Where to Measure Mash Temperature: At the Inlet
Measure mash temperature at the inlet, where recirculated wort flows back into the mash tun. Why it beats the outlet or the middle of the grain bed, how to set it up on any mash tun, and what to do without a pump.
In this post
The short answer: measure mash temperature at the inlet, where the recirculated wort flows back into the mash tun. That is the hottest wort your grain will ever see, so run your controller from there and set it to your mash temperature. It works on any recirculating mash tun: all-in-one, RIMS or HERMS, any size. The outlet under the false bottom is a useful second reading, and a thermometer in the middle of the grain is for spot checks.
| Where | What it reads | Use it for |
|---|---|---|
| Mash inlet: the return line, a RIMS tube outlet or a HERMS coil outlet | The wort going into the grain | Controlling the mash. Set your controller to your mash temperature here |
| Mash outlet: under the false bottom or grain basket | Wort that has been through the grain | Confirming the whole grain bed has caught up |
| Handheld thermometer in the grain bed | One spot, and every spot is different | Spot checks, and mashes without a pump |
| Hot liquor tank on a HERMS | The water around the coil | Keeping the HLT just above your mash temperature |
This question comes up a lot with our customers. You read one number on a handheld thermometer, another under the mash tun plate, and a third on the controller. So which one is right?
At Bräu HQ I have a lot of probes, and when I'm designing a new system I can never have enough: one on the mash inlet, a few in the grain bed, one under the mash tun and one on the outlet valve. On a 3-vessel system I add one on the HERMS coil output and one in the body of the hot liquor tank. Even calibrated, no two read exactly the same. But watching them all together shows you what is really happening in a mash, and it's why I put the control probe at the inlet.
Why the inlet is the best spot
- It's the temperature you're actually applying. Your enzymes react to the hottest wort they touch. Beta-amylase, the enzyme that makes your wort fermentable, works best below about 150°F (65°C) and breaks down quickly above that. If wort goes in at 158°F to help a grain bed that still reads 150°F at the bottom, the top of the bed is getting a hotter mash than your recipe asked for.
- The outlet lags. While you heat, the wort coming out from under the grain is cooler than the wort going in, because the grain is still soaking up heat. A controller reading the outlet keeps the heat on until the outlet hits your target, and by then the inlet has overshot. The bigger and denser the grain bill, the bigger the lag.
- It's a clean, repeatable reading. A probe in flowing wort reads steadily. The grain bed has hot and cold pockets where the liquid barely moves, so a probe in the grain reads whichever pocket it happens to sit in. Stirring evens it out, but it disturbs the grain bed and clouds your wort.
- It works on every mash tun. Wherever the heat comes from, an element under a basket, a RIMS tube or a HERMS coil, the wort passes the inlet on its way back to the grain.
The outlet still has a job
Keep a second probe under the false bottom or grain basket, or check it with a handheld. When the outlet reads within a degree or so of the inlet, the whole grain bed is at temperature and your rest has really started. If the gap stays big, the flow is too slow or the wort is channelling around the grain instead of through it.
How to set it up on any mash tun
- Put the probe in the return line, as close to the mash tun as you can. Wort cools in the tubing, so measure it just before it goes back in. A 1.5" tri-clamp instrument tee with a thermowell holds the temperature sensor in the flow without it touching the wort.
- Run your controller from that probe, set to your mash temperature. Not a few degrees higher to speed things up: that is exactly the overshoot you're trying to avoid.
- Keep the recirculation steady. A steady flow gives a steady reading. If the flow drops, the grain bed may be compacting; see how to avoid a stuck mash.
- For a step mash, just raise the setpoint. The inlet climbs to the next rest without overshooting it, and the outlet tells you when the grain has caught up.
- Check your probes against each other. Put them in the same pot of water before brew day. If one reads a degree off, note it and correct for it.
On an all-in-one system like the Unibräu
In a single-vessel system with a pump, the wort is pulled from under the grain basket, through the pump and back to the top of the grain bed, and it's heated under the basket on every pass. The wort under the basket is the wort that goes straight back in, so the Unibräu's kettle probe reads close to the inlet, and on a system this size you won't see much difference between the two.
If you add the element upgrade kit, the probe comes out of the bottom of the kettle. Put it in a tee right after the pump, on the line going back to the top of the grain:
That spot has a bonus. After the boil, move the tee to the outlet of your wort chiller and you can read your pitching temperature as you throttle the ball valve.
On RIMS and HERMS systems
On a RIMS, the heat is in the tube, so measure where the wort leaves the tube: that is your mash inlet. On a HERMS, the coil outlet is the mash inlet. Put your control probe there, and keep the hot liquor tank just above your mash temperature so the coil can hold it. A second probe in the HLT tells you how much heat you have in reserve. More on how the two vessels work together in how a HERMS mash tun works.
Without a pump
In a cooler mash tun or a bag on the stove, there is no inlet, so your strike water is your inlet. Get the strike temperature right (the Brew Day Planner works it out for you), stir well at dough-in to break up any dough balls, wait a few minutes, then check three spots in the middle of the grain bed and average them. After that, leave it alone.
Frequently asked questions
Should I measure mash temperature at the top or the bottom?
At the top, where the wort goes back in. That's the temperature your grain is getting. Use the bottom to confirm the grain bed has caught up.
Why do my two probes read different temperatures?
While you're heating, the outlet lags the inlet because the grain is still absorbing heat. Once the mash settles they should be within a degree or two. If they're not, check your probes against each other and check your flow.
Where do I put the thermometer if I don't recirculate?
In the middle of the grain bed, after a good stir at dough-in. Check a few spots and average them.
Does probe placement matter on a small system?
Less. On a small single-vessel system the inlet and the bottom of the kettle read close together. On a bigger system, a dense grain bill or a HERMS, the gap grows, and that's when measuring at the inlet really pays off.
Related guides
Mash temperature chart, step mashing and how a HERMS mash tun works. Plan your strike temperature and heat-up times with the Brew Day Planner.