Notes from the Trenches with Rick Schabener

A 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 meter.

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 and accuracy.


Resolution refers to how a digital meter displays information, specifically, in what amount of detail.

  • A meter that has a resolution of 1 will display only in increments of whole numbers: 9, 10, 11, 12, etc.
  • A meter with a resolution of 0.1 will display in increments of tenths: 10.0, 10.1, 10.2.
  • A meter with a resolution of 0.05 will display in increments of five one-hundredths: 10.00, 10.05, 10.1, 10.15.
  • A meter with a resolution of 0.01 will display in increments of hundredths: 10.00, 10.01, 10.02.
  • A 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."

TDS Meters:

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 in between.

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 accuracy.

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."


  • A 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.
  • A 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, or vice-versa.

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.

Reefkeeping Magazine™ Reef Central, LLC-Copyright © 2008

A Lesson in Meters & Monitors: Resolution and Accuracy by Rick Schabener -