Lesson in Meters & Monitors:
Resolution and Accuracy
Whether saltwater is our hobby or
our business, we all know the importance of monitoring our
aquarium's water parameters. The ideal specifications (limits)
of many of those parameters still have not been exactly determined.
There are, however, many agreed upon guidelines for successful
reefkeeping. As I ventured into this hobby, I quickly learned
that while some of these water parameters are critical, much
debate remains about which basic dip strip or titration test
kits, if any, were accurate and reliable enough to measure
some of these parameters. I learned that I had to accept that
it was okay simply to know that I was somewhere within
an acceptable range.
Some time after I recovered from the initial expense of setting
up a reef tank, I could finally afford to look at more expensive
testing equipment such as meters and monitors in order to
nail down my water's exact parameters. My first purchases
were pH and temperature monitors. Next, I purchased a TDS
Along the way I have heard, and read, many people stating
their meter's readings while they are trying to solve a particular
problem. "My pH is ___," "My temperature is
___;" "My TDS is ___." It is my belief that
many people are putting their faith in meters that are simply
not accurate. They are trusting that what their meter is displaying
is exactly what their water's parameter actually is. Depending
on the meter, that may or may not be true. My goal in this
article is to educate aquarists on what to look for when considering
the purchase of a meter or monitor with regard to its resolution
Resolution refers to how a digital
meter displays information, specifically, in what amount of
meter that has a resolution of 1 will display only in increments
of whole numbers: 9, 10, 11, 12, etc.
meter with a resolution of 0.1 will display in increments
of tenths: 10.0, 10.1, 10.2.
meter with a resolution of 0.05 will display in increments
of five one-hundredths: 10.00, 10.05, 10.1, 10.15.
meter with a resolution of 0.01 will display in increments
of hundredths: 10.00, 10.01, 10.02.
meter with a resolution of 0.001 will display in increments
of thousandths: 10.000, 10.001, 10.002.
When choosing a meter, ensure that the meter will display
the information in the amount of detail (resolution) you require.
The accuracy of these devices is the
main reason I am writing this article. Many people do not
look at, question or understand the accuracy specification
of a meter or monitor. As you will see, understanding the
accuracy specification can give hobbyists great confidence
in their water's parameters or it can lull them into a very
false sense of security.
For the purpose of this article, the terms accuracy and tolerance
mean the same thing and are used interchangeably. They refer
to the amount of deviation the meter can have from the displayed
reading (even after it has been calibrated); in other words,
the amount the meter can be "off." The symbol +/-
is spoken "plus or minus."
The advertisement states, "extremely
accurate: +/- 2%," and the consumer thinks, "2%
looks good," and purchases the meter. Two things must
be done with the accuracy specification before making a purchase.
1. Find out, 2% of what?
2. Do the math.
As an example, one particular TDS meter states: The "measurement
range = 0 - 999 ppm." The measurement range (scale) states
the limits of what the meter is designed to display. It is
designed to measure (and display) from 0 to 999, and anything
Its accuracy is stated as: "+/- 2% full scale."
Many people are under the assumption that this means +/- 2%
throughout the meter's entire range; in other words, +/- 2%
of whatever the meter is displaying at the time. For
instance, if the meter is displaying 4, then 2% of 4 is .08.
So the accuracy is +/- .08 for this particular reading. This
is not true!
Full scale refers to the maximum (highest) number
the meter can display. The meter's accuracy is +/- 2% of
the full scale. The full scale is 999 ppm. It's time to
do the math. Two percent of 999 is 19.98; let's round it to
20 for the sake of discussion. The meter's accuracy is actually
+/- 20 ppm! That means the actual amount of TDS in your water
could be up to 20 ppm lower than the displayed value or up
to 20 ppm higher than the displayed value. That does not mean
that the meter will be off by 20. It means that it
could be. You'll never know.
Let's say the meter is measuring the TDS of the water coming
into a house. The TDS meter reads 400 ppm. In actuality, the
water's TDS could be anywhere from 380 - 420 ppm. Some aquarists
would probably think, "That's not so bad. I can live
with that tolerance."
Now, suppose this meter is testing the purity of RO, DI or
RO/DI water. In general, a TDS reading of 0 is good, 1 or
2 is questionable and 3 suggests the water might be contaminated.
Remember that "+/- 2% full scale?" That +/- 20 ppm?
Even though the meter is reading 0, the TDS could actually
be as high as 20 ppm, and the user doesn't know it! Instead,
he's comfortable and confident in his water quality because,
"The meter says I have 0 TDS."
Again, please understand that it does not matter where within
the meter's range the displayed number falls; the tolerance
is not +/- 2% of the displayed value. If the reading is 400
ppm, the tolerance is not 400 times 2%, or 8 ppm. It is +/-
20 ppm, no matter what the display reads. Several manufacturers
verified this information.
Now, imagine the following situation. You're trying to figure
out a problem you're having with your tank. You "know"
your water change and top-off water is good because you have
a TDS meter, and it's reading 0. So, you start taking other
measures in an attempt to fix the problem. You start adding
supplements to raise "this" or lower "that,"
not knowing that the only problem is that the water is actually
polluted with TDS of 10, 15 or even 20 ppm. Your "corrective"
actions have probably created new problems and you still haven't
discovered the original problem. That's not good.
Another meter's advertisement, a popular in-line model, states
an accuracy of "+/- 2%" and a range of 0-999, with
a second range of 0-9990. Since it states only +/- 2%, and
not 2% of what, the worse case scenario must be assumed
and expect that it means +/-2% of full scale. Again,
do the math. In the high range, 2% of 9990 is +/- 200 ppm.
That's whatever the display reads +/-200 ppm! If the display
is reading 1000, the actual TDS could be anywhere from 800-1200.
Yes. In the low range the accuracy is still +/- 20 ppm. My
point with this meter is: before making a purchase, find out
2% of what. I did. I called the manufacturer. It is "+/-
2% of full scale."
Different companies state their accuracy in different ways.
Some state "full scale," some simply state the tolerance
(+/-2%). You should immediately assume +/-2% of full scale.
If unsure, call the meter's manufacturer, not the distributor.
The distributor has only the information that the manufacturer
supplied. Arm yourself with the information above and ask
the manufacturer's technical department to explain the meter's
I am certainly not suggesting that these less accurate (and
less expensive) meters do not have a place in the hobby. While
much more accurate meters are available, of course, they cost
more. It all boils down to what will the meter be used for,
and in what amount of detail must the information be presented?
If the desire is to test the condition of an RO membrane (the
times 10 factor explains this), then a basic, inexpensive
meter might suffice. If the goal is to know whether RO or
DI water's purity is at 0, 1, 2 or 3 ppm, then consider (and
find out) whether the meter will perform with the amount of
detail and accuracy necessary for that task.
Temperature, pH and Conductivity Meters:
Two popular, inexpensive water
temperature meters do not specify an accuracy range anywhere
on their packaging or website. So I called their manufacturer.
Their accuracy is +/- 1° C. That equals +/- 1.8° F.
If the meter reads somewhere in the middle of the safe range,
say 79°, that's okay; there's room for error there. If
it reads 76°, the tank's water could be at 76°, as
high as 77.8° or as low as 74.2°. Would 74.2°
be okay? If you glanced at your meter and it read 83.2°,
your action would probably be to keep a closer eye on your
tank until the temperature dropped. But, if you knew your
tank was actually at 85°, would you take a different (immediate)
action? Again, understand the meter's limits and use these
meters as a rough guide. Very accurate digital temperature
meters are available but, yes, they cost a lot more.
pH and conductivity meters typically state accuracies
that are more straightforward and much easier to understand.
They typically quote tolerances in the unit being measured,
such as +/- 0.1 pH or +/- 0.01 pH and +/- 0.1µS or +/-
0.01µS. Since they state actual pH and µS limits,
rather than a percentage, simply add and subtract the tolerance
from the displayed reading, and that is how much the reading
can be "off."
pH meter with a stated accuracy of +/- 0.1 pH is displaying
8.0. The water's actual pH can be 7.9, 8.0 or 8.1.
pH meter with a stated accuracy of +/- 0.01 pH is displaying
8.00. The water's actual pH can be 7.99, 8.00 or 8.01. This
is much more accurate.
The same goes for conductivity meters that display in µS.
Again, it is simply a matter of balancing the desired amount
of accuracy with the desired amount of cost.
On a side note: A very inexpensive, quite accurate bulb
and tube type thermometer is available. It's accuracy is rather
tight at +/- 0.4° F. I use this thermometer about once
a month as a reference against my digital thermometer. I know
that my tank is always 1.5° F lower than the reading on
my big, easy to read, inexpensive, digital thermometer.
In my opinion: Equally as dangerous as not knowing a parameter
at all is having a false sense of security from a meter that
says the water is good, when it actually could be very bad,
It's been said before, but I'll say it again: Many people
have added chemicals trying to increase "this" or
decrease "that" because their test kit or meter
shows they're outside the acceptable limits. If you don't
understand your meter's accuracy, the only thing that may
be wrong is your having faith in the meter itself! You could
find yourself taking a perfectly sound tank and send it spiraling
downhill by trying to fix a problem that doesn't exist. The
old adage "You get what you pay for," usually still
applies. It always costs more to build something better (more
accurate, more complex) than something basic. If the price
sounds too good to be true, it usually is. If all you want
is an inexpensive meter, and you understand what and how it
will display, that's great. But, if you're depending
on it to produce a specific amount of accuracy, make sure
you understand what the meter will, and will not, do before
you buy it.
In closing, I must point out that I am not suggesting that
any of these devices will be off by their stated tolerances.
They may in fact be dead on. They may be off by a miniscule
amount. Or, they may be off by a lot. The manufacturers designed
them and with design comes tolerances. They are telling
you what their accuracy is. They are telling you by how much
their meters can be "off."
My goal in this article was to clarify and help you understand
the resolution and accuracy specifications to aid you in being
an informed and educated consumer and hobbyist. I hope I have
achieved this goal.
If you have any questions about this article, please visit
my author forum
on Reef Central.