Having a problem with your water quality sonde? OK, so you call your manufacturer and you work with them to start troubleshooting your issues, and hopefully the two of you can resolve the problem, quickly and easily.
But, what about other options, are there any? Yes, you could read this mini blog series put together by our experts!
We have worked with our product managers, sales team, and technical support specialists, to identify the top 5 most costly mistakes made with water quality sondes and we'll share these over the course of the next few weeks.
Let's get started: “Common, Costly Mistake #1.”
Mistakes with Calibration
Frankly, calibration is the trickiest part of using sondes for the vast majority of people, and it’s also the most critical for collecting quality data.
There are so many detailed nuances to each step of a calibration that can be easily overlooked by a novice user, not to mention that some sensors require you to prepare your own standards used for calibration and that can cause a whole host of other issues.
So let’s narrow the scope of the discussion to some of the mistakes we see most often.
a. Using Expired Standards - calibration standards have an expiration date...and each bottle of standard is labeled with this information. See blog post 'YSI Calibration Solutions and Buffers Expiration Times'
As soon as a standard is manufactured, it has a lifespan – this is typically 2 years for pH standards for example, but will vary from standard to standard due to the varying chemical compositions. However, once opened, that lifespan shrinks significantly. So for the pH example, once opened it will last approximately 6 months.
To put this in practical terms – expired standards are kind of like expired milk. Yes, you can probably drink some that is a few days old, but should you take the risk? After you pass the expiration date, you’re adding a layer of uncertainty into the accuracy of your data. In a multi-point calibration, even using one expired standard can affect the calibration, since calibration points often build off one another behind the scenes.
If you are using sondes infrequently throughout the year, it is worth writing the open date on the bottle so you can keep track of when it will expire to avoid any potential confusion.
b. Unclean Equipment
So while we’re on the topic of contamination, the next thought that came to mind is the cleanliness of the equipment we’re calibrating. It’s one of the biggest calibration errors we see with turbidity probes. It’s not uncommon for our technical support team to get calls asking about negative values for turbidity. There must be something wrong with this probe – it’s reading negative numbers. Well in most cases, this ties back to calibration.
Zero point calibrations for optical sensors, but turbidity in particular, are susceptible to contamination during calibration. If there’s any residual dust, grime, biofouling left on the probes, sonde bulkhead, or sensor guard (Basically anything that will be in contact with the standard), then essentially we’re calibrating our 0 NTU cal point using a 1 or 2 NTU standard. In some cases, this difference may be so small that it will not matter. In environments like drinking water reservoirs, where there are very low turbidity levels – improper cleaning will manifest itself in the field when you see something like a -1 to -3 NTU turbidity reading.
So what’s the lesson here?
Thoroughly clean all of your equipment before you calibrate and when you think you’ve cleaned it well enough, clean it some more. If you’re working in these low turbidity environments that I was just talking about, it also may be handy to have a second sensor guard available that you use just for calibrations because it takes one source of potential contamination off the table. My main point here is: Don’t let something as simple as cleaning impact the integrity of your data.
c. Taking Shortcuts
The simple truth is that just about everyone reading this has a to-do list a mile long. It’s the new normal these days. So it shouldn’t be a surprise that we’re all looking for shortcuts in just about everything in life. Calibration, however, is not the time to cut corners.
In a standard calibration process, our equipment manuals always recommend rinsing the calibration cup, sensor guard, and probes with your next standard at least twice prior to performing a calibration. Some groups’ standard operating procedures may even be more thorough, so be sure to follow your own organization’s rinsing policy. Ultimately, the rinsing process ensures that there are no remnants of any previous standards left on the equipment before moving on to the next point in a multi-point calibration or the next sensor that needs calibrated.
To give a practical example here: Let’s say you need to calibrate a Turbidity probe and a pH probe for a day of sampling. After you’ve successfully calibrated the Turbidity probe with NTU standards, it’s important to rinse the calibration cup with your first pH buffer at least twice before proceeding with a pH calibration. This ensures that none of that Turbidity standard is left to contaminate your next cal.
d. Choosing the Right Calibration Points and Standards. This is another common area where users can struggle.
With conductivity probes for example – If you’re monitoring in freshwater you want to use a 1000 micro-siemen standard, in brackish water a 10000 micro-siemen standard, and in seawater a 50000 micro-siemen standard. Calibrating with the proper standard tunes the probe to be more accurate at those ranges. Also, one quick note on conductivity standards – even if your monitoring site runs far lower than 1000 micro-siemens, always calibrate with a 1000 standard. Anything else is susceptible to contamination.
…But what about when you’re starting to monitor a new field site? Sure, it’s easy to decide whether or not I’m monitoring fresh water, salt water or somewhere in between and make a decision on a conductivity standard, but what something less obvious like pH or turbidity?
Our advice, if you’re unsure about the environment you’re going to be measuring, always calibrate to the full range of the probe. If we’re talking about pH – perform a 3-point calibration at pH 7, 10, and 4. If we’re talking about turbidity – perform a 3-point calibration at 0, 126, and 1010 NTU. This way the probe is ready to provide the highest quality data for whatever happens in the field.
OK, that was a lot, so to summarize what you just read:
- Don’t use expired standards
- Read standard labels carefully
- Make sure to clean your equipment thoroughly before calibrating
- Avoid taking shortcuts in calibration like skimping on rinsing
- …And if you’re not sure which standards and calibration points to use, try to get a better read on your monitoring location – if all else fails, calibrate over the full range of the sensor
To learn more about calibration, check out EXO University, for on-demand training on our to properly calibrate your sonde.
Additional Blog Posts of Interest:
5 Tips To Prevent Costly Mistakes With Your Sondes | Tip 2 of 5
5 Tips To Prevent Costly Mistakes With Your Sondes | Tip 3 of 5
5 Tips To Prevent Costly Mistakes With Your Sondes | Tip 4 of 5
5 Tips To Prevent Costly Mistakes With Your Sondes | Tip 5 of 5