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WERF Research Digest (continued)

On-Line Nitrogen Monitoring and Control Strategies

Summer 2006

Table 5 summarizes instrument capital, installation, support system and annual maintenance materials costs based on information provided by the manufacturers for the five instruments that were field tested.† Included in Table 5 is the number of maintenance and calibration events that were required during the 12 week field test.


†It can be assumed that the most cost-effective and reliable instruments are the instruments that require the least number of maintenance and calibration service events and the instruments that had the least number of out-of-compliance events.† In accordance with the test protocol, an instrument is considered to be out-of-conformance when the instrument is reading outside the upper or lower control limits of +/- 1.0 mg/L of the laboratory conformance measurement, in the same direction, for 4 consecutive laboratory conformance measurements (i.e., 4 consecutive readings that are higher or lower than the laboratory conformance measurements).



Field Test (continued)

Note (a) :† Capital cost and annual maintenance materials costs data was provided by manufacturers and are for the year 2004.† Installation costs are variable and are dependent on the site and application. All cost data is shown in U.S. dollars.

Based on this field test, the Endress + Hauser Stamosens most consistently and most accurately measured the nitrate concentration in the test tank when compared to the laboratory conformance measurement.† The mean deviation between the nitrate concentration measured by the Endress + Hauser Stamosens and by the laboratory conformance test was less than 0.1 mg/L and was within +/- 1.0 mg/L of the conformance measurement for about 95% of the measurements made during the test.† The Endress + Hauser Stamosens output showed a very high correlation (R2 = 0.92) between the analyzer output and the laboratory data.


The Hach Nitratax instrument displayed the highest correlation between test instrument output and laboratory conformance result (R2 = 0.97) during the test; however, the mean difference between the analyzer output and the conformance test (0.68 mg/L) was higher for the Hach Nitratax than for the Endress + Hauser Stamosens and the percentage of values within +/- 1.0 mg/L was lower (82%).


The Royce S::can Messtechnik had a mean deviation between instrument output and laboratory compliance measurement (0.26 mg/L) that was lower than the Hach Nitratax instrument (0.68 mg/L) but higher than the Endress + Hauser Stamosens (-0.08 mg/L). The analyzer output correlated as well to the conformance test result as the Endress + Hauser Stamosens (R2 = 0.91); however, fewer of the Royce S:can Messtechnik output values were within the target range of +/- 1.0 mg/L (69%) than either the Endress + Hauser Stamosens or the Hach Nitratax.


The Wedgewood Stip Scan had a lower mean deviation between its output and the conformance measurement (0.55 mg/L) than the Hach Nitratax, but had a significant lower correlation to the conformance measurement over the test range (R2 = 0.62) than the Endress + Hauser Stamosens, the Hach Nitratax or the Royce S::can Messtechnik.† Slightly more than half (54%) of the output values of the Wedgewood Stip Scan were within +/- 1.0 mg/l of the laboratory conformance measurement.


There was no significant correlation between the output of the Hach Evita instrument and the laboratory conformance measurement (R2 = 0.02).† The mean deviation between the Hach Evita output and the laboratory conformance measurement was more than 1.0 mg/L (2.88 mg/L) and only about one-third (36%) of the measurements were within the target of +/- 1.0 mg/L.† For periods of time during the test, the output of the Hach Evita instrument was zero (week of 10-24-05) or 12.0 mg/L (full scale; weeks of 11-28-05 and 12-05-05) and the instrument was non-responsive to changes in the test basin nitrate concentration.


The Endress + Hauser Stamosens did not require recalibration or maintenance during the test period and did not experience a non-conformance as defined by the test protocol (four consecutive test measurements that differed from the laboratory test by more than +/- 1.0 mg/L and always in the same direction).† The Hach Nitratax experienced one non-conformance event during the test and required one maintenance event (a manual activation of the self-cleaning wiper).† The other three instruments (Hach Evita, Royce S::can Messtechnik and Wedgewood Stip Scan) all experienced four non-conformance events during the test.† The Royce S::can Messtechnik was recalibrated three times during the test but required no other maintenance.† The Wedgewood Stip Scan instrument was recalibrated five times and required one maintenance event.† The Hach Evita required three maintenance events on-site by a factory representative.† After one of these maintenance events, the Hach Evita underwent a complete re-calibration.


This research report includes presentations and discussions of comparative analyses for performance and observations of instrument design and features but does not conclude or select one instrument over the other since each treatment facilityís circumstances will determine the selection of the best instrument for their application.† For example, one treatment facility may have the staff resources to perform routine instrument maintenance and calibration and might select an instrument that is the easiest to use but requires more staff time to perform maintenance. Another treatment facility might select an instrument that requires a high level of skill to maintain and operate requiring specialized training but does not require as much maintenance.† Therefore, a complete review of this research report is required for an end-user to be able to weigh their circumstances against the benefits and limitations of each instrumentís performance and reliability presented herein, to make the best selection for their application.

The Water Environment Research Foundation (WERF) On-Line Nitrogen Monitoring and Control Strategies (03-CTS-8) research project final report can be obtained through WERF.† Please contact WERF at 703-684-2400 or† email WERF directly to obtain a copy of the WERF 03-CTS-8 report or visit http://www.werf.org




WERF 03-CTS-8 Project Report


Obtaining a Final Copy

Table 5.† Field Tested In-Situ UV Nitrate Instruments Estimated Cost Data


Model/ Sensor

Instrument Capital Cost (a)

Instrument Installation Cost (a)

Support System Cost for both Purchase and Installation(a)

Annual Maintenance Materials Cost(a)

Number of Field-Test Maintenance & Calibration Events

Endress + Hauser

Stamo-Sens CNM750/CNS70







Evita Insitu 5100 (Danfoss)







NITRATAX plus sc


Self installation available $309 (in-situ mounting hardware) $1,000 (ex-situ flow through cell) $100 to $200 (for 25 to 100 ft extension cables)


(sc 100 controller)



s::can Messtechnik GmbH as distributed by Royce Technologies

82 N Nitrolyser sensor / 8580 Constat analyzer

Various options, features & additional parameters available.† Price ranges from $9,995 to $13,995.

Approximately $25 for mounting pipe and pressure hoses

If potable water or instrument air not available, compressor system available for $995



Wedgewood Analytical/E+H


$17, 160 with stand and touch panel PC.