS at most if the membrane is functioning correctly. The OUT probe on the RO/DI should always be 0 TDS , if it starts to show TDS after it has been running a minute or two its time to change the resin or cartridge since things like phosphates, silicates and nitrates are weakly ionized and the first things to be released by exhausted DI resin.
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