Laptop + USB + Old PC

I bought a wireless thermometer so I could monitor the greenhouse temperature from my kitchen. The unit displays temperature and humidity, plus high/lows for the day. It's great, but I was really curious to see how the temperature changed through the day/night cycle, and how modifications like bubble wrap, grow lights, or circulation fans impacted the temperature and, ultimately, heating costs.

The setup:

• Old laptop running Windows XP
• Wireless, so that I could view the temperature from any machine
• A TEMPer2 USB temperature probe ($15)
• ThermoHID software (free / supported by donations)

Here's a picture of the TEMPer2 USB probe. It has two temperature sensors - one in the plastic housing, the second as a remote probe. I figured I could use this to take the temperature in two different areas of my greenhouse, such as the floor and ceiling.

The software that came with the USB stick didn't provide the basic features that I needed for my project. I considered setting up a Linux box since I found drivers for the TEMPer2 probe, but as I dug into the solution I figured it was going to be a lot of work. In my googling I found the ThermoHID software - it provided everything in one single app, including a few extras, like a buit-in web server and the ability to read temperature for my city from an Internet source.

Here's a typical snapshot of the last 6 hours of my greenhouse:

Let me explain:

• The green line is the air temperature at 5' level. Right now it's in the middle of a plant
• The yellow line is the remote probe, currently at 18" from ground level
• The dark red line (bottom of the graph) is the outside temperature. The software connects to an outside source for local temperature and humidity for my area.
• The bright red line (top of the graph) is the outside humidity, also from an outside source.
• Temperature axis is on the left, humidity axis is on the right, time is on the bottom.
• In the lower right-hand side is the current outdoor weather conditions (partly cloudy)

The lines are a bit squiggly - that's the heater cycling on and off. It's a bit hard to see on this graph, but a grow light turns on at 6:00 am, slightly raising the temperature. The light cycles off by 8:30 am. My house is up against the mountains, so sunlight doesn't hit the greenhouse until about 10:30 am - that's when the temperature starts to climb.

Here's another shot showing the last 24 hours:

See the two dips in near the left of the graph? That's when the greenhouse got too warm (about 95) and the automatic vent fan turned on for a few minutes - just enough to take some extra heat out of the greenhouse. As night arrived the heater turned on, keeping the temperature around 50 degrees. It's a bit hard to see with all of the squiggly lines, but that's because I'm recording the temperature every 30 seconds. If I sampled less frequently the lines would be a bit smoother. 

I sample frequently so that I can estimate cost. Follow along for a bit. Here's the current temperature trend for the last thirty minutes:

Starting on the left side, you can see the ramp up for 7 dots. Each dot is a 30 second period, so the heater ran for 3.5 minutes, turned off for a while, then came back on for another 3.5 minutes. So, over the past thirty minutes, the heater has run twice, for a total of 7 minutes.

Right now the heater is running 14 minutes per hour, and it's a 5500watt (5.5kW) heater. My local electricity rate is $.07 per kW/hr. Thus, I'm using ((14/60) * 5.5kW) = 1.28 kW per hour, at a cost of (1.28 * $.07) = $.09 cents per hour. If I assume that the heater runs approximately 16 hours per day, that's a cost of $1.44 per day, or $43 per month. 

But that's just an estimate - the actual cost varies with the weather. We had colder temperatures in December, where lows were around 10-15 degrees at night. On snowy days the sun doesn't heat the greenhouse, so the heater runs more frequently. I figure that during the coldest, darkest part of the winter I'm probably adding $80-$100 to my electric bill. I'm also running a 1kW high pressure sodium light for a few hours in the morning and at night so I can get 15 hours of light per day.

Enough geeking for now. If you want to see the current six hour trend for the greenhouse, click here.