# pH pens and low KH



## Sacha (6 Aug 2014)

Been having a few issues trying to get an accurate pH profile so I'll try and make this as clear as possible...

I have a cheap pH pen from eBay. From what I can tell, it works extremely well. My reason for saying this is that, even after 3 months of sitting unused and dry, it still gets the pH reading perfect when I put it into buffer solution. I put it into buffer solution 7, and the pH pen reads 7 or 7.1. I put it into buffer solution 4, and the pen reads pH 4 or 4.1. So it clearly holds its calibration.

However when I tested the tank water with it this morning, it read 6.6. I find this hard to believe. The reason being that when I tested it, the Co2 had been off for 15 hours, and there is enough surface agitation to drive off any residual Co2 during that time. The KH of the tank water is 2.2. Based on the KH of 2.2, a pH of 6.6 would mean a Co2 concentration of around 17-20 PPM. That clearly isn't right, considering all the Co2 should have gassed off during the night. The concentration should definitely be below 10, meaning a pH of at least 6.8 or 6.9.

Secondly, I have taken pH profiles in the past with this pen. According to the pen, my pH took about 7 hours to drop from 7.2 to 6.4. That is clearly awful. However, the fish started to suffer at the end of the photoperiod, so I was left with no choice but to reduce the Co2 injection, and accept an absolutely terrible pH profile, with what I assume to be poor Co2 dissolution.

Now I'm starting to think that maybe the pH profile is bogus, and the pen is inaccurate.

So I suppose my question is this: is it possible that the pen is giving incorrect readings in the tank, despite the fact that it is extremely accurate at measuring the pH 4 and pH 7 buffer solutions? Will the pH 4 buffer solution have a higher KH than the tank? Is the pen unable to accurately measure pH of water with such low KH?

Or is it just that my pH really is 6.6? And if so, why is the Co2 concentration so high after a night of no Co2 and moderate surface agitation?

If the pen is giving me correct readings, then I have a serious problem with my Co2 injection, because I can't even achieve a full pH drop after 7 hours of injection.

If the pen is unable to accurately measure pH in such soft water, would a more expensive pen like this one be able to give me accurate and reliable pH readings?

I hope this all makes sense and I'm very grateful for any responses.[DOUBLEPOST=1407329293][/DOUBLEPOST]Also, let's say my pH is really 6.6. If I wanted to achieve a full pH drop before lights on, that would put the Co2 concentration well above 100 PPM. 

I've never really understood this "full pH drop" gospel. In most tanks with a KH above 2, a full pH drop would put the Co2 well above 50 PPM and into lethal amounts. So why do we aim to have a full pH drop when that means having lethal doses of Co2?


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## GlassWalker (6 Aug 2014)

I remember reading on another forum that Seneye units had difficulty reading pH with low KH. I think in that case, it was around 4 where it started to have problems. I have to wonder if something similar is going on here.

I also have a cheap pH pen from ebay. That also calibrates ok, but I don't trust the readings it gives. I do have one more observation, which is to leave it submersed in the liquid to be measured for some time, at least a minute. I don't know if it is temperature or some other effect, but it does take a while to stabilize.

As hard as they may be to read, if you have one, a liquid based pH test may be useful as a 2nd opinion?


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## Sacha (6 Aug 2014)

That was my very long rant about Seneye, about 2 years ago it started. Took such a long time for me to get a straight answer out of them!


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## GlassWalker (6 Aug 2014)

Ok, so you've been though this before  I debated moving my Seneye into my to be CO2'd tank but then remembered that so thought it wasn't worth it. I'm running on the low side of 4 KH so I guess it would be unreliable in that condition. It can stay in my marine tank where I struggle to keep KH above 7.

I do have and use a TMC pH controller now. Well, I use it as a meter, not a controller. These aren't cheap new but seem to go for peanuts used, which is how I got mine. As their common application in with marine aquariums, the probe condition is important to look out for which may be part of that depreciation. When I got mine, the cables and connectors had corroded badly. I replaced them and it seems to work fine after a calibration. Probes are also expensive to replace, so it was a bit of a gamble. It has two point calibration, where you adjust offset then slope. I found I could do either 4 and 7, or 7 and 10, but not all 3 at the same time. Error was up to 0.15 units for the point not calibrated so not that bad anyway.

Having written all that, I've not verified it yet at low KH. I was using it to observe tank pH prior to adding CO2. I estimated levels were about 5ppm based on reported pH and measured KH, so if that is about right for a non-CO2 tank it can't be far out.


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## Sacha (6 Aug 2014)

My pH pen is now telling me the pH is 6.1. If that's true, then the Co2 concentration in the tank is around 50 PPM. Can that be correct, considering the fish aren't struggling?


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## Iain Sutherland (6 Aug 2014)

With low kh you shouldn't have an issue dropping your ph. My tank sits at kh3-4 ish and ph drops from 6.6 - 3.5ish within 30 mins of the co2 coming on. 

That's not to say my co2 is great in 30 mins, it still runs for 3 hrs prior to lights as the plants tell me that's what I need and the fauna are happy. 

I don't understand the science side of it but I'm a sceptic to the charts as in theory all the fish should be dead and not sparring happily.

I would trust what's in your tank more than the pen and charts for sure.


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## Sacha (6 Aug 2014)

The fauna are happy at Co2 levels well in excess of 200 PPM?


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## Andy Thurston (6 Aug 2014)

Tom barr states kh/ph/co2 charts will always underestimate co2 levels in the tank. Theres an explaination in this thread
http://www.barrreport.com/showthread.php/12022-CO2-pH-KH-table[DOUBLEPOST=1407350094][/DOUBLEPOST]And don't trust that Chinese ph pen, i've got one and its rubbish compared to the hannah one i broke being heavy handed


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## Sacha (6 Aug 2014)

Hmm interesting. The Hanna one I linked to- will that be reliable in low KH water? 

According to my pen, I have only achieved a 0.7 point drop today, from 6.6 to 5.9. Also it's taken about 7 hours to drop that far. What could cause the pH drop to be so slow?


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## Andy Thurston (6 Aug 2014)

The pen you linked is the one Iain uses. You could always buffer your water with extra kh so you know your pen is giving a good reading. Charts and test kits are inaccurate i only trust my tds meter for water quality, i use my ph pen to time my co2 period and find the lowest ph value, the drop checker is a quick day to day indicator.  with a 1.4ph drop my fish were happy and drop checker was green yellow about 3hours after lights on(crappy delay on colour change)


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## X3NiTH (6 Aug 2014)

From observation using a Weipro pH controller the probe has difficulty getting a reading in 0KH and TDS 0. It's fine with 0KH when there's a TDS reading. Your KH of 2.2 may not be accurate as you see that you do not correspond to the chart. From my own experience I deduced my tested KH not to be accurate by looking where I was on the charts in relation to my pH drop and what the drop checker was telling me, my measured 2.5 was in reality 0.5 or less according to the charts, my pH drop from start point at lights on to getting the drop checker green still correlated with the charts but I had to shift my results to make them fit. Very confusing, so I stopped looking at the charts and instead concentrated on the drop checker that had a known KH and just concentrated on making sure it went green. Through adjusting a few things to ensure enough oxygen was in the water before adding any co2 and that I get the one point+ drop in pH by lights on my plants and fish are fine and the drop checker is green/yellow. This all sounds easy but to get the tank where it needs to be I'm now chewing through a soda stream bottle in 15 days for a 28L volume than 30 days previously just by ensuring the increase in oxygenation of the water to stop the fish gasping.


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## Spnl (7 Aug 2014)

KH has no effect on performance of the pH meter , but TDS (or conductivity) does.
You probably notice that the reading stabilises much faster in the calibration buffers than in water, because the buffers are strong solutions of ions.
It is usually said that at TDS of less than about 40 or so pH pens don't work, but mine is not very reliable even at around 90 or 100ppm, the reading depends a lot on how much the water is stirred. I can get an accurate reading but only with care.
PH pens are high maintenance, and as "test kits" go, they don't get much flakier. There is an excellent post on Apistogramma by "Regani" on how to maintain them properly.

I have to say I think a pH of 3.5 due to bubbling CO2 into the tank is not possible, you would need to increase the pressure to >1 atmosphere of CO2 to achieve that.
The pH/KH/CO2 charts are ok for natural water, but because they assume there are no sources of alkali other than KH and the only acid is from CO2, they give meaningless values for fish tanks (or even tap water!).

You read a lot of strange stuff about pH on various fish forums, I wonder how much of it is down to erroneous pH pen readings.
If the results look unbelievable I always follow up with a chemical test kit, they are not perfect either but at least you have two independent measurements.
Personally I don't understand why people don't use drop checkers, which seem to be out of favour for some reason.


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## ceg4048 (7 Aug 2014)

Increase your KH to get a better pH reading.

Cheers,


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## Sacha (7 Aug 2014)

I appreciate all the replies. 

But the problem still stands. Every single pH profiling thread on this site lists pH and KH readings which, according to the charts, equate to lethal amounts of Co2. Does that mean that the charts are horrifically inaccurate? Or that everyone's pH meters are broken? 

Like I said, in most tanks, a "full pH drop before lights on" means a lethal dose of Co2. So I don't know why we are encouraged to aim for that.


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## ceg4048 (7 Aug 2014)

No Sacha, your interpretation of the information provided to you is horrifically inaccurate. I have never instructed anyone to use the data in the charts to determine CO2 concentration levels. In fact I always tell people to NOT use the chart to determine absolute values of the CO2 concentration level. What we are using the charts for is to determine the differences. Mathematically, by using differences in the chart readings we are essentially subtracting the error associated with reading the charts as a single point calculation. 

When we use the expression "ph drop" we are talking about the pH difference.
See the post http://www.ukaps.org/forum/threads/reliability-of-drop-checkers.19499/page-2#post-198360

Cheers,


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## Sacha (7 Aug 2014)

I fully understand all of that. 

Using the charts as an extremely rough guide, it's clear to me that in most water (above KH 2.5), a full pH drop before lights on means we have injected a dangerous amount of Co2. 

Can't you see that? In most water, a full pH drop means we have injected well over 30 PPM.


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## dw1305 (7 Aug 2014)

Hi all,





Spnl said:


> KH has no effect on performance of the pH meter , but TDS (or conductivity) does.
> You probably notice that the reading stabilises much faster in the calibration buffers than in water, because the buffers are strong solutions of ions. It is usually said that at TDS of less than about 40 or so pH pens don't work, but mine is not very reliable even at around 90 or 100ppm, the reading depends a lot on how much the water is stirred. I can get an accurate reading but only with care. PH pens are high maintenance, and as "test kits" go, they don't get much flakier. There is an excellent post on Apistogramma by "Regani" on how to maintain them properly.


 That's the one, "Regani" is excellent on water chemistry <http://www.apistogramma.com/forum/threads/a-note-on-ph-meters.14870/>, I've learnt a lot from him.

You can add a salt like NaCl (has to be NaCl, "table salt" has magnesium carbonate (MgCO3) added to it) to the water sample, this doesn't effect the pH but makes the reading stabilise much more quickly.  Have a look at this one (again via the excellent Apistogramma forum) <http://www.apistogramma.com/forum/threads/ok.12048/>.





Sacha said:


> Every single pH profiling thread on this site lists pH and KH readings which, according to the charts, equate to lethal amounts of Co2. Does that mean that the charts are horrifically inaccurate? Or that everyone's pH meters are broken?





Spnl said:


> The pH/KH/CO2 charts are ok for natural water, but because they assume there are no sources of alkali other than KH and the only acid is from CO2, they give meaningless values for fish tanks (or even tap water!).


 This is the reason in "Spnl"'s very informative post.

Additionally I'd be very surprised if many people pH meters are accurate in softer water, pH is quite a complicated measurement and a pH value on its own doesn't tell you very much. That is one of the problems that occurs when people extrapolate from pH readings in buffered systems (like the Rift Lakes or Marine) to soft water systems. You get this a lot on some Cichlids forums, where certain aspects of keeping _Apistogramma_ etc. and Lake Tanganyika cichlids coincide (diet, low BOD water, stocking etc), but they come from water at either end of the carbonate hardness spectrum and often successful Tanganyikan cichlid keepers attribute problems in soft water to "_lack of pH stability_".

In buffered systems pH will be fairly stable and changes in pH indicate significant changes in water chemistry, but in systems with few salts, and little buffering, pH is a "moveable feast".

cheers Darrel


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## ceg4048 (7 Aug 2014)

> Can't you see that? In most water, a full pH drop means we have injected well over 30 PPM.



No, you are missing a couple of key points. The charts are a very rough guide because they assume that the KH of the water is due to Carbonates - and that's not really true for most tap water. Your KH test kit does NOT measure KH, it measure alkalinity which is affected by a lot more ions than Carbonate. So right off the bat there is n overestimate of CO2 concentration on the charts. You will never have the opposite situation where the charts underestimates the CO2 concentration level. The second issue is that the pH readings are corrupted by acids in the water so the pH reading again, always overestimates the CO2 concentration.

So, both parameters are overestimates and that's why you're mistakenly assuming that we're adding well over 30 ppm. The 1 unit pH drop for medium to medium-high KH readings is about as good an estimate as we can get because no one knows exactly how much the non-Carbonate ions are contributing to the KH reading but generally that difference accounts for the error.


Cheers,


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## GlassWalker (7 Aug 2014)

https://www.kickstarter.com/projects/4612189/oco-the-worlds-first-co2-meter-for-aquariums

Anyone here sign up for one of these? Direct measurement of CO2 would eliminate the problems described here using pH as an indicator. While the operation method wasn't stated last time I looked, in research I found a method to monitor the spectral absorption of CO2 which sounds like a likely candidate to be used here.


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## dw1305 (7 Aug 2014)

Hi all,
First of all I should say I've never added CO2 and have no intention of ever adding it, but because of the combination of 4dKH/bromothymol blue fluid and air gap, a drop checker will give you the actual CO2 concentration as shown on the CO2/dKH/pH chart (making the assumption that the chart itself is accurate).  

Bromothymol blue is just a narrow range pH indicator.



 
*Air gap*
Because of the air gap you are only then looking at the HCO3 ~ CO2 (as H2CO3) equilibrium, and other acids, sources of alkalinity etc in the tank water are irrelevant. The downside is the time taken to equilibrate, which means that the CO2 level shown is not the current level.
*
DIY 4dKH solution*
If you wanted to be absolutely sure you could make up your own drop checker fluid from NaHCO3 (12g NaHCO3 in 1litre of RO, diluted 1:100 in RO for the drop checker, will be ~ 4dKH)  and bromothymol blue pH indicator (you can buy bromothymol blue indicator for £5 from "Better Equiped" or similar).

cheers Darrel


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## ceg4048 (7 Aug 2014)

GlassWalker said:


> I found a method to monitor the spectral absorption of CO2 which sounds like a likely candidate to be used here.



Spectral absorption probes already are available and cost lots of money. Mass spectrometry is one of the techniques used in £1000 equipment.

Cheers,


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## GlassWalker (7 Aug 2014)

The point of that kickstarter was to develop a unit at a more affordable price for use in planted aquaria. I didn't sign up for it, but recall some on another forum who did so will wait to see how it performs. If it is any good, I can consider getting one later.


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## X3NiTH (7 Aug 2014)

Plantbrain has a co2 probe in the works.

http://www.barrreport.com/showthread.php/14164-CO2-probe-meter


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## GlassWalker (7 Aug 2014)

$125-200 lasting a year or two, plus you need a pH meter to plug it into... doesn't get me excited! The other CO2 monitor I linked to I think has more long term potential, assuming it performs to expectations.


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## Sacha (7 Aug 2014)

Thanks for all the input guys. 

I've always thought that it would be great to have a more accurate drop checker. What I mean by that is one similar to the JBL one, but with a very precise colour chart which indicates the estimated Co2 level in PPM. So you could compare the colour of the bromo to the colours behind it and check the PPM of Co2.


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## dw1305 (8 Aug 2014)

Hi all,





Sacha said:


> I've always thought that it would be great to have a more accurate drop checker. What I mean by that is one similar to the JBL one, but with a very precise colour chart which indicates the estimated Co2 level in PPM. So you could compare the colour of the bromo to the colours behind it and check the PPM of Co2.


 You could make your own colour chart using the pH indicator chart for bromothymol blue and CO2/dKH/pH chart.  Something like the CalAquaLabs one, but with actual figures.




cheers Darrel


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## Iain Sutherland (9 Aug 2014)

You already have an accurate drop checker.... Your plants.  Ppm is pretty irrelevant if your plants are failing... Play with co2 and flow until your tank is going the way you want.
While I can see some use for a co2 devise I can also see the innumerable number of threads created by being tunnel visioned by it and still having issues (same as drop checkers now)


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## Jaap (2 Sep 2014)

So how would you go about measuring the CO2 concentration in an aquarium. For example I have a kH of 9 and a pH of 7 and after 1 hour I have a pH of 6.5...what would be approximately my CO2 ppm then? 



ceg4048 said:


> No, you are missing a couple of key points. The charts are a very rough guide because they assume that the KH of the water is due to Carbonates - and that's not really true for most tap water. Your KH test kit does NOT measure KH, it measure alkalinity which is affected by a lot more ions than Carbonate. So right off the bat there is n overestimate of CO2 concentration on the charts. You will never have the opposite situation where the charts underestimates the CO2 concentration level. The second issue is that the pH readings are corrupted by acids in the water so the pH reading again, always overestimates the CO2 concentration.
> 
> So, both parameters are overestimates and that's why you're mistakenly assuming that we're adding well over 30 ppm. The 1 unit pH drop for medium to medium-high KH readings is about as good an estimate as we can get because no one knows exactly how much the non-Carbonate ions are contributing to the KH reading but generally that difference accounts for the error.
> 
> ...


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## ceg4048 (2 Sep 2014)

Please review Iain Sutherland's response in post #27. If the plants are dying then the CO2 concentration is too low. If they are thriving then the CO2 concentration is high, if they are doing fair to middling then the CO2 concentration is fair to middling. The number is irrelevant. We only use the numbers as a guide. We only use the DC as a guide. No number and no DC can tell you more than you can see with your own two eyeballs.

Cheers,


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## Jaap (2 Sep 2014)

ceg4048 said:


> Please review Iain Sutherland's response in post #27. If the plants are dying then the CO2 concentration is too low. If they are thriving then the CO2 concentration is high, if they are doing fair to middling then the CO2 concentration is fair to middling. The number is irrelevant. We only use the numbers as a guide. We only use the DC as a guide. No number and no DC can tell you more than you can see with your own two eyeballs.
> 
> Cheers,


Can you suggest a reasonably priced good ph pen?


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## ceg4048 (2 Sep 2014)

Check these from Hanna Instruments. The three at the top of the page are excellent but pricey.
http://www.amazon.co.uk/s/ref=nb_sb...words=hanna ph meter&sprefix=hanna+ph,aps,230

Cheers,


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## Jaap (2 Sep 2014)

ceg4048 said:


> Check these from Hanna Instruments. The three at the top of the page are excellent but pricey.
> http://www.amazon.co.uk/s/ref=nb_sb...words=hanna ph meter&sprefix=hanna+ph,aps,230
> 
> Cheers,


What is TDS that the third one measures?


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## ceg4048 (2 Sep 2014)

TDS = Total Dissolved Solids = Conductivity (more or less) .

Cheers,


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## John P Coates (3 Sep 2014)

ceg4048 said:


> If the plants are dying then the CO2 concentration is too low.


There is a real danger of people reading and interpreting this statement in isolation. It gives the impression that CO2 concentration is everything. That clearly is not the case. I signed up to this community because I want to better understand the science behind successful growing of aquatic plants. On a personal level, I am not strong enough to get embroiled in arguments. I just wish we could discuss things on an equal terms basis.

There is no evidence to suggest that insufficient CO2 is an issue in the problem that I have been recently reporting.

JPC


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## John P Coates (3 Sep 2014)

Jaap said:


> Can you suggest a reasonably priced good ph pen?


Hi Jaap,

Today, I took delivery of an HM Digital PH-200 pH pen. Please take a look at:

http://www.tdsmeter.com/products/ph200.html

It is too soon to say that I made the right choice - so watch this space!

When you say 'reasonably priced', what did you have in mind? It has to be said that pH meters are, by their very nature, complex pieces of kit. It is possible to pay very little for a pH meter but its accuracy is not likely to be particularly good. If, for example an accuracy of +/- 0.2pH meets your needs, then select one accordingly. If auto-calibration is important to you, then add that to your requirements specification. Also, be aware that the sensing part of a pH meter - the pH electrode - may give you just 18 months' service after which it will need to be replaced. So, choose a meter that will enable you to do this. If you take a look at the above link, it will hopefully give you some ideas of what you need to consider.

JPC


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## Jaap (3 Sep 2014)

John P Coates said:


> Hi Jaap,
> 
> Today, I took delivery of an HM Digital PH-200 pH pen. Please take a look at:
> 
> ...


Would you use it to measure ph reliably?

Would you use it to measure pars?


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## John P Coates (3 Sep 2014)

Jaap said:


> Would you use it to measure ph reliably?
> 
> Would you use it to measure pars?


Hi Jaap,

As I said, it is too soon to know just how reliable the PH-200 is.

I don't understand your second question. By 'pars', are you referring to lighting? The PH-200 measures pH and temperature but not lighting.

JPC


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## Jaap (3 Sep 2014)

John P Coates said:


> Hi Jaap,
> 
> As I said, it is too soon to know just how reliable the PH-200 is.
> 
> ...


I m sorry i messed up the threads...both questions were for the seneye?


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## John P Coates (3 Sep 2014)

OK, not to worry. Perhaps you could put these questions on the seneye thread?

JPC


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## Jaap (4 Sep 2014)

I am between the HM PH-200 and the Milwaukee PH55

http://www.osmotics.co.uk/products/HM-Digital-PH%2d200:-Waterproof-pH-Meter.html

http://www.ebay.co.uk/itm/Milwaukee..._Measurement_Equipment_ET&hash=item19f272d477

The first is more accurate than the second....any suggestions? Are they both reputable companies?


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## John P Coates (4 Sep 2014)

Jaap said:


> I am between the HM PH-200 and the Milwaukee PH55
> 
> http://www.osmotics.co.uk/products/HM-Digital-PH%2d200:-Waterproof-pH-Meter.html
> 
> ...


Hi Jaap,

Yes, the PH-200 is specified as having an accuracy of +/-0.02pH, whereas the PH55 quotes +/-0.1pH. In the UK, they are priced at £50 and £40, respectively. I don't know much about the company Milwaukee Instruments. When I unboxed my PH-200 yesterday, I was disappointed to find that there were only a few droplets of storage solution in the protective cap even though the cap was attached firmly in place. I chose to calibrate it using the sachet of cystals, which I dissolved in RO water.

JPC


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## chandler (9 Jan 2015)

Am I off the mark thinking that if the reason for buying a pH pen is that one wants to do a pH profile to study the CO2 injection it does not really matter if the pen is correctly calibrated/cheap/expensive or not? I mean, in this case we are mainly interesting in the change as opposed to the exact pH level and that should a cheap chinese pen handle too, right? 

Hope so, anyway, since I just bought one


Skickat från min iPad med Tapatalk


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## foxfish (9 Jan 2015)

Umm you will have to calibrate it mate, it wont tell you much if you don't!
Cheap pens like, £10, work fine but if it did not come with any calibration fluid you will have to get some...


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## chandler (9 Jan 2015)

Cheers, foxfish! May well be wrong but I think I've read somewhere that the fluid that comes with a standard "narrow" pH test (6 to 7.6 or something) can be used for calibration? 


Skickat från min iPad med Tapatalk


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## GlassWalker (9 Jan 2015)

The cheap pens have single point calibration which works as an offset. So if you're looking to get a certain relative drop, it could be ok without calibration. You just wont necessarily know the absolute value.

With only a single point calibration there could be slope error regardless if you correct for the offset or not. It can only be accurate at the pH value you calibrate at, with errors potentially building as you go away from that. Probably not to any significant degree within typical aquarium ranges of say 6 to 8 though, but they could build up if you go far from that.


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## Eugine Thomas (20 Feb 2015)

ceg4048 said:


> Check these from Hanna Instruments. The three at the top of the page are excellent but pricey.
> http://www.amazon.co.uk/s/ref=nb_sb_ss_c_0_8?url=search-alias=pets&field-keywords=hanna ph meter&sprefix=hanna ph,aps,23



These are good pens for what we need as aquarists (you might see one of these being used at a school): it'll last a year before you'll need to replace it (two at the very most), and the fluids will cost another £100.



ceg4048 said:


> If the plants are dying then the CO2 concentration is too low. If they are thriving then the CO2 concentration is high, if they are doing fair to middling then the CO2 concentration is fair to middling. The number is irrelevant. We only use the numbers as a guide. We only use the DC as a guide. No number and no DC can tell you more than you can see with your own two eyeballs.



I'm afraid that's probably as good as it'll ever get: too many variables. It's not science: it's (yuck!) an art.

The really basic pH readers that you may have used at school cost a couple of hundred pounds (when you total up the cost of the various different fluids that you need to look after them): you really have to baby these things, and even then, if you want it to remain accurate, you'll need to re-calibrate it at least once a week (or before an important reading); the electrodes also need to be replaced every year or two at the very latest, and usually cost about the same as buying a new pen; there are separate fluids for cleaning, calibrations, storage, and so forth. The better pH testers used for basic university experiments (not that you'd need anything this good for the average aquarist) can cost several hundred pounds, and the professional pH testers that are used for highly accurate readings may cost over a thousand pounds. Those "ebay pens" (the yellow one, right?): put it in the bin before you screw up your tank. A pH reader is a pretty serious piece of scientific equipment and, if you want to look after it, you'd have to be shown how to use it by a laboratory technician in order not to break it straight away: it's not an ipod.

Regards,

Eugine


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## Eugine Thomas (20 Feb 2015)

chandler said:


> it does not really matter if the pen is correctly calibrated



Throw it away: for the love of God!


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## pepedopolous (20 Feb 2015)

Just how inaccurate can an un-calibrated, poorly looked-after pH pen be? I leave mine with the electrode immersed in the aquarium 24/7. I switch it on every now and then and do pH profiles occasionally. I calibrate it every couple of months and bought a new electrode after about a year when in fact the problem was that the batteries were low! I honestly don't see much difference in the results when I calibrate it or when I changed the electrode over.

P


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