Buying a pH Meter
pH meters are electronic devices that measure how acidic or basic a solution is.
pH is such a useful measurement that it is used to evaluate an astonishingly broad
range of substances, including beer and wine, battery acid and blood, soil and swimming
pools, coffee and cheese, and much more. Almost all modern pH meters are digital.
How good a pH meter do I need?
Because pH meters range in price from tens of dollars to tens of thousands of dollars,
one of the first questions that comes to mind when selecting and buying a pH meter
is “How good a pH meter to I need?” The good news is that even inexpensive meters
that are carefully calibrated and well maintained can give surprisingly accurate
results. The answer to the question, though, ultimately depends on a number of factors,
including how much accuracy, precision, durability, and adaptability you need.
- Accuracy means the meter’s readings are exact. If the measurements you take
with the meter are exactly right (for example, the pH of a calibration standard
with a pH of 4.00 gives a reading of 4.00), the meter is accurate.
- Precision means the meter’s readings are repeatable. A pH meter that reads
4.00 on the first reading but 4.90 on the second reading is of little use.
- Durability means how much abuse can the meter take and how long it will last
A pH meter designed for professional use in the field or in harsh industrial environments
will be more durable than a home-use meter, but it will cost more. As a rule, more
expensive instruments are more durable, but cheap pocket pH meter may take more
abuse than an expensive benchtop pH meter. The probe is the heart of the pH meter
and rather delicate, so replaceable probes also figure into durability; cheap, pocket
and “pen” meters usually don’t have replaceable probes.
- Adaptability means the range of inputs, outputs, accessories and other variables
that are available with the meter. Cheap pH meters usually give no more than a pH
reading on a digital display but are perfectly adequate for casual use. Expensive
benchtop pH meters can usually take a variety of probes, and can be connected to
computers, controllers, and data networks.
Can I just use a cheap pH meter? What about just using a pH test kit?
For most people who are taking routine measurements outside of a laboratory—measuring
the pH of wine or of an aquarium for example—an inexpensive but carefully
calibrated and well maintained pH meter is perfectly adequate.
A pH test kit can give results that are just
as accurate as a simple pH meter and that are more accurate than a poorly maintained
or poorly calibrated meter. The drawbacks of test kits are that they can be messy
and time-consuming, and that they’re not good for continuous monitoring. The advantage
of using a water or soil pH test kit is that they are inexpensive, easy to find
at pet stores and garden suppliers, and require no calibration or maintenance.
For the curious and electronically skilled, building a pH meter is an option, from
the simplest possible pH meter
to a pH measurement and control system.
What is pH, anyway?
The short answer to the question, “What is pH?” is that it’s a measure of how acidic
or basic a solution is. The longer, more technical answer is that pH is a measurement
of the concentration of hydrogen ions in a solution. The name “pH comes” from the
German for (roughly) “power of Hydrogen.” The more acidic a substance is, the lower
its pH number. (Vinegar has a pH of about 2.) Basic (or “alkaline”) substances have
pHs from just above 7 up to a maximum of 14. (A solution of lye has a pH of about
12, depending on how concentrated it is.) Neutral pH is 7. Pure water has a pH of
about 7. (However, water cannot be used to calibrate a pH probe since its pH can
be affected greatly by substances dissolved in it. Always use calibration buffers
to calibrate your pH meter.)
How does a pH meter work?
A pH meter uses a probe with a very delicate tip made of thin glass to measure the
concentration of hydrogen ions in a solution. Small voltage changes proportional
to changes in the pH are produced by the probe and then processed and displayed
by the meter circuitry. Because the probe is the origin of the signal measured by
the pH meter, to get accurate pH measurements, it must be kept very clean and must
be carefully calibrated.