Introduction: Digital Temperature/Hygrometer Modification

Picture of Digital Temperature/Hygrometer Modification

I have a temperature/RH (Relative Humidity) meter which reads the inside temperature and humidity. An outside wired probe measures the outside temperature.
Since I needed to know the RH in my "wine cellar" refrigerator (where I ripen my homemade cheeses), I needed an outside probe for measuring the RH.
This instructable describes the easy steps it takes to accomplish this.

Step 1: What You Need

Picture of What You Need

- digital Thermo/Hygrometer.
- a length of (preferably) shielded cable (e.g. from an old PS-2 mouse).
- cap of empty cutter blades box (see pic in step 3, anything similar will do).
- drop of super glue (10 seconds glue).
- soldering iron.
- hot glue gun.
- small screw driver.
- cutting knife.

Step 2: Preparation

Picture of Preparation

- begin by taking the battery out of its compartment.
- turn around the thermo/hygrometer and remove the back cover.
- find an unused PS-2 mouse and remove its cable, cut of the PS-2 connector. Any other thin cable with 2 wires will do. If more than 2 wires, cut off the extra ones.

Step 3: Modification Process

Picture of Modification Process

- locate the humidity sensor and remove it using the soldering iron.
- take the plastic cap of an empty plastic box cutter container (that's what I had lying around, but any small gadget will do).
- drill several 1.5 mm holes in 3 sides of this cap: front, left and right side. No need to drill the top or back.
- glue the RH sensor into the cap with a tiny bit of super glue, so that it will hold in place. Do not spill any glue on top of the humidity sensor (the side where the metal contacts show).
- solder 2 wires from the cable to the 2 leads of the humidity sensor.
- solder the other end of these wires to the PCB (Printed Circuit Board) where you removed the humidity sensor. There is no polarity to be observed: it doesn't matter which one is soldered left or right).
- hot-glue the cap to the cable, taking care that no glue covers the sensor element (the metal strips on one side of the sensor) and let the glue cool/harden.
- within the meter housing, guide the new cable so that it can exit next to the temperature probe cable, enlarging the existing hole in the casing with a cutting knife.
- re-assemble the unit and put the battery back in its compartment.
- test to see if it functions correctly by blowing onto the remote humidity probe. This should result in a temporary increase of the RH% reading.

Enjoy.



Comments

sagart2 (author)2015-09-07

I required such type of instrument. From where I get it.

HoldOnTight (author)2011-02-01

Wow, good job. Can you comment about the accuracy change before and after adding the extra wire? Your initial photo showed 50% RH and then, I saw another that said 65%. I believe adding the wire added capacitance to the sensed input, and as a result, the RH reads (15%) higher. To improve accuracy, you could recalibrate the meter or you could shorten the wire as much as possible.

I like your idea and I'm going to try to do this project!!

Thanks for the 'ible.

memkuk (author)HoldOnTight2011-05-29

Sorry for the very late reply. I actually started checking the accuracy after I made the modification, using one of those wet/dry double bulb thermometers, from which it is possible to calculate the RH %. I hope you know what kind I'm talking about, because I had never heard about it myself before either. It turned out to be pretty accurate, at least for my purpose (cheese ripening and curing meat). Maybe its accuracy is also due to the fact that the wire I used to make the modification is very well shielded.

Take care.