Practical guides

Measurement and calibration

An uncalibrated thermometer produces confident wrong numbers, and a moisture guess is not a measurement. These protocols turn the instruments from the buyers guide into numbers you can act on — and record, using the calibration record.

Which measurement for which decision

Calibrating a thermometer

Do this monthly, after any drop or impact, and whenever a reading surprises you. Record results in the calibration record.

1. Ice point (32 °F / 0 °C) Fill a cup with crushed ice, top with a little water, stir, and wait a minute. Insert the probe into the middle of the ice, not touching the sides. Stir gently and read after 30 seconds. Record the reading — the difference from 32 °F is your ice-point offset.
2. Boiling point (212 °F / 100 °C at sea level) Boil plain water (never salted), insert the probe mid-pot without touching the bottom, and read after 30 seconds. Altitude correction: subtract about 1 °F for every 500 ft above sea level (1 °C per 300 m).
3. Compute the offset If the ice point reads 34 °F, your instrument reads 2 °F high — subtract 2 °F from every future reading, or adjust the instrument if it has a calibration nut. A linear instrument should show the same offset at both points; if the offsets differ, use a two-point correction or replace the instrument.
4. Record and apply Write the offset on a sticker on the instrument and in the record. An unrecorded offset helps nobody.

Verifying dryer temperature

Dial thermostats are frequently 10–15 °F out, and bottom-heated stack dryers run much hotter near the element. Map your machine once and you will trust every schedule since.

1. Set the dial to your usual working temperature (e.g. 135 °F) and let the machine settle for 20–30 minutes empty.
2. Survey the tray positions Place a probe (or several, if you have them) at the centre of each tray position in turn, mid-air not touching anything. Record each position.
3. Compare with the dial Note the offset at each position. You now know the real set-point map: “tray 1 = 141 °F, tray 3 = 133 °F…”.
4. Repeat under load Run the survey again with a loaded machine — temperature drops and airflow changes under load, and the loaded map is the one that matters.
5. Use the map Put thicker or slower-drying pieces on the hottest positions; rotate based on the real differences, not guesses.

Oven loss-on-drying: a real moisture measurement

The cheapest accurate moisture test. You need a scale reading to 0.01 g, an oven, and a few hours.

1. Sample Take a representative sample — pieces from several tray positions, ground or finely chopped so it dries evenly. Weigh 20–50 g exactly (W₁).
2. Dry to constant weight Spread in a thin layer on a pre-weighed dish and dry at 100–105 °C (212–221 °F) for 2–4 hours. Cool in a dry place, weigh, return to the oven for another 30–60 minutes, weigh again. When two successive weighings differ by less than ~0.05 g, you are at constant weight (W₂).
3. Compute Moisture (wet basis) = (W₁ − W₂) / W₁ × 100 %. Cross-check the batch: predicted finished weight comes from the yield calculator.
4. Convert to aw Feed the measured moisture into the water activity estimator to see whether the batch is plausibly at aw ≤ 0.60.

Approximating water activity without a meter

Water activity is, by definition, the equilibrium relative humidity above the food (aw = ERH/100). That gives you a slow but genuine measurement:

1. Enclose a sample Place a small representative sample in a sealed jar with an accurate hygrometer — a small digital humidity logger is ideal — without the sensor touching the food.
2. Wait for equilibrium Keep the jar at stable room temperature. Small samples equilibrate in 12–24 hours; large or whole pieces can take 2–3 days. Temperature swings stretch this out.
3. Read The hygrometer's RH is the ERH; divide by 100 for aw. 58 % RH → aw 0.58 → shelf-stable by the aw limits.
4. Cross-check the scale Test the setup once with a known reference — salt slurry checks are the classic lab method (saturated NaCl ≈ aw 0.753 at 25 °C). If your jar-and-hygrometer disagrees wildly, trust the salt.

This method costs almost nothing and is accurate to roughly ±0.02–0.03 aw with a good hygrometer — enough to decide whether a batch is safely dry. For anything you sell regularly, a dedicated water activity meter pays for itself: it reads in minutes and is defensible to buyers and inspectors.

Moisture–protein ratio for jerky

The USDA-style stability target for jerky is MPR ≤ 0.75:1 — water mass divided by protein mass on a dry-matter basis. You cannot easily measure protein at home, so:

• Home route: treat MPR ≤ 0.75:1 as achieved by (a) a verified 160/165 °F lethality step, (b) drying to a finished weight near the lean-meat yield in the food guide, and (c) a firm, dry, fibrous texture — then keep finished jerky refrigerated anyway unless you have measured it.
• Commercial route: send a composite sample to a lab for moisture and protein (Kjeldahl or Dumas nitrogen × factor). One test per recipe change is usually sufficient; use the result to set a finished-weight target you can check on a scale for every batch after.

Measuring airflow and humidity

Air velocity (anemometer)

1. Survey the tray area Hold the vane at mid-height across several positions on an empty tray rack; record the spread. You are looking for roughly 1–2 m/s (200–400 ft/min) across the food.
2. Check for dead zones Corners and centre-bottom of stack dryers are typically slowest — those are the spots that stall.
3. Re-check loaded Trays of food throttle airflow; a machine that reads 2 m/s empty may deliver 0.8 loaded. Under-load figures are the ones that matter.

Ambient and exhaust humidity (hygrometer)

• Inlet RH tells you the drying potential of your air: above 65 %, expect stalls (humidity control).
• Exhaust RH tells you whether the machine is working: exhaust should read noticeably more humid than inlet. If inlet ≈ exhaust, the air is not picking up water — airflow or load problem, not a temperature problem.
• Room RH overnight predicts whether conditioning jars will sweat in humid climates.

Checking a 0.1 g scale

1. Zero check With nothing on the pan, the display should read 0.0 after taring; drift over a minute means batteries or a draught.
2. Known weight Use a calibration weight, a sealed coin of known mass (check current mint specs), or a measured volume of water (1 ml = 1.00 g). Record any offset.
3. Linearity spot-check Weigh the reference, then the reference plus a second object; the difference should match the second object's known mass. Cheap scales drift at the top of their range.
4. Use it properly Still air, level surface, and always the same corner of the bench. Cure dosing at 0.25 % of meat weight is exactly the kind of measurement where a sloppy scale is dangerous — see nitrite handling.

Turning measurements into records

A measurement that is not recorded cannot be verified, compared or defended. Log every protocol result in the calibration record, and every batch reading in the batch record; track weigh-ins interactively with the batch tracker. Records are what let you answer, months later: “was it the machine, the method, or the material?”