How to Compare Horizontal Mud Mixers for HDD: A Buyer’s Guide

Choosing the right horizontal mud mixer is one of the highest-leverage equipment decisions on an HDD rig — get it right and your rig stays running; get it wrong and you’re fighting downtime waiting on mud. This guide breaks down what actually separates one horizontal mud mixer from another, from shear quality to how manufacturers measure GPM, plus a full look at the STS lineup.

Key takeaways

• The right mixer size is driven by how fast your rig consumes fluid, not just total tank capacity — undersizing costs you rig downtime waiting on mud.
• Shear quality determines whether bentonite fully yields or leaves clumps that clog your system downhole — this matters more than raw horsepower.
• Power source (gas, diesel, electric) should match your jobsite constraints, not just what’s cheapest up front.
• Confirm whether a mixer can agitate the tank and supply the drill simultaneously — some systems force a choice between the two, which matters most on long bores.
• Treat published GPM numbers with caution: ask whether it’s measured at the pump or delivered to the drill, and against clean water or field-viscosity bentonite.
• STS’s line ranges from a 2″ hopper system (CAD $4,349) up to the 1,500-gallon M-1500 (CAD $47,850), covering utility HDD through large-diameter bores.

What actually matters when comparing a horizontal mud mixer

Every mixer on the market claims to be fast and reliable. The criteria below are what actually separate a mixer that keeps your rig running from one that becomes the bottleneck on site.

Shear quality, not just mixing. A mixer that just stirs bentonite leaves clumps (“fish-eyes”) that reduce fluid effectiveness and can clog nozzles or the drill string downhole. A true high-shear system tears the bentonite particles apart as they hit water, so the polymer hydrates fully and fast — that’s the difference between “mixed” and “yielded.”

Output rate versus rig demand. Tank capacity alone doesn’t tell you whether a mixer keeps up with your rig. What matters is how fast a full batch actually yields — a mixer with a smaller tank that yields in half the time can outperform a larger tank that yields slowly, especially on jobs with frequent short bores.

Tank capacity versus jobsite footprint. A larger-tank horizontal mud mixer means fewer mixing cycles, but also a bigger, heavier unit to move, site, and store between jobs. Utility and compact-rig work generally favors smaller, more maneuverable systems; larger bores and maxi-rig work justify the footprint of a bigger tank.

Recirculation and pump/plumbing size. Larger-bore pumps (STS’s line runs 2″ through 4″) move fluid faster during fill and discharge, which shortens cycle time — but also add cost and physical size. Recirculation capability matters for keeping mixed mud in suspension between uses rather than letting solids settle out.

Simultaneous agitation and supply, or one-or-the-other. Some mixers can only do one job at a time: agitate/recirculate the tank to keep bentonite solids in suspension, or pump supply out to the drill — not both at once. If agitation has to stop while supplying the rig, solids can begin settling out during a long pull, degrading fluid quality exactly when you need it most; if supply has to stop to agitate, the rig sits idle. Ask directly whether a system can agitate and supply simultaneously, or whether it’s one-or-the-other — this matters more on longer bores where a single continuous pull can run well past the point where a static tank starts to settle.

How the GPM number is actually measured. A published GPM rating can mean very different things depending on how a manufacturer tests it, and two questions are worth asking before comparing numbers across brands. First: is the GPM figure measured at the pump itself, or delivered to the drill after running through hoses, fittings, and head-pressure losses — pump-rated GPM is almost always higher than what actually reaches the rig. Second: is the number based on clean water or on bentonite mud at a standard field viscosity (commonly measured by Marsh funnel time)? Pumping viscous bentonite fluid takes meaningfully more effort than pumping water, so a clean-water GPM rating can overstate real-world output by a wide margin. A mixer rated for a given output “in under 8 minutes” or “at X GPM” is only a fair comparison against another mixer if both numbers were measured the same way — ask the manufacturer directly rather than taking the headline number at face value.

Power source flexibility. Gas, diesel, and electric configurations each solve different jobsite problems — diesel for remote sites without easy fuel access matching other equipment, electric where noise or emissions restrictions apply, gas for straightforward cost and simplicity. The right choice depends on your jobsite constraints more than any inherent performance difference.

Durability and cleanout. Bentonite is abrasive and cures hard if left to sit. A design that’s easy to fully drain and clean between uses avoids the compounding problem of hardened residue reducing tank volume and fouling components over time.

Who built it, and on what experience. A mixer designed by people who’ve actually run drilling fluid systems on real jobsites tends to solve the problems contractors actually have — uptime, ease of use, consistent fluid quality — rather than problems that look good in a spec sheet.

STS horizontal mud mixer lineup at a glance

Pricing and specs as listed on stsmixers.com; confirm current figures before publishing, since these change.

The M-750 in detail

The M-750 is positioned as STS’s flagship — the site describes it as “the original king of mixers.” Confirmed specs:

• Tank capacity: 750 gallons
• Output: produces a full 750 gallons of fully yielded bentonite mud in under 8 minutes
• Flow rate: 237 GPM, tested with plain water — as with any mixer, actual field output will be lower with bentonite mud at working viscosity
• Mixing system: 4″ proprietary STS four-point shear system — mixing, shearing, agitation, and delivery happen simultaneously rather than as separate steps
• Simultaneous agitation and supply: yes — agitates the tank and supplies the drill at the same time, so bentonite doesn’t settle out during long pulls and the rig isn’t waiting on the mixer
• Dimensions: 92″ W × 90″ L × 75″ H
• Weight: approximately 2,010 lbs
• Configuration: available in the original square layout or a longer, narrower footprint for tighter jobsites
• Power: available in gas, diesel, and electric configurations, consistent with the rest of the STS line

Why field-built design matters

STS was founded in 1992 by people with over 25 years of hands-on drilling experience at the time, with a singular focus from day one: build a horizontal mud mixer that actually holds up on real HDD jobsites. After their drill manufacturing division was acquired by Ingersoll Rand in the late 1990s, the company narrowed entirely to mixing technology — three-plus decades of that focus now sit behind the patented four-point shear system used across the current line. That background shows up less in the spec sheet than in the details contractors notice on site: how fast a batch actually yields, how easy the tank is to clean out, and whether the thing still runs the way it did on day one after a season of abuse. For broader guidance on trenchless installation practices industry-wide, the North American Society for Trenchless Technology (NASTT) maintains training and standards resources for HDD contractors.

Frequently asked questions

How much bentonite mud do I need for a typical HDD bore? It depends on bore length, diameter, and soil conditions — sandier or looser soils generally consume more fluid volume. Rather than guessing at capacity, size your horizontal mud mixer to your rig’s actual fluid draw rate on your typical bore profile, and talk to your mixer manufacturer about matching output rate to that draw.

What’s the difference between a 2″ and a 4″ high-shear mixer? The number refers to pump/plumbing bore size, which drives fill and discharge speed. A 2″ system is lighter, more portable, and suited to utility-scale work; a 4″ system moves significantly more volume per cycle, suited to larger bores and maxi-rig operations where cycle time matters more.

Does it matter whether a mixer can agitate the tank and supply the drill at the same time? Yes — on a long or continuous bore, if a mixer has to choose between agitating the tank and supplying fluid, you either risk bentonite settling out mid-pull or you stall the rig waiting on the mixer. Every mixer in the STS line, including the M-750, is designed to do both at once. That’s not a given across the industry — confirm it directly with any other manufacturer you’re comparing against.

Why does GPM vary so much between manufacturers’ published specs? Because “GPM” isn’t a standardized measurement across the industry. It can be measured at the pump before losses, or as delivered volume at the drill; and it can be tested against clean water or against bentonite mud at a representative field viscosity. These produce very different numbers for what might be the same physical pump. Always ask which measurement a published spec represents before using it to compare mixers.

Gas, diesel, or electric — which power option should I choose? Match it to your jobsite, not just sticker price. Diesel suits remote sites already running diesel equipment. Electric suits sites with noise or emissions restrictions, or where power is already run to the site. Gas is often the simplest and lowest-cost option for straightforward access.

How fast should a mud mixer yield bentonite? Faster full-yield time directly reduces rig downtime waiting on mud. As a reference point, STS’s M-750 yields a full 750-gallon batch in under 8 minutes — use that as a benchmark when comparing other systems’ stated yield times, but confirm what conditions that figure (and any competitor’s) was tested under.

Can I upgrade from a smaller mixer as my jobs get bigger? Many contractors start with a compact system and move up as job size grows. Talk to the manufacturer directly about trade-in or upgrade paths rather than assuming resale is your only option.