APERA INSTRUMENTS / TN400 / AI481

Infrared turbidity in the field.
Five standards in the case.

The TN400 is Apera’s portable infrared turbidimeter. An 860 ± 30 nm LED and a detector at 90° measure scattered light by the nephelometric method that ISO 7027 and DIN EN 27027 describe, across 0–1000 NTU with automatic range selection: 0.01 NTU resolution below 20 NTU, 0.1 NTU to 99.9 NTU, and 1 NTU above. It runs on four AA batteries in an IP67 housing and ships in a carrying case with five AMCO calibration standards and six numbered sample vials.

Choose AI481 when your procedure calls for the infrared method and a written log is enough. The TN400 takes a single reading or a 10-reading average and shows it on a color TFT screen; Apera documents no onboard data storage or computer export. If you need stored, exportable records, the TN480 is the infrared model with logging.

Infrared, ISO 7027 method

The light source is an infrared LED at 860 ± 30 nm, with the detector at 90° to the beam. Apera states compliance with ISO 7027 and DIN EN 27027. White-light meters built to EPA 180.1, such as the TN420 and TN500, use a different optical method; choose by the procedure you report against.

Normal or 10-reading average

Press MEAS/ENTER for a normal reading, shown after about 10 seconds. Long-press it for average mode: ten consecutive readings, one every 8 seconds, then the average on screen. Apera recommends average mode for samples that settle quickly or keep changing.

Five-point AMCO calibration

The meter calibrates at 0, 20, 100, 400 and 800 NTU, and the kit supplies all five standards. It leaves the factory calibrated across the full range; afterwards you can calibrate two or more points, as long as your measuring range sits between them.

Know what arrives
in the carrying case.

Kit list follows Apera’s TN400 instruction manual, section 3.2, and Apera’s US product page.

Know the sample holder.
Line up the arrows.

The vial sits in a holder under a flip cover. Take out the dust-proof plug before measuring, and close the cover for every reading. Each vial cap carries an arrow; line it up with the arrow on the holder every time, because a vial reads slightly differently at different rotations.

Set the meter on a flat, level surface and leave it there. Apera’s manual says not to hold it during calibration or measurement, and to wait one to two minutes after seating a vial, since moving the vial sets the liquid in motion.

Prepare the vial.
Bubbles and smudges read as turbidity.

Clean a vial with detergent inside and out, rinse it several times with distilled or deionized water, then rinse it twice with the sample. Fill to about four-fifths, roughly 18 mL, and close the lid. Turn the vial gently to mix; do not shake it hard. Let it stand two to five minutes until the bubbles clear.

Wipe the outside dry with the microfiber cloth. Every several days, or about once a week, spread a single drop of silicone oil over the glass to mask fine scratches, then polish off the excess with filter paper or good tissue; leftover oil also affects the reading. Hold vials by the cap and base so no fingerprints land on the glass, and keep each vial with its own numbered cap.

Calibrate the points you need.
Select the right standard.

For low-range or high-accuracy work, warm the meter up first: long-press MEAS/ENTER with no vial in place, wait three to five minutes, then press CAL/ESC. Seat the 0.0 NTU vial, press CAL/ESC to open the calibration menu and MEAS/ENTER to calibrate; the menu marks the point OK. Repeat with 20.0 NTU, and with 100, 400 and 800 NTU if your range calls for them, then press CAL/ESC to return to measurement.

The meter does not recognize which standard is in the holder. If you select the wrong point in the menu, the calibration and every reading after it will be wrong; recalibrate with the correct selections to fix it. Formazin can be used instead of AMCO standards, but only when freshly prepared.

Verify the points.
Then decide how often to recalibrate.

After calibrating, read each standard back as a sample. Apera’s reference tolerances are 0.05 NTU or better at 0 NTU, ±0.2 NTU at 20 NTU, ±2 NTU at 100 NTU and ±5 NTU at 400 and 800 NTU. If a point is outside its tolerance, calibrate it again.

Above 2 NTU, Apera recommends calibrating weekly or checking a standard close to the sample value and recalibrating if the error is large. Below 2 NTU, read the 0.0 NTU standard first; if it is not close enough, calibrate at 0.0 and 20.0 NTU and measure in vial 1# or 2#, which Apera reserves for low-turbidity work. Using the same vial for the zero point and the sample removes vial-to-vial differences.

Sealed nonzero standards

The 20, 100, 400 and 800 NTU vials cannot be opened. The AMCO polymer does not settle, so normally it needs no mixing; if a vial has sat unused for a long time, turn it over slowly twice and let it stand two minutes. Replace the set when it expires.

Refillable zero

The 0.0 NTU vial can be opened. Refill it with AMCO 0.0 NTU solution, which Apera rates for 6–12 months, or with fresh lab-grade distilled water, which is good for only a couple of days. Rinse the vial three times with distilled water before refilling, and do not shake or flip the zero standard.

Storage

Store the calibration solutions between 5 and 30°C and do not let them freeze. The meter itself can be stored from −40 to 60°C and operated from 0 to 50°C.

Read once, or average ten.

In normal mode the meter shows a progress bar and displays the result after about 10 seconds; press MEAS/ENTER again to repeat. For average mode, long-press MEAS/ENTER until the meter beeps. It takes ten readings, one every 8 seconds, counting them on screen, and finishes with the average marked on the display.

Measure as soon as the sample is mixed and free of bubbles, before it settles or changes temperature. Avoid diluting samples where you can, keep the meter out of direct sunlight, and use the same mixing and waiting routine whenever two results need to be compared.

Blue to measure, green to calibrate.

The color TFT screen uses a blue background in measurement mode and green in calibration, with on-screen prompts for the next step and a progress bar while a reading is taken. In the calibration menu, an OK beside each standard value shows which points are done.

Four keys run the meter. Power switches it on and off, and it turns itself off ten minutes after the last key press. CAL/ESC enters and leaves calibration, and a long press switches the unit between NTU and FTU. MEAS/ENTER takes readings and confirms calibration points, and the arrows choose the standard in the calibration menu. A battery icon warns when the four AA cells need replacing; replace all four together.

Check the specifications.

Apera specifies accuracy of ±2% of reading plus stray light, stray light of 0.02 NTU or less, and repeatability of ±1% of reading or 0.02 NTU, whichever is greater. Resolution is 0.01, 0.1 or 1 NTU depending on the range; resolution is the display step, not the accuracy.

The detector is silicon photovoltaic. Vials are Φ25 × 60 mm high-borosilicate glass holding about 18 mL. The meter operates at 0–50°C and up to 90% relative humidity at 30°C (80% at 40°C, 70% at 50°C), non-condensing. It measures 90 × 203 × 80 mm and weighs 385 g, carries CE and RoHS marks, and is IP67 rated.

Order replacements by part number.

Apera lists four TN400 consumables: T500-2, AMCO 0.0 NTU solution in a 100 mL bottle; T200-1, the 20/100/400/800 NTU AMCO set specified for the TN400 and TN480; T500-3, a set of six numbered sample vials with lids; and TN400-S3, 10 mL of silicone oil.

The T500-1 set on this site is assigned to the white-light TN420 and TN500. Matching NTU values alone do not make a standard set interchangeable, so order T200-1 for the TN400.