Published: January 31st, 2020 Last Modified: April 14th, 2020
Do you have a favorite heat block in your lab? One that you avoid like the plague? Are you a religious believer in the nirvana-like stability of the water bath? Well folks, have I got some food for thought for you! Simply sticking a thermometer in your block/bath doesn’t tell the whole story when we talk about stability or accuracy of an incubator. There’s the almost invisible dimension of time and little minutia like probe setup and placement. What the hell am I talking about? You know, it’s easier to explain with a graphic, here’s what our old analog heat block looks like when you graph the temperature over about 6 hours:
Sweet merciful baby Jesus, that’s a lot to take in. We’re getting overshoot, all sorts of temperature fluctuations, other madness! Let’s get into it!
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Unless you’re particularly concerned with the cosmic fate of heat blocks and their kin (I sure am!) you may not hear the word hysterisis thrown around very often. It’s a phenomenon that occurs when the state of the system, in our case the current temperature of an incubator, is dependent on the history of said system which can include:
1) The time you turned it on
2) The size of your incubator
3) The time of day, season
4) The location and size of your sample in the incubator
5) The physical distance between the temperature sensing element and the heater
6) The path of heat conducted between those points through which medium, Air? Water? Metal?
And those are just the obvious ones I can think off the top of my head. S0, suffice to say it gets complicated, and some of those details you can’t do a damn thing about. There are a few interesting and perhaps actionable points to come out of my investigation, though.
1) A bare probe will give you a different picture of temperature than a probe with some thermal mass
This one should be pretty obvious, but how often do you see a bare glass thermometer sticking out of a incubator? Our 37C dry incubator is “calibrated” that way, but what do the samples actually see? What about the dry block? Here are the probes I used, bare and in 1.5 mL of water. Also, the incubators in question:
Well, how about that? There’s about a 1C difference between bare probe/1.5mL tube when we look at the dry block, and 2C when we look at the dry incubator. The other shock for me was that even though the glass thermometer in the dry incubator is reading 37C, a bare metal probe reads about 35.5C, and the 1.5mL tube reads only 33.5C!!! MY E.COLI ARE GONNA GET COLD!!! Take home message? Place your thermometer/probe in a way that mimics the environment of your samples.
2) Digital dry blocks are more stable than analog, but still require a secondary thermometer
Here’s a digital dry block that someone threw out. Don’t laugh! Don’t! I couldn’t find all the aluminum blocks, okay?! How does it fare against the old analog?
Despite the jankyness of my setup, and dubious provenance, the digital heat block wins in terms of stability! Note both heat blocks have “overshoot”, where they overheat before cooling down and stabilizing. The digital heatblock has some brains in it which allows it to intelligently and quickly understand its own hysterisis and compensate. After the digital settles down, it’s rock solid. BIG caveat here though, I set the digital block at 37C, but my samples will only see 35C! So even a digital block needs a secondary thermometer to keep it honest. This is due to the physical distance between the temperature sensor of the machine, and the location of your sample.
3) Large incubators with no air movement SUCK
Here’s our old incubator that we use for less common organisms that require lower temps, like agro and yeast.
GO HOME, YOU’RE DRUNK!!! YOU’RE NOT EVEN TRYING!!! I knew this incubator was swingy, I thought I had it dialed in to an average of 28C…but yeah, didn’t think it was this bad. Bit better at lower temps, but either this incubator is broken or it’s size and lack of airflow inside is an absolute killer.
4) Water baths are very stable for incubation, but floor shakers or digital dry blocks come really close
Ooooh, controversial! Here’s our water bath and floor shaker:
So, water bath, yay or nay? Overall I would still say water baths are the most stable form of incubation we’ve come across, but by a fairly thin margin and with some caveats. There’s still a bit of undershoot/overshoot with the bath, but minimal, and once it settles in it’s rock solid. The floor shaker is actually almost as stable, maybe with even less overshoot, however you can clearly catch someone opening the door! Do note, the digital water bath was set at 37C, but your sample sees 35.5C, not a huge difference but worth noting, a secondary thermometer is a good idea. Also, don’t forget water baths need regular cleaning or otherwise antimicrobials added. Are the water baths clean around your parts?
So, there you have it, you can have quite a bit of fun and get some good insights by logging the temperature of your incubators over time! What gadget did I use to gather the data?
Elitech GSP-6 Temperature and Humidity Data Logger Recorder (?)
Buy one (or more!) with my affiliate link and support me 🙂
And how did I make the pretty graphs? Here’s my hacked together R code, and sample data:
library(ggplot2)
library(png)
library(gganimate)
library(hrbrthemes)
library(dplyr)
library(viridis)
library(gifski)
hystdata <- read.csv("RStudio/bathvsblockvsshaker.csv")
animation <- hystdata %>%
ggplot( aes(x=time, y=temp, group=location, color=location)) +
geom_line() +
geom_point() +
xlab("Time (x10 seconds)") +
ylab("Temperature (°C)") +
scale_x_continuous(breaks=seq(0,2000,180)) +
scale_y_continuous(limits=c(33,38), breaks=seq(33,38,1)) +
theme(legend.position = c(0.8, 0.1),
axis.title=element_text(size=14,face="bold"),
axis.text=element_text(size=14,face="bold"),
legend.text=element_text(size=14,face="bold"),
legend.title = element_blank()) +
transition_reveal(time)
animate(animation, height = 400, width =800)
anim_save("bathvsblockvsshaker.gif")