V-Notch vs Rectangular Notch Weir: Which to Buy

Every year I get the same call from a lab in-charge, usually two weeks before the equipment budget closes: “We have room for one notch weir apparatus this cycle. V-notch or rectangular?” It sounds like a small decision. It isn’t. Pick wrong and half your fluid mechanics batch struggles to read the head, or your research scholar can’t push flow high enough for a proper Cd curve.

TL;DR: For a diploma or first-year BTech lab where flows are small and you want clean readings, buy the V-notch (90 degrees, sharp-crested). For a senior BTech or research lab that needs a wider flow range and comparison with broad-crested behaviour, buy the rectangular. Ideally you buy a combined bench that mounts both plates, and one caveat holds either way: without a properly levelled approach channel and a still-water head gauge, neither weir will give you the coefficients your syllabus expects.

The short answer

If I had to spend one line-item, I’d pick the V-notch for teaching. Head varies with the 5/2 power of discharge, so at low Q the head reading is generous and students actually see the change. The rectangular notch is the honest workhorse for higher flows and for anyone doing a real Cd-vs-Reynolds study. In most engineering colleges I’ve supplied, the right answer is “both, on one bench” rather than choosing one.

Head-to-head at a glance

Working principle: how each one operates

A V-notch is a sharp-crested triangular plate, almost always at 90 degrees for teaching (60 and 45 exist for very small flows). Water in the approach channel rises until it spills over the vertex. Because the flow area widens with head, discharge scales with H raised to the 5/2 power. That non-linearity is exactly what makes it good at low flow: double the head, discharge goes up more than five times, so the hook gauge reading is never cramped. Coefficient of discharge sits around 0.58 to 0.62 for a well-machined plate with proper ventilation under the nappe.

A rectangular notch is a flat sharp-crested plate with a horizontal sill of length L. Discharge scales with H to the 3/2 power. That gentler curve is a liability at trickle flow (the head barely lifts off the sill) and an asset once you push serious water across the crest. Cd typically lands 0.60 to 0.63 for a suppressed rectangular notch. If you contract the sides you need the Francis correction (subtract 0.1 H per end contraction from L). Half the “wrong answer” reports I see in student journals come from ignoring that correction.

V-notch vs rectangular notch weir: which for which department

Diploma polytechnic (HSBTE, PSBTE, MSBTE, DGT-linked ITIs). V-notch every time. The polytechnic fluid mechanics syllabus caps at Cd determination and simple flow measurement. You want a curve students can actually plot in a 90-minute practical, and the V-notch delivers that at pump flows a small bench can supply.

BTech first and second year (AKTU, GTU, VTU, MAKAUT, BTEUP). Combined V-plus-rectangular on one bench. The course expects both derivations and both experiments. A single Scientico hydraulic bench with swap-in plates saves a purchase order and a workbench.

Senior BTech, MTech, research (INC-adjacent hydraulics labs, PhD scholars). Rectangular is the honest choice, ideally suppressed and contracted plates so you can teach the Francis correction with real data. Pair it with a venturi and pitot apparatus to triangulate against a differential measurement. This is also where you should ask for ISO 17025 traceable calibration of the head gauge, not just a factory sticker.

Nursing and pharmacy colleges (INC, PCI). Neither. Wrong department. Look at the nursing lab equipment guide or the pharmacy lab equipment guide.

Common mistakes when choosing

1. Buying the notch plate without the bench. A notch is only as good as the still water upstream of it. If your approach channel is short, turbulent, or unlevelled, your Cd will drift by 5 to 8 percent and no one will trust the lab report. Budget for the bench first, plates second.

2. Ignoring the nappe. Sharp-crested notches need air under the falling sheet. If the downstream side seals against water, the nappe clings, pressure drops, and discharge goes up without your head reading knowing about it. A proper apparatus has a vented sill. Ask for it in writing.

3. Reading head at the crest. Head must be measured at least 3 to 4 times the maximum head upstream of the plate, in a still-water pocket. A hook and point gauge in a stilling well is the right answer. A ruler taped to the tank is not.

4. Confusing sharp-crested with broad-crested. Different physics, different Cd, different syllabus. If your course lists broad-crested weir, a standard notch plate will not do the job. Say so at the enquiry stage.

Scientico’s recommendation

For most engineering colleges I specify the combined notch tank as an accessory to the Multi-Pump Test Rig FluidoSurgeX 294 or a dedicated FluidoSurge hydraulic bench. You get both a 90-degree V-notch and a rectangular plate (suppressed and contracted) with a machined stilling well, hook gauge to 0.1 mm, and a vented downstream chamber. Cd values in our factory acceptance test land inside the textbook bands. For a broader fluid mechanics setup, walk through the complete fluid mechanics lab setup guide before you finalise the PO. The full category is here: fluid mechanics notch weir apparatus and related benches.

Manufactured in Ambala since 1993, shipped to 60+ countries, built under ISO 9001:2015 with CE marking where the destination requires it. That last bit matters for EU and MENA buyers more than most Indian buyers realise.

Cost and delivery framework

I won’t invent a price. Notch plates alone are a low-tier line item. A bench with combined plates, hook gauge, and pump is a mid-tier line item, and combined kits with data logging move into the higher tier. Ask for a quotation against your exact syllabus, not a catalogue number.

Incoterms. Ex-Works Ambala is the default. FOB Nhava Sheva or Mundra is available for container shipments. CIF is possible for buyers who want a single landed figure but expect a small premium for the freight forwarder margin. For African and MENA orders I usually recommend CFR and let the buyer handle marine insurance locally.

Lead time. Standalone plates ship in a couple of weeks after payment clears. A full bench with pump and instrumentation takes several weeks of production. Add sea transit on top: roughly 2 to 3 weeks to Gulf ports, 4 to 6 weeks to East and West Africa, 5 to 7 weeks to Latin America. Air freight is possible for the plates but almost never worth it for the bench.

Documentation. Factory acceptance test report, calibration certificate for the hook gauge, packing list, invoice, certificate of origin, and a fumigation certificate on wooden crates for countries that ask for it. If your tender needs a specific format, share the template at the enquiry stage.

Ready to specify?

Send me your syllabus page (or your tender BOQ), your department, and your destination port. I’ll come back with the right notch configuration, the matching bench, and a firm quotation with realistic delivery. Start here: contact Scientico.

Frequently Asked Questions

Is a V-notch more accurate than a rectangular notch?

At low flows, yes. The V-notch head varies with the 5/2 power of discharge, so small changes in Q produce a readable change in H. At higher flows the rectangular notch pulls ahead because the V-notch plate starts to submerge and lose resolution.

Can one hydraulic bench take both notches?

Yes. A Scientico FluidoSurge hydraulic bench with a combined notch tank accepts a 90-degree V-notch plate and a rectangular plate (suppressed or contracted) as swap-in inserts, sharing the same stilling well and hook gauge.

What Cd should I expect during the practical?

For a well-machined 90-degree V-notch, roughly 0.58 to 0.62. For a sharp-crested rectangular notch with proper ventilation, roughly 0.60 to 0.63. Values outside these bands usually point to a clinging nappe, wrong head-measurement location, or a poorly levelled sill.

Do these apparatus meet Indian and international standards?

Scientico builds under ISO 9001:2015 with CE marking on request. The notch geometry follows the IS 1500 family and references ISO 1438 for sharp-crested weirs, and head gauges can be supplied with ISO 17025 traceable calibration for research labs.

What is the typical lead time for export?

Plate-only orders ship in a couple of weeks after payment. A full bench with pump and instrumentation takes several weeks of production. Sea transit adds another 2 to 7 weeks depending on the destination port.