Getting Started With The Teledyne ISCO GLS Sampler
The Teledyne ISCO GLS sampler is a submersible, peristaltic pump device used for collecting groundwater samples from monitoring wells. It connects to an ISCO remote monitor like a 3700 or 4200, runs on 12V DC, and uses disposable tubing to pull water up and into sample containers. The manual covers installation, programming, calibration, maintenance, and troubleshooting. If you are reading this because your field technician called in saying the sampler pulled nothing but air, this is probably useful. The basic workflow involves lowering the sampler into the well, priming the tubing, running a purge cycle, then starting the sampling program. The GLS does not have an internal pump motor — it relies entirely on the peristaltic head squeezing the tubing against a rotor. That means tube condition is everything. A single nick or a stretched section near the roller will kill your flow rate and introduce real variability into your composite samples.
Unlock The Power Of Groundwater Sampling Mastering The Teledyne Isco Gls Sampler Manual
The manual itself is organized by task, not by concept, which makes it fine for quick lookups but thin on the why. I found it more useful to treat it as a reference than a textbook. The programming section under the 3700/4200 interface is where most people waste time. The GLS parameters — flow rate, volume per interval, purge time, and tube replacement schedule — are set through the host instrument menu system. You do not program the GLS directly as a standalone unit. Here is what actually matters in practice. Flow rate on the GLS 100 is rated at roughly 40 mL/min with fresh tubing. That drops to maybe 25 mL/min once the tube has been running for a few days and starts to compress. If your program calls for 200 mL every 4 hours, budget accordingly. A new tube gets it done in about 8 minutes. An aged tube might take 13. That matters when you are chasing event-based sampling windows. I ran into a specific problem last fall on a project in northern Michigan where the well had a static water level around 18 feet and the sampling depth was set for 22 feet. The GLS was pulling fine during commissioning, but after three weeks of intermittent duty cycles, the sample volumes dropped by nearly 40 percent. No alarms triggered. The instrument logged normal cycle counts. I spent a morning tracing the issue and realized the tubing had developed a subtle constriction point right where the first roller made contact on the return side. The pump was still moving fluid, just not at the expected rate, and the volume integration from the peristaltic pulse counting was underreporting.
The fix was straightforward but not obvious from the manual alone. I replaced the tubing, yes, but also switched to rotating the tube orientation by 90 degrees at each replacement so the wear patterns never aligned with the same roller. It extended tube life from roughly 2 weeks to about 3.5 weeks on that particular well, which had a slightly abrasive silica sand presence in the borehole water. Worth noting: ISCO does not officially recommend this in the manual, but it is a field-honed practice that shows up in discussion threads among people who run these things constantly. Calibration is another area where people get sloppy. The GLS does not have a built-in flow sensor. Volume determination relies on pulse counting from the peristaltic head rotation. That means there is no active feedback loop correcting for tube wear or viscosity changes. You calibrate by running a known volume and adjusting the calibration factor in the host instrument. I recommend doing this every time you replace the tube, not just on installation. A 5 percent error in calibration factor translates directly into a 5 percent error in your reported concentration timeseries, which is the kind of thing that looks fine on a QC chart until someone asks you to defend your data at a regulatory review. One counter-intuitive thing about the GLS: lower is not always better for submersion depth. The manual states an operating depth range, and staying within it is important, but running the unit too deep introduces hydrostatic backpressure that the peristaltic pump has to work against. On a well with a 30-foot dynamic head, dropping the GLS to 25 feet submerged can cut your effective flow rate significantly compared to keeping it at 12 feet. The pump moves water by squeezing, not by pushing. There is a practical limit to how much backpressure it can overcome before the rollers start slipping against the tube rather than closing it completely. Check your well specs and position the sampler accordingly.
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Another common mistake is ignoring purge volume calculations. The GLS manual walks through how to compute purge volume based on well casing volume and static water level, but field crews often skip it and just pull a few hundred milliliters because that is what the default program uses. If your monitoring well has a 4-inch casing and a 10-foot static level, that is roughly 2 gallons of water standing in the casing. Pulling 500 mL and calling it purged is not defensible. Standard practice is three casing volumes minimum before starting your composite sample. For a 4-inch by 20-foot well, that is closer to 18 gallons. Factor that time into your program or your sampling window will collapse. Maintenance intervals from the manual suggest tube replacement every 1 to 4 weeks depending on application. In my experience, sandy or acidic waters push that toward the 1-week end. Clean municipal monitoring wells might let you stretch to 4 or 5 weeks. The telltale signs are volume drift, increased cycle times, and occasional air ingestion when the tube collapses completely under the roller. Do not wait for an alarm. The GLS will not alarm on volume deviation unless you have configured that explicitly, and even then the threshold settings are easy to misconfigure. Speaking of alarms, the alarm logic is configurable but defaults are not always sensible. The low voltage alarm, for example, triggers at a preset threshold that may be too conservative for battery-powered deployments. If you are running off a 12V lead acid in a remote site, you might want to lower that threshold slightly so the sampler finishes its last cycle rather than cutting off mid-sample. Conversely, the tube lifetime counter is just a timer-based estimate and has nothing to do with actual tube condition. Treat it as a scheduling reminder, not a diagnostic tool.
Data management is handled through the host instrument, not the GLS itself. Samples are logged as events with timestamps, volumes, and any alarm conditions. Exporting that data via RS-232 or USB gives you a CSV-style file that most lab QA teams can parse without issues. If you are integrating with a central database or cloud platform, the Teledyne telemetry modules work but require separate configuration. The manual touches on this briefly but assumes you already know the broader ISCO ecosystem. If you are new to it, expect to spend an afternoon cross-referencing the GLS manual with the host monitor manual. The biggest limitation of the GLS system is probably the peristaltic approach itself. Tubing wear is unavoidable. Flow rate degrades predictably but not linearly. Viscosity and temperature changes affect throughput. Air entrainment can occur if the intake is too close to the water surface or if the well draws down rapidly during pumping. None of these are fatal flaws, but they mean the GLS is not a set-and-forget device the way some modern ultrasonic or diaphragm-pump samplers claim to be. It requires attention, regular tube changes, and a operator who understands what the pulse-counting volume method actually implies for data quality. If you need continuous, unattended sampling in a harsh environment with high sediment loads, you might look at alternatives like the ISCO 3700 with a diaphragm pump option or a dedicated submersible pump sampler. The GLS excels in relatively clean wells where composite sampling over days or weeks is the goal and where sample contamination from internal pump materials is a concern — the tubing is the only wetted path, and it is disposable. For trace organics or metal analyses where cross-contamination between samples is a real risk, the GLS design is genuinely advantageous.
The manual is available from Teledyne ISCO's website. It is dense, occasionally contradictory between revisions, and missing some of the practical details that field experience fills in. But it is the authoritative source for wiring diagrams, parameter ranges, and alarm codes. Keep it open while you work. Pair it with the host instrument manual. And replace that tubing before it fails you, not after.
