EXO NitraLED - Frequently Asked Questions (FAQ)


Nitrate Sensor Technology

Q: How does the EXO NitraLED Sensor work?

The NitraLED Sensor uses one LED (235 nm) to measure nitrate and accounts for interferences in the signal with a second LED (275 nm) that measures Natural Organic Matter (NOM) interference. The influence of particulate matter on the absorbance at both LED’s is corrected through the use of the EXO Turbidity Sensor. Inside the NitraLED sensor are two LEDs and a UV-robust photodiode that captures the light transmitted across the 10mm pathway. Internal microprocessors compute the nitrate value based on the nitrate LED's reading and the sources' interferences, as mentioned earlier.

Q: Which wavelengths does the sensor measure at?

The sensor measures nitrate with a 235 nm LED and corrects for Natural Organic Matter (NOM) using a 275 nm LED.

Q: Do I need an fDOM sensor to make EXO NitraLED work?

The fDOM sensor is unnecessary in the computation of nitrate when using the NitraLED sensor. Natural Organic Matter (NOM) breaks down into a category called Dissolved Organic Matter (DOM), which is then further categorized into Colored Dissolved Organic Matter (CDOM) and fluorescent Dissolved Organic Matter (fDOM). While it seems that fDOM and NOM should be relatable because fDOM is a subset of NOM, this is not the case. As the species in a water sample changes, so does the percentage of fDOM that makes up NOM. Also, merely knowing the fluorescence does not mean that we automatically know the species' absorbance characteristics. There are too many variables to consider, and relying on the absorbance from the NitraLED LED's and calculating NOM from those provides a more accurate measure of nitrate.

Q: What is the range and accuracy of the sensor?

The current measurement range is 0-10 mg/L-N. The accuracy is ±0.1 mg/L-N or 5% of the reading, whichever is greater, if the sample temperature is within 2°C of the calibration temperature. The accuracy is ±0.4 mg/L-N or 5% of the reading, whichever is greater, for the full temperature range. The accuracy applies to analytes comprised of nitrate in pure water only and absent of all other absorbing species. We expect to improve upon these initial specifications via additional firmware updates to NitraLED in the coming months.

Q: What is the minimum detection limit for EXO NitraLED?

The minimum detection limit is 0.005 mg/L-N in nitrate standards.

Q: Will my results be similar to the values I get when I collect manual grab samples for lab analysis?

First, laboratory analysis is still highly relevant and should not be replaced entirely from your monitoring program. Second, the NitraLED sensor's values should be comparable if you apply a site-specific correction. However, please note that certain situations may cause the sensor to drift from the site-specific correction coefficients (such as sediment composition change, seasonality, amount of algae/plant life, etc.). It is crucial to review site-specific corrections if you notice significant drift between laboratory and continuous data.

Q: Is EXO NitraLED equivalent or supplemental to the more expensive nitrate analyzers I have now?

This sensor will report values similar to the continuous analyzers that you may have deployed in the field. For the most accurate data, you'll need to perform a site-specific correction to account for any effects of Turbidity or NOM on the nitrate reading.

Q: How does the UV LED technology in EXO NitraLED compare to an ion-selective electrode, colorimeter, photometer, or spectrophotometer?

Ion-selective electrodes (ISE's) are typically not intended for long-term deployment due to the significant drift over short periods. Other instruments, such as a colorimeter or photometer, are not designed for long-term deployment. They are typically used for discrete measurements and rely on reagents to determine a value. Spectrophotometers are typically lab instruments, but some have been designed for long-term deployment to monitor nitrate. These spectrophotometers are extremely expensive, have large power budgets, and are incredibly complex to install and maintain.

Q: If no site-specific correction is applied to NitraLED, what are the impacts on data accuracy?

The data will trend relatively well, but will not be as aligned with any grab samples collected and may not give the full picture of nitrate fluctuations at the site.

Q: Are there instructional videos available online that show how to use the EXO NitraLED Sensor?

Instructional videos are coming soon! They'll cover the sensor and accessory installation, routine calibration, and site-specific calibration. Stay tuned!

Q: Is this sensor compatible with any other technology platforms besides EXO Sondes? Maybe a 6-Series Sonde or the ProDSS Handheld Meter?

EXO NitraLED is only available for the EXO Monitoring Platform. YSI designed the sensor for continuous deployment, and currently, EXO Sondes are the only instruments we offer for continuous environmental monitoring.

Q: Are additional sensors needed to collect data with EXO NitraLED?

You will need an EXO Turbidity sensor installed along with the NitraLED sensor, as it is necessary to compensate nitrate data for the influence of particulate matter. However, if you deploy an EXO2 or EXO3 Sonde for continuous monitoring, you will need a third EXO sensor installed for the alignment ring to remain seated on the sensors correctly.

Q: Can I run a sonde deployment off internal batteries, or does an EXO Sonde with NitraLED require an external power source?

You can deploy an EXO sonde with NitraLED using internal batteries alone. You can also choose to power the instrument externally. External power is often supplied by connecting the instrument to a solar-powered system.

Q: Will EXO NitraLED require excessive power like other UV nitrate sensors?

No! Our new sensor requires the same amount of power that other EXO sensors do. It does not call for an external battery or power source for a single sensor, unlike many available options on the market today.

Q: Do I need a data logger to capture my EXO NitraLED data?

The EXO sonde platform allows for a user to log data internally and also gives the ability to connect to an external data logger if desired. When deploying the sonde, internal data logging is always an option. Even if the data logger loses power, as long as the sonde has batteries installed, it will still log data. Data saved onto the sonde can be easily downloaded via Kor Software and copied into any data record.

Q: Can the nitrate readings from EXO NitraLED be transmitted via SDI-12 or other external means?

Yes! The EXO sonde can be connected to a data logger and transmit data via SDI-12, RS-232, or Modbus. You can find additional information about EXO connectivity in the EXO User Manual.

Nitrate Monitoring

Q: How expensive is the EXO NitraLED Sensor compared to other solutions for in-situ nitrate monitoring?

The price of NitraLED is a fraction of the cost of other field spectrophotometers or wet chemistry analyzers. YSI engineers designed EXO NitraLED for field use, whereas other companies have retrofitted laboratory instruments for the field. This product development strategy adds a tremendous amount of cost to their equipment and restricts the number of scientists that can afford to collect continuous nutrient data. Before NitraLED, an environmental monitoring program could afford to monitor nitrate at a few select locations. Now, you can afford to monitor nitrate, at scale, throughout your entire watershed!

Q: How do I request a quote for an EXO NitraLED Sensor?

Visit YSI.com/NitraLED and fill out the Request a Quote form with your contact information. Your local YSI representative will follow-up to discuss the sensor and provide pricing information.

Q: What is required to deploy an EXO NitraLED Sensor on an EXO sonde?

For an EXO1 Sonde, you'll need the NitraLED sensor, an EXO Turbidity Sensor, and nitrate calibration standards. For an EXO2 or EXO3, you'll need the NitraLED sensor, an EXO Turbidity Sensor, a Central Wiper, a Central Wiper Brush Replacement, an Alignment Ring kit, and nitrate calibration standards.

Q: Can the sonde still be deployed in a flow cell with an EXO NitraLED Sensor installed?

Yes! You can deploy sondes with EXO NitraLED in standard EXO flow cell systems. No changes to existing site infrastructure are required.

Q: How difficult is it to deploy an EXO NitraLED Sensor in the field?

It's easy! The sensor is small enough to fit in your pocket for easy transport to the field. Once you arrive at the site, it's a simple installation of the sensor and required accessories. Once the software-guided calibrations are complete, you're ready to collect high-quality nitrate data!

Q: Will I still get nitrate readings if I don't have an EXO Turbidity Sensor installed in my sonde?

You must use an EXO Turbidity Sensor along with EXO NitraLED to maintain the specified accuracy of the sensor. Turbidity compensation is a critical part of collecting quality nitrate data.

Q: Is this sensor just as easy to calibrate as other EXO Sensors?

The calibration of this sensor is a simple two-step procedure, just like many other EXO sensors. EXO NitraLED requires a calibration point at 0 mg/L-N using distilled water and a calibration point at 5 mg/L-N or 10 mg/L NO3-N using YSI standards.

Q: Do I need any special equipment for the calibration procedure in standards?

No, you only need the sonde guard and calibration cup used in all other EXO Sensor calibrations.

Q: How long does it take to calibrate EXO NitraLED?

The two-point calibration does not take long to complete. Our engineers added Auto-stabilization functionality into Kor Software, so you'll know as soon as the sensor has stabilized in standard, and the calibration process can continue to subsequent steps.

Q: What calibration solutions are necessary to calibrate EXO NitraLED?

The standard calibration for the NitraLED sensor is a two-point calibration. The first point requires Type I water (0 mg/L). Distilled water with no added minerals is a substitute for Type I water, but there must be no interfering species in the water. The second calibration point is determined by the user depending on expected site conditions. YSI offers two solutions for this second calibration point: 5 mg/L and 10 mg/L NO3-N.

Q: Can I use the nitrate standards used with EXO ISE Sensors?

The NitraLED sensor requires a particular standard made with nitrate diluted into pure water. The ISE sensor's nitrate standards have an added salt to increase the standard's conductance to allow the ISE to stabilize faster. The addition of this salt could cause interference with the NitraLED sensor calibration, so we recommend using the specific NitraLED standard.

Q: What is the shelf life of the NitraLED calibration standards?

The NitraLED calibration standards have a shelf life of one year.

Q: What type of solutions can I use for the NitraLED 0 mg/L-N calibration?

YSI recommends using Type I water for the 0 mg/L-N calibration point. Distilled water may also be used, however, make sure there are no minerals added.

Q: Where can I find MSDS sheets for NitraLED calibration standards?

You can find the MSDS sheet for our 5 mg/L NO3-N standard here and our 10 mg/L NO3-N standard here.

Q: How long can the EXO NitraLED Sensor be deployed in the field before another calibration is necessary?

Calibration intervals are dependent on factors such as the deployment setup and environmental conditions the sensor is exposed to. This sensor is an optical sensor, so it has a high level of stability. If deploying at a 15-minute interval, we expect a 30-day deployment to have 0.2 mg/L-N of drift or less typically.

Q: Can you calibrate multiple EXO NitraLED Sensors with EXO's concurrent calibration feature?

No, you cannot calibrate more than one NitraLED Sensor at a time. The NitraLED must be calibrated alongside the specific turbidity sensor it will be deployed with into the field.

Q: What is a site-specific correction, and how does it make my EXO NitraLED data even better?

A site-specific correction is just what it sounds like – a correction that affects the coefficients inside the NitraLED sensor to account for the Turbidity and NOM at a particular site. Each site has a different mix of constituents in the body of water that affects nitrate readings. For example, in a reservoir affected by algae, there may be an increase of NOM compared to what is programmed into the sensor by default. The nitrate data becomes even more accurate by allowing the user to compensate the sensor to the site's conditions.

Q: Do I need any special equipment to perform a site-specific correction with EXO NitraLED?

The site-specific correction does require additional equipment to perform the procedure. We recommend using a stir plate and stir bar for all steps of the correction process and using a ring stand to hold the sonde in the solution above the stir plate. We also recommend using a vacuum pump to pump the raw sample water through a 0.45µm filter to get the filtered sample water for the filtered water portions of the correction process.

Q: Do I need a third-party nitrate sensor to calibrate EXO NitraLED?

For the calibration in standards, an external nitrate sensor is unnecessary. However, step one of the site-specific correction requires that a user input the actual value of nitrate in the water body. This value can be recorded from an ion-selective electrode, photometer, or colorimeter. An ISE or laboratory-style photometer will provide the best data for the initial site-specific calibration value.

Q: I do not have an external nitrate sensor, but I send samples to the lab. Can I preserve my samples, wait for the lab results to come back, and then perform the site-specific correction for EXO NitraLED?

Yes, as long as the sample is adequately preserved, there should be no issue. Proper preservation calls for holding the sample chilled at or below 4°C. Ensure that around 2L of the sample is reserved for the site-specific correction procedure. Our testing indicates that nitrate in properly preserved samples changed very minimally over the course of a month. Hence, a sample held for even a month can still allow for a proper site-specific correction.

Q: Once I perform a site-specific correction, will my NitraLED sensor read the same in standards?

Yes, once the site-specific correction is performed, the sensor should still read 0 mg/L-N in Type I water and 5 mg/L-N or 10 mg/L-N in the corresponding standards.

Q: How often do I need to perform a site-specific correction with EXO NitraLED?

There is no definitive answer to this question because many factors affect the need for site-specific corrections. For example, as algae grow in a reservoir, additional Natural Organic Matter (NOM) in the water will influence the sensor during summer months. However, as the algae die off in cooler temperatures, and the water at the top of the reservoir in the measuring location becomes clearer and free of NOM, the site-specific correction may need to be completed again.

Seasonality will likely affect the frequency of site-specific corrections, as alluded to in the example above, and also because some seasons are more likely to see increased runoff from storm events. When the sediment type changes at a monitoring location, we also recommend performing additional site-specific corrections. For example, if summer baseflow conditions at a site are extremely low, and the sediment type is sand, this will give different values than elevated baseflow conditions where the sediment type is clay. Experimentation is needed to determine how often these corrections need to occur at each site. Contact us at info@ysi.com, and we will be more than happy to discuss site-specific corrections further.

Q: What's the average lifespan of an EXO NitraLED Sensor?

A long time! Like our other optical sensors, the NitraLED sensor is designed to last for an extended time in the field. At YSI, we perform an extraordinary amount of testing on each sensor before shipping it to ensure the highest quality standards.  We also offer a 2-year warranty on this sensor to protect your investment.

Q: Are there any changes to Kor Software related to EXO NitraLED?

There are several changes to Kor software from previous versions. The first, of course, is the addition of the NitraLED sensor to the recognized sensors list. When you connect a NitraLED sensor to an EXO sonde, the sensor will appear in the Calibration and Instrument & Settings tabs. When navigating to File à Settings, the NitraLED sensor will also appear as an option in the menu. In the Calibration menu for the NitraLED sensor, there are two options now available. There is one option for the calibration in standards and another option for the site-specific correction.

The software update also includes an update to the central wiper that allows a user to designate between Nitrate Mode and Normal Mode. To change the wiper mode, navigate to the Calibration menu, select the wiper, select Calibrate, and choose Normal or Nitrate.

Q: Is the small wiper brush for the EXO NitraLED sensor as effective as the big wiper brush for the other sensors?

The new wiper brush has been tested in heavy fouling environments for efficacy and is as effective in cleaning the sensors as the old wiper brush. The short arm wiper brush intended to wipe the nitrate sensor has bristles on the top and bottom of the arm to wipe the photodiode lens and the LED lens effectively.

Q: Will the wiper affect the sensor adjacent to the NitraLED sensor?

YSI recommends installing a non-optical sensor in the port adjacent to the NitraLED sensor. The NitraLED wiper arm will park on top of this adjacent sensor and impact any optical readings. Sensors such as the pH, pH/ORP, other ISE's, or the optical dissolved oxygen sensor will not be affected by the new wiper brush positioning.

Q: Are there special storage requirements for the EXO NitraLED Sensor?

Like other optical sensors, this sensor can be stored completely dry. However, we recommend saving the sleeves used in shipping to protect the sensor faces and the pins during any storage period. The sensor can also remain installed on a sonde when placed into storage.

Q: Once I set my EXO Sonde up for use with NitraLED, can I use it without the new sensor?

Of course! You can use the sonde without the NitraLED by merely removing the sensor and "recalibrating" the central wiper to wipe in "Normal" mode instead of "Nitrate" mode.

Q: If I uninstall NitraLED, can I put other sensors in that sensor port?

Yes! The NitraLED sensor is just like any other EXO sensor, and having it installed does not change the sonde's inner workings. If you remove the NitraLED sensor and install any other sensor, the new sensor will be recognized. That new parameter will be reported in place of nitrate.

Q: What are the minimum software and firmware versions needed to use NitraLED?

The minimum requirements are as follows: KorEXO: 2.3.10, EXO Sonde: 1.0.84, NitraLED Sensor: 1.0.58, Turbidity Sensor: 3.0.4, Central Wiper: 3.0.11, Handheld: 1.0.63

Q: Does EXO NitraLED work in salt or brackish water?

Unfortunately, the sensor does not work in salt or brackish at this time.

Q: Why will EXO NitraLED not work in saltwater?

There are too many other species in saltwater that interfere with the nitrate absorbance. For example, iodide and bisulfide absorb in the wavelength that we measure at, which could heavily impact nitrate readings.

Q: What is the max salinity value the sensor will operate in?

There is no definite answer because there could be many interfering species in the water. A safe bet would be to avoid areas with salinity greater than 2.5psu.

Q: Can I use EXO NitraLED Sensors for spot sampling?

Due to the need to develop specialized coefficients for interferences at each site, the NitraLED may be more applicable for long-term deployments.

The sensor can be used for spot sampling. However, we recommend only spot sampling at field sites with low turbidity (<15 FNU) and low NOM interference. At sites with higher levels of interference, you'll need to decide whether to perform the site-specific correction at each location or just make comparison measurements using the sensor calibrated in standards. If making comparing measurements without an SSC, you can correlate the sensor readings with nitrate grab samples to help establish trends at your sampling sites.

Q: Can EXO NitraLED be used in groundwater applications?

Most groundwater applications call for a sonde with a diameter no greater than 2”. If the NitraLED sensor is installed on an EXO1 Sonde with a Temperature/Conductance Sensor and a Turbidity Sensor, the sonde will monitor nitrate in a groundwater well. Please note there is no wiper option on the EXO1 system, so data could be impacted by bubbles left on the sensor faces if there is a great amount of aeration in the well.  In this case, it may be best to use an EXO2 or EXO3 with a central wiper using a flow cell setup.

Q: What water quality parameters would you recommend for monitoring Harmful Algal Blooms (HABs)?

For HAB monitoring, we would recommend the EXO2 Sonde equipped with Temperature/Conductance, pH, Optical Dissolved Oxygen, Turbidity, Total Algae: Chlorophyll/Phycocyanin, and Nitrate (NitraLED), using the central wiper for anti-fouling purposes.

Q: Will the UV light from EXO NitraLED harm me in any way?

The UV light emitted from the NitraLED sensor is in the UV-B and UV-C spectrum and can cause irreversible damage to the body if care isn't taken. When uninstalled and independent from an EXO sonde, the sensor is harmless. However, when the sensor is installed and a power source is available (such as internal batteries), it activates and emits light. Do not look directly into the sensor light pathway or put any body part in the light pathway. Also, be aware that harmful UV light can be reflected if any reflective material is placed in the sensor path. If the light pathway requires cleaning, wear nitrile gloves to prevent the UV-B and UV-C light from coming into contact with the skin. You can find more information on our UV Safety page.

Q: Are the calibration standards for EXO NitraLED safe?

There is no known harm from the calibration standard coming into contact with the skin. MSDS information is provided with the standard for reference if it is splashed into the eyes or ingested.