Humidity measurement


Thread Starter

Fernando Gutierrez


I need to measure and control relative humidity and temperature in a drying chamber for salami production.I am looking for a sensor or transmitter for this job. The sensor must work wall mounted in the drying chamber. We need an accuracy of +- 1% R.H and 4..20mA output. In normal conditions temperature range will be between 40 and 60 =BAF (5 to 15 =BAC) and humidity range 70 to 90 RH. There are fatty acids in the environment.
What could be the effects of fatty acids in the sensor? Anyone have any experience in an application of drying salami or similar?
Which type of sensor could be applicable?
Does this type of sensors need or accept any kind
of cleaning?
Does the sensors need any periodic calibration and in this case is it a simple task?
Do I need special equipment to do calibration?

Please suggest type of sensor and possible suppliers.


Fernando Gutierrez


Garrett Socling

I have had good experiences with rotronic's RH sensors. We use them in our vacuum kilns, and when we retrofit the control systems of older kilns. Some of the operating enviroments we've put them in include temperatures from 60'F - 180'F with RH from 10 to 100%. Kilns have lots of nasty chemicals in them, mostly tannic acid. I've never had a problem with these sensors. I've gotten nasty water on them, and all you have to do is rinse them off with some distilled water. They come in wall and duct mount models. 4-20mA output. Oh, we also use them in pressures down to about 40 torr. Never had a problem. They don't seem to drift, although the oldest one is only about three years old. We've used a number of RH sensors, but are most happy with this brand. The model number we use is Rotronic F2C-D, I believe.

For temperature, just use a good old platinum RTD.
Hola Fernando,

Check out Vaisala. The address of their online catalog is:
This page has links to contact information. They have several different types of sensors, so I am reluctant to make a recommendation. Further, I am not sure of what your temperature units are (I've never heard of BAF - is that from a
Spanish translation for degrees F?). You might check out the HMP231 on the website. It claims to be resistant to harsh chemicals, so you could probably do a periodic washdown of the sensor head with a caustic to remove any fatty acid
buildup. It has +- 1%RH accuracy up to 90%RH, two analog outputs (you can select which parameters to output) and can withstand temperatures up to 60 degrees Celsius.

As for calibration, you can buy a calibration kit that includes a hand-held calibrator, probe, and a cable for connection to your measurement sensor. It is easy to use, and the hand-held unit is fairly easy to calibrate using salt solutions. A salt solution calibration will only give you +- 2% accuracy however.

Dean Reimer
BPB Westroc Inc.

Kiel-1, Larry

Hello. You have asked several questions that I think might be better answered by the companies that make the humidity sensors and transmitters.
Two companies you might want to try are Vaisala and Mamac Systems. Vaisala claims to meet the 1% accuracy you need but Mamac only claims to be 2%
accurate. You should probably also be worried about the repeatability of the measurements and not just the accuracy. Both web sites are listed
below. I would call their local sales reps and ask them the questions you asked on your original post.
Larry Kiel
Kennedy Space Center

Fernando Gutierrez

Hi Dean,

Thank you very much for your answer. Yes temperature units are degrees Fahrenheit. The BA appears from a translation of my spanish keyboard.

Fernando Gutierrez
Its been my experience that the question of accuracy in a humidity transmitter is very problematic. If you are fortunate enough to find a
reliable source that can produce a humidity bath to 1% you will discover that its only good for 1 temperature and not the range in which your
interested. Further, most humidity sensors are as good a temperature sensor as they are a humidity sensor. So unless the system is temperature compensated, any accurate calibration effort is of no value. Your best bet is to get a repeatable sensor and not worry too much about its actual calibration as long as you can depend on its repeatable performance at your set point.

Erich Mertz