1. Is the duration known in advance?

If the interval is fixed — a reagent must stand for exactly fifteen minutes, a sample must cure for four hours — you need a countdown timer that signals at zero, because the operator should not have to watch the display. Only the HW30 provides a countdown, up to ten hours.

If the duration is the quantity you are measuring — how long a tank takes to drain, how long a reaction runs to completion — you need a count-up stopwatch, and any of the 365510, STW515 or HW30 will do it.

2. How long is the longest single run?

This is the specification most often overlooked and it eliminates models outright. The HW30 counts to 9 hours 59 minutes; the 365510 and STW515 count to 23 hours 59 minutes. A test that runs a full working day plus setup will roll the HW30 over and quietly lose the total.

For anything approaching or exceeding ten hours — overnight soak tests, long curing intervals, extended pump or generator runs — specify the 365510 or STW515.

3. Where will it be read?

If the instrument will be used in anything less than good light, the STW515 is the only handheld stopwatch in the range with a backlight, and retrofitting a torch to the job is how splits get missed. Night pours, shaft work, plant rooms, cold rooms and pre-dawn starts all fall here.

If the reading has to be visible from across a room rather than in the hand, the CTH10A with its 109 mm digits is the only model designed for that distance.

4. Do conditions need to be recorded with the time?

Where a result is temperature or humidity dependent — concrete workability, adhesive cure, moisture equilibration, calibration records — capturing conditions on a second instrument invites transcription errors and mismatched timestamps.

The HW30 puts the interval and the conditions on one handheld device. For a fixed location that must be monitored continuously rather than during a test, the 445702 with max/min memory or the wall-mounted CTH10A is the better fit.

Resolution: 1/100 second

Resolution is simply the smallest increment the display can show. A reading of 04:32.17 tells you the instrument resolves to a hundredth of a second. It says nothing about whether that hundredth is correct, and it is not a claim of measurement uncertainty.

Accuracy: ±3 to ±5 seconds per day

This is drift of the quartz timebase over 24 hours of continuous running. Three seconds a day is roughly 35 parts per million. Scaled down, a five-minute interval accumulates well under one hundredth of a second of timebase error — far below the display resolution.

The error that actually dominates

For manually started and stopped measurements, human reaction time is the largest error term, typically 150 to 250 milliseconds per press and worse with gloves, fatigue or a distracted operator. On a short interval this dwarfs instrument error by two orders of magnitude.

The practical consequence: for intervals under about ten seconds, do not trust the second decimal place regardless of which model you buy. Repeat the measurement and average, or use an automatically triggered timing method.

Heat stress on South African sites

Summer conditions in the Lowveld, Limpopo, the Northern Cape and the KwaZulu-Natal coast regularly push apparent temperature well above dry-bulb readings once humidity is included. The HW30's heat index alarm gives a supervisor a defined trigger for rest and rehydration rather than an argument.

Note the limit honestly: heat index is derived from air temperature and humidity in shade. Formal occupational thermal stress assessment uses Wet Bulb Globe Temperature, which also accounts for radiant heat and air movement, so a heat index device will under-represent risk in direct sun or beside a furnace or kiln. Treat the HW30 as a screening and awareness instrument, and specify a WBGT meter where a documented assessment is required.

Timing through load shedding

Scheduled supply interruptions have made two timing tasks routine on South African sites: verifying generator and UPS changeover intervals, and measuring how long a battery bank or inverter actually sustains a load against its rated runtime.

Both are count-up measurements that can run for hours, so the 23-hour capacity of the 365510 or STW515 is the correct specification. Changeover testing frequently happens in a dark plant room during an outage, which is where the STW515's backlight stops being a luxury. All five instruments are battery powered and unaffected by the outage itself.

Concrete, materials and civil testing

Slump tests, initial and final setting times, mixing intervals, curing checks and permeability tests all depend on reliable elapsed-time measurement rather than high-precision timing, which puts them comfortably within the 365510 and STW515.

Because workability and cure rate are both temperature and humidity dependent, and a Highveld winter morning and a coastal summer afternoon are very different curing environments, the HW30 is worth specifying where the conditions at the time of test must be recorded on the same sheet as the timing.

Laboratory and cold chain monitoring

Reagent stores, sample rooms, incubators, wine cellars and vaccine or pharmaceutical fridges all need a visible record of the conditions a stored item has actually experienced, not just the conditions at the moment someone happened to look.

The 445702's max/min memory with reset answers exactly that: reset it at the start of a period and the extremes reached over a weekend, a long weekend or a public holiday are still on the display when staff return. Its −10 to 60 °C range covers ambient rooms and cool stores, though not deep-freeze applications.

Time zone and clock setting

South Africa observes South African Standard Time (SAST), which is UTC+2 throughout the year with no daylight saving transition. On any Extech clock in this range that offers an automatic daylight saving adjustment, that setting should be left off — leaving it enabled will shift the clock by an hour at a Northern Hemisphere transition date that has no local meaning.

The CTH10A's radio time-code receiver is designed for the North American WWVB broadcast and has no usable signal in southern Africa, so it is set manually here. This does not affect the clock's timekeeping in normal use.

Delivery and support

Extech stopwatches, timers and clocks are supplied from our Cape Town base throughout South Africa and to Namibia, Botswana, Zimbabwe, Mozambique, Lesotho and Eswatini.

For advice on which model suits a particular test method, or to discuss calibration certificate requirements before ordering, contact us on +27 21 911 5835, or email sales@ecotao.co.za or aqua@ecotao.co.za.