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How to use a tachometer and get one stable reading

Updated: October 2026

You have a tachometer out of its box, a machine in front of you, and a number on the display that keeps jumping or looks half what it should be. Learning how to use a tachometer starts with knowing which kind you are holding, because a laser photo meter and an inductive hour meter follow different procedures, and a mistake in either looks like a broken meter. This page walks through both, then explains the readings that are setup faults rather than defects. If you are still choosing a model, the [main tachometer comparison](/hub/saw-accessories/tachometer) lists all twelve amazon.co.uk listings, and the [tachometer buying guide](/hub/saw-accessories/tachometer/sub/tachometer-buying-guide) helps you pick a family first.

You have a tachometer out of its box, a machine in front of you, and a number on the display that keeps jumping or looks half what it should be.

Our top picks

YOOREAL YOOREAL YR-HM011F hour tachometer

Best overall

YOOREAL YOOREAL YR-HM011F hour tachometer
7.1/10about £13.99
View offer
DANOPLUS DANOPLUS DP-401 2-in-1 tachometer

Alternative

DANOPLUS DANOPLUS DP-401 2-in-1 tachometer
6.7/10about £29.99
View offer
NInE-ROnG NInE-ROnG RL-HM032R hour tachometer

Rank 3

NInE-ROnG NInE-ROnG RL-HM032R hour tachometer
7.3/10about £18.59
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On this page
  1. 1.Which procedure you are in
  2. 2.A non-contact reading
  3. 3.An inductive reading
  4. 4.When the number is not a fault

Which procedure you are in

If the box holds a handheld meter, a lens, and reflective tape, you are using a photo tachometer. Rubber or plastic heads make it a 2-in-1, so you can also measure by contact. A small box with a wire or a clip and no lens is an inductive meter. It reads spark pulses in a petrol plug lead. The methods do not overlap. An inductive meter cannot read a lathe, a drill, or an electric motor. A laser can, in principle, see a petrol flywheel, but holding a beam next to a blade or chain is not a safe way to tune. Use the procedure that matches the meter, and skip the other section. For a 2-in-1, the laser or contact tachometer page covers which mode to choose. The steps below assume you have already chosen.

A non-contact reading

Start with the machine stopped and isolated. Stick a small square of reflective tape on a clean patch. The dark area around it should be much larger, because the meter counts each flash as one revolution. On a shiny shaft, put black tape down first, or the bare metal adds extra counts. Most photo meters here state 50–500 mm. NAMVO also prints 50–200 mm. Hold the meter steady inside that range and shade the target. A UK QWORK buyer saw the figure jump from hundreds to thousands on a lathe. A larger dark target with one mark is the quick check. None of these listings prints a laser class. Keep the beam below eye level. If you would lean over a moving cutter, stop.
YOOREAL YOOREAL YR-HM011F hour tachometer

Testsieger

YOOREAL YOOREAL YR-HM011F hour tachometer
7.1/10about £13.99
View offer

An inductive reading

Switch the engine off and let it cool before you route anything near the exhaust. The NInE-ROnG RL-HM032R says four to five turns of its wire around the plug lead. The YOOREAL YR-HM011J uses a clip, so no wrap is needed. Other meters do not print a turn count. Follow the manual in the box. Keep the wire off hot metal and moving parts, and confirm a reading before you fix the meter. Hours may show until the engine starts, then rpm, and the counter may not move for the first few minutes. Choose the firing pattern against a known figure, such as a bike's own tachometer. One UK reviewer found adhesive pads failed on a mower and used the screw holes. If a chainsaw chain still moves at idle, stop and use a dealer. The small engine tachometer page explains why the target comes from that saw's manual.

When the number is not a fault

A figure that is exactly half or double the expected rpm usually means the wrong firing pattern. A UK YR-HM011E buyer saw 3,000 rpm while the bike showed 6,000, until the wasted-spark setting was chosen. A blank photo meter often means no battery. QWORK and NAMVO omit the 9 V cell. One UK NAMVO buyer reported smoke after a reversed cell. If a fresh battery still does nothing, return the meter under the Consumer Rights Act 2015. If you buy through our links, we may earn a commission at no extra cost to you.

Common questions

Where do I put the reflective tape on a tachometer?
Stick a small square of reflective tape on a clean, flat spot on the rotating part, with the machine stopped. The surrounding area should be dark and much larger than the tape. On a shiny or polished shaft, put black tape down first and the reflective piece on top. That stops bare metal from reflecting the beam and giving extra counts.
Why does my laser tachometer reading jump around?
The usual causes are a target that is too small, a shiny surface reflecting the beam, bright light hitting the target, or a hand that drifts during the reading. Use a larger dark patch with one reflective mark, shade the target, and hold the meter steady inside the stated distance. If a big, matt target still gives random numbers, the meter may be faulty.
Why is my inductive tachometer reading half the rpm?
A reading that is exactly half or double the expected figure usually means the firing-pattern setting does not match the engine. Some engines fire the plug once per revolution and others every second revolution. Change the pattern in the menu and compare the result with a known speed, such as a vehicle's own tachometer, until the two agree.
How many times should I wrap the wire around the plug lead?
Follow the manual for your meter. The NInE-ROnG RL-HM032R listing says four to five turns, while the YOOREAL YR-HM011J uses a clip on the plug and needs no wrap. Other listings in this set do not print a turn count. Keep the wire close to the plug cap and clear of the exhaust and any moving parts.
Why do the hours stay at zero after fitting an hour meter?
Many hour meters show time to one decimal place, as the 9,999.9 h service timer on the YOOREAL YR-HM011F shows, so the figure may not change for the first few minutes of running. Some models also show the hour total only while the engine is off and switch to rpm once it starts. Run the engine for ten minutes or so and check again before deciding the counter is not working.
What should I do if my tachometer will not switch on?
First check the battery. Several budget photo meters on amazon.co.uk ship without their 9 V cell, and a cell fitted the wrong way round will not work and may damage the meter. If a fresh battery fitted correctly still does nothing, the unit is likely faulty. Return it to the seller rather than open the housing.

Related pages

Tachometer

Best tachometer UK: two lists, not a single winner

Digital tachometer buying guide: photo or hour meter

Contact vs non-contact tachometer: which mode to use

Small engine tachometer hour meter for petrol tools

Sources

Sources checked for this comparison; last retrieved on 16 October 2026.

  1. GOV.UK, laser radiation safety advice (retrieved 16 October 2026)
  2. HSE, lasers (retrieved 16 October 2026)
  3. HSE, work-equipment inspection (retrieved 16 October 2026)
  4. HSE, training and competence (retrieved 16 October 2026)
  5. Consumer Rights Act 2015, section 9 (retrieved 16 October 2026)
  6. Consumer Rights Act 2015, section 22 (retrieved 16 October 2026)
  7. Compact Instruments, optical tachometer manual (retrieved 16 October 2026)
  8. STIHL MS 170/180 instruction manual (retrieved 16 October 2026)