One-point in-situ calibration – the fast, easy option

Sanna Lehtinen calibrating HMT330 with HM70 Hand-Held
Tiina Vainio
Marketing Manager
Industrial Manufacturing and Processes
Industrial Measurements

There are many ways to approach the calibration and adjustment of relative humidity instruments. In a perfect world all instruments would be checked and certified in accredited laboratories to the highest standards. In the real world we must balance time, cost, technical requirements, expertise, and the unique needs of each individual application.

Users often ask manufacturers for a recommended calibration interval. This is a reasonable starting point when thinking about how often to calibrate your instruments, but in practice the performance of an instrument may degrade at different rates depending on the conditions it’s being used in, so it’s important to establish an appropriate calibration interval.

As far as performing the calibration itself, there are several options open to you: you can set up your own calibration lab, send your instruments to a third party for lab calibration, or perform a field check or one-point in-situ calibration yourself.

One-point in-situ calibration uses a working standard to calibrate a fixed field instrument without having to take it out of service – for example, when a technician uses a hand-held device to check a wall-mounted transmitter. The main advantage of this method is that it is both fast and easy. There is no downtime for the field instrument and no additional labor required for dismounting and reinstallation. However, the drawback of this method is the lack of a second calibration point to indicate the correct dynamic response of the field instrument. If the measured conditions are stable over a short period, it may be adequate to gently breathe on the sensor and observe the instrument’s response and recovery; if the measured conditions are highly variable, we recommend a multi-point calibration.

The Vaisala HUMICAP Handheld Humidity and Temperature Meter HM70 is designed for demanding humidity measurement in spot-checking applications. It is also ideal for field checking and calibration of Vaisala's fixed humidity instruments. The following transmitters can be calibrated using the HM70: HMDW60/70, HMW61/71, HMD/W20/30, HMP130, HMP230/240 series, HMT320/330 series, HMM210 series, and the DMW19 transmitter.

When calibrating in the field, be sure to check and adjust a fixed transmitter’s reading against a calibrated reference probe connected to the HM70. One or two-point calibrations can be performed on industrial transmitters and modules (HMP230/240 series, HMT320/330 series, and HMM210 modules) using a Vaisala MI70 indicator and Vaisala’s HMK15 humidity calibrator.

In the video below we go through how to perform one-point in-situ calibration and adjustment for the Vaisala HMT330 using a Vaisala HM70. See other HMT330 calibration alternatives here.


Ismael Moda

Sep 25, 2021
Gostaria de adquirir sensores Humicap para aplicação em nossos medidores de ponto de orvalho.

Johnny Zhuang

Sep 30, 2021
we use one point calibration method for EMS sensor calibration, some clients and auditor challenge us that all T/H sensors calibration should follow 3 points calibration method. could you help to explain if the one point calibration method meets the related regulation or GMP policy? these sensors are used for T/H monitoring and data record for ambient and cold rooms, these rooms are used for drugs storage.


Dec 3, 2021
Dear Johnny Zhuang,

Adjustment and calibration
Calibration means comparing the instrument reading with a reference. Adjustment is correcting the instrument to minimize deviation from the calibration reference. It is important to understand calibration uncertainty when deciding whether or not to make adjustments to your measurement equipment. The amount of calibration uncertainty varies depending on multiple factors, including temperature differences, the thermal mass of the instrument, and the measurement technology used. For this reason, it may not be wise to make small adjustments, because you may end up adjusting the random error and adding noise to your measurements as a result

Single-point calibration vs. multi-point calibration:
In single-point calibration, the calibration is performed at one point against the working standard by placing the reference instrument as close to the unit under calibration (UUC) as possible. It is important to allow sufficient stabilization time so that there is a temperature equilibrium between the working standard and the UUC. Single-point calibration is an effective way to maintain a sensor’s performance in applications where the operating conditions do not vary greatly. Sensors that operate at a stable temperature, humidity, pressure, and so on are ideal candidates for single-point field calibration.
Using a working standard and generator or chamber, which is able to produce various points enables the user to perform multi-point field calibration. The difference between one-point field calibration and multi-point field calibration is that the UUC must be removed from the process.

Laboratory calibration vs. field calibration
Laboratory calibration is the most accurate method of calibrating measurement equipment. It offers lower uncertainties than field calibration, environmental effects are minimal, and the number of factors that can influence the calibration process is significantly reduced. Field calibration provides the opportunity to perform rapid checks and diagnostics without the need to remove measurement equipment.
Handheld devices are especially good for quick pass/fail checks, but with a bit of extra care and attention, they can also be used to perform single-point calibration and adjustments in the field.

The GMP regulations specify only that sensors should be calibrated, and do not provide any guidance as to the specific calibration method. It is up to you to decide which calibration method is suitable for your application and will provide you with confidence in your measurements. Auditors and clients may have preferences or requirements of their own that exceed the GMP regulations, so you should have a solid rationale, including a risk assessment, for any calibration method you choose. If you have questions about the benefits and risks of a given calibration method, your best course is to consult a metrologist for advice.

Best regards,
Sanna Lehtinen and Paul Daniel


Dec 3, 2021
Dear Ismael Moda,
Our dewpoint probes contain DRYCAP sensor. Unfortunately, DRYCAP sensor is not sold as a spare part as it is not easy to replace. You may either send your existing probe to Vaisala service center for sensor replacement (Select CRS Product | Vaisala Store) or buy a new dewpoint probe. If you are referring to our humidity products, then we do have INTERCAP sensor that is user changeable: INTERCAP® Humidity Sensor 15778HM | Vaisala Store. It is compatible with following product: HMD40, HMD50, HMW40, HMW50, HMD42, HMW53, HMW80, HMP60, HMP50, HUMITTER, HMD60Y, HMD60U, HMD70U, HMD70Y, HMT330.

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