Taking The Lab Outdoors
Environmental remediation of soil and water, construction sites, municipal and industrial wastewater treatment, monitoring of wastewater-receiving waterways, groundwater testing, natural resource management, aquaculture, fisheries, and mining are just a few applications that rely on field pH measurements. “If you’re out measuring water, you’re probably measuring pH,” notes Laura St. Pierre, senior product manager for water quality at YSI (Yellow Springs, OH), a Xylem brand.
Field measurements make sense when sampling is impractical, real-time results are desirable, or there are concerns regarding sample integrity during a long trip to the home lab. Improperly stored samples may experience significant changes in pH, which in turn could alter concentrations of metals or even dissolved organics. Consequently, samples containing volatile organics or metals may be pH-adjusted or stabilized in the field to lock in concentration levels before transport.
Portable pH meters are also used in near-process environments for quality control for food and beverages, and even in laboratories—although these need not meet the specifications for field ruggedness.
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Outdoor water testing often involves trade-offs in performance or capabilities. “One thing you lose is being around other instruments,” notes Tim Larson, inside sales manager at Hanna Instruments (Woonsocket, RI). This limitation fades somewhat with multiparameter meters, which allow testing of two or more water properties while saving trunk space for other equipment.
“The other issue is the opportunity to do multipoint calibration. Fieldworkers will often do a single-point calibration, especially when time is of the essence.” But one can surely draw an infinite number of lines through a single point. “True, but a lot of fieldwork involves obtaining ballpark measurements or looking for trends—spikes or dips in a meter reading. Environmental work will sacrifice accuracy for time saving.” According to Larson, +/- 0.1 pH is more than adequate for fieldwork, while 0.02 is closer to what one would expect from a benchtop meter.
Features to look for
According to Angela Stuetz, product manager, SBU pH Lab at METTLER TOLEDO Analytical (Schwerzenbach, Switzerland), potential buyers of field-worthy meters should insist on portability, mechanical robustness, battery operation and sufficient battery life for typical workflows, adequate data storage (particularly for multichannel instruments, e.g., pH and conductivity), one-hand operation, and built-in protection from water and dust. “As for consumables, buffer solutions for calibration should be supplied in accessible, handy packaging,” she adds.
Multiparameter meters, Stuetz says, reduce workloads and save time. “You only need one meter but are able to measure two different parameters in parallel, within the same sample.”
First and foremost, a field-worthy pH meter must be rugged enough to withstand field conditions and travel. “You should be able to throw one into the back of a truck as you would any other piece of equipment,” says St. Pierre. The instrument should be completely waterproof as well. “And that means not just with the cable on and battery compartment closed, but with the cable off and the battery out, because users often change these components in the field.” Meters are also subject to unexpected mishaps like falls, which is why YSI field devices are drop-rated at one meter to concrete. Readability of the display panel in all lighting conditions, including bright sunlight by workers wearing polarized sunglasses, is an added touch.
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Cables on field pH meters take a lot of abuse, with users sometimes using them as ropes to swing probes to a measurement location. St. Pierre recommends that cables attach to meters through connections that easily handle this type of usage. Additionally, YSI subjects its field instrument cables to flex testing.
Readers familiar with laboratory pH probes recognize they’re delicate and even under ideal conditions have a finite life. Reference solutions leach out and eventually need replacement. That is why St. Pierre recommends systems with field-replaceable electrodes. “You don’t want to deal with downtime or to have to find special tools for replacing electrodes.” Ease of calibration and auto-buffer recognition allow sampling within minutes of electrode replacement.
Since pH electrodes are glass, they need protection during jobs that require lowering and raising probes into and from turbulent water or throwing them any distance. YSI ships systems with a mechanical guard over the tip and a larger guard over the whole probe assembly. “So when you throw the probe into a river and it hits a rock, the probe is fine,” St. Pierre adds.
Field-worthy instruments increasingly rely on global positioning systems (GPS) to support the integrity of the data they collect. In the case of field water measurements—a good deal of it regulated or subject to legal action—ensuring the location of data acquisition becomes critical. Long-term projects that generate thousands of measurements are similar. A GPS with the capability of logging data, date/time stamping it, and providing geographic coordinates proving where it was generated makes data handling simple.
With the appropriate computer and communications interface for transmitting data, logging calibrations, sensor value, calibration buffer, and general data maintenance, field pH meters can satisfy even the most stringent standards for Good Laboratory Practices. These capabilities become more significant for multimode meters that measure two or more parameters every time the probe is deployed.
A spec is not always a spec
St. Pierre notes that multi-probe meters are great, but for water measurements all sensors should be connected to the meter through a single cable so technicians can be sure they are measuring all parameters at the same exact location and not ten or 20 feet apart. Studies that relate pH with, say, dissolved oxygen may not be consistent if one probe is near the surface and another falls into a rotting log, similar to pH and temperature at different depths.
Instrument designers achieve collocation through the use of a bulkhead that can hold as many as four sensors, which could measure either pH, temperature, conductivity, dissolved oxygen, depth, or turbidity.
St. Pierre cautions buyers to ask serious questions about pH meter specifications. Specifically, does the vendor specify accuracy for the meter only, for the electronic printed circuit board that relates conductivity to pH, or for the entire system, including cables and probes? “Focus on the latter. Additionally, ask if those specs were generated over temperature or over all cable lengths, because pH measurements can be finicky across those variables, especially from analog systems.” Digital meters are not affected by cable length. Similarly, watch for accuracy of specs generated under ideal conditions or with a fieldimpractical configuration (e.g., very short cable).
For additional resources on pH meters, including useful articles and a list of manufacturers, visit www.labmanager.com/ph-meters