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pH pens and low KH

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?
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
 
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


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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?


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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.
 
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.

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
 
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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