Salt and water hold sealed air at 75%
A salt test is a calibration check that uses damp table salt in a sealed bag to hold the air at a known 75% relative humidity. Mix salt with a few drops of water until it looks like wet sand. No puddle. Seal it in a bag with the hygrometer, keep it at room temperature, and wait.
How long? Boveda asks for at least 24 hours with its kit. Give a salt bag the same. Read the meter before you open the bag, because room air starts moving the number right away.
The full step-by-step method is in our guide on how to calibrate a hygrometer.
Why 75 and not some other number?
Because saturated sodium chloride sits at about 75% RH across normal room temperatures. The National Bureau of Standards (now NIST) measured it. The figure barely moves with temperature, which is what makes it a good home reference.
| Room temperature | Saturated salt RH | Uncertainty |
|---|---|---|
| 20 °C (68 °F) | 75.47% | ±0.14 |
| 25 °C (77 °F) | 75.29% | ±0.12 |
The salt is accurate to a few tenths of a point. Your bag seal and your patience matter far more. The calculator uses 75% for a salt test.
Offsets for common salt test readings
An offset is the number of points you add to, or take off, every reading. Find what your meter showed in the salt test, then read across to see what a humidor display really means.
| Salt test reading | Offset | Display 65 is really | Display 69 is really | Display 72 is really |
|---|---|---|---|---|
| 70% | +5 | 70% | 74% | 77% |
| 71% | +4 | 69% | 73% | 76% |
| 72% | +3 | 68% | 72% | 75% |
| 73% | +2 | 67% | 71% | 74% |
| 74% | +1 | 66% | 70% | 73% |
| 75% | 0 | 65% | 69% | 72% |
| 76% | -1 | 64% | 68% | 71% |
| 77% | -2 | 63% | 67% | 70% |
| 78% | -3 | 62% | 66% | 69% |
| 79% | -4 | 61% | 65% | 68% |
| 80% | -5 | 60% | 64% | 67% |
Note the offset or turn the dial
You have two choices once you know the error, and Boveda lists both: record the difference and apply it to every later reading, or adjust the meter itself to match the reference. Analog dials often have a small screw or ring on the back. Some digital units have a calibrate setting. Many have neither.
No adjustment? Write the offset on a piece of tape and stick it on the meter. Done.
How far off is too far?
That depends on how tight you run your humidor, so the calculator lets you pick the tolerance. Our own rule of thumb, not a standard: within 2 points, live with it and use the offset. Off by 5 or more, retest, because a bad seal or a cold room can fake an error.
One test checks one point. A meter can be right at 75% and still drift at 65%. A second reference at a lower RH, such as a kit at another level, tells you whether the error is the same across the range. Boveda calls that a two-point calibration.
Recheck twice a year
Boveda tells kit owners to recalibrate every six months and to test even a new, pre-calibrated meter before trusting it. Sensors drift.
Your humidity target also depends on the room. The 70/70 humidity calculator adjusts it for a warmer or cooler house, and the humidor seasoning guide covers what to expect from a box that is still drinking water.
Salt test and hygrometer offset questions
What should a hygrometer read in a salt test?
75%. Saturated table salt holds sealed air at about 75.3 to 75.5% RH at room temperature, so any other number is your meter's error. Subtract the reading from 75 to get the offset.
My hygrometer reads 80 in the salt test. What does that mean?
It reads 5 points high. Subtract 5 from everything it shows, so a humidor display of 70% is really about 65%.
How often should you calibrate a hygrometer?
Every six months, per Boveda's calibration kit instructions. Test a new meter before you trust it too, even one sold as pre-calibrated.
Why do my two hygrometers disagree?
Each one carries its own error. Put both in the same salt bag or kit, work out each offset, and the corrected readings should land within a point or two of each other.
Where the 75% reference comes from
- Humidity Fixed Points of Binary Saturated Aqueous Solutions (Greenspan, 1977): sodium chloride table, Journal of Research of the National Bureau of Standards
- Hygrometer Calibration Kit (75% RH): instructions, Boveda