If you have 18/45 trimix, you have 18% O2, 45% He and 37% N2.
(The math phobic can skip to the next section.)
You need to figure the weight of the gas by volume but the molecular weight is in moles so you need the number of moles per cubic foot.
1 cubic ft of gas contains 1.26414494 moles at standard temperature and pressure (70 degrees F, 1 ATM)
Then you need the weight of 1 mole of each gas:
1 mole of O2 = 31.99 grams
1 mole of He = 4.003 grams
1 mole of N2 = 28.01 grams
Consequently, in 1 cu ft (1.2641 moles) of 18/45/37, you'll have:
.2257 moles of O2;
.5689 moles of He; and
.4677 moles of N2.
the weight of the gases in 1 cu ft of 18/45/37 is then:
O2 = 7.2 grams
He = 2.28 grams
N2 = 13.10 grams
For a total of 22.58 grams, or .0498 pounds per cubic ft of 18/45.
In comparison air is 21% O2 and 79% nitrogen (for all practical purposes)
1 cu ft still equals 1.26414494 moles of gas
1 mole of O2 = 31.99 grams
1 mole of N2 = 28.01 grams
and, in1 cu ft of air you'll have:
.2655 moles of O2; and
.9987 moles of N2.
Consequently, the weight of the gases in 1 cu ft of air is:
O2 = 8.49 grams
N2 = 27.97 grams
Total = 36.46 grams, or .0804 pounds per cubic ft of air.
32% nitrox isn't much different:
.4045 moles and 12.94 grams of O2;
.8596 moles of N2 and 24.08 grams of 02;
for a total of 37.02 grams, or .0816 pounds per cubic ft for 32% nitrox.
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I'll assume we're doing a fairly normal normoxic trimix dive with double LP 95s filled with the 18/45, a stage of nitrox as a travel mix/first deco gas, and a deco bottle with O2.
A pair of cave filled LP 95s and an AL 80 stage holds 335 cu ft of gas. That gas weighs:
16.68 pounds if it's 18/45; and
26.93 pounds if it's air.
Consequently, at the bottom early in the dive you need about 10 pounds less lift in the wing to achieve neutral buoyancy. That's where we'll notice the lighter weight of the helium.
As noted by a poster above, the difference at the end of the dive is much less significant. With 200 psi or so in the stage the difference between air and 18/45 trimix is insignificant.
With 1550 psi (one third of the total gas in the back gas and stage) left in the doubles, you have a total of 111.5 cubic feet of 18/45 remaining. That gas weighs 5.6 pounds, compared to 9 pounds for a comparable volume of air, a difference of only 3.4 pounds.
The reality is that most divers with double LP95s, an empty stage of bottom mix, a 1/3 rd full stage of nitrox and a deco bottle of O2 are not going to need any additional weight to hold their 10 and 20 ft deco stops, even with trimix, unless they are diving in very cool water with heavy undergarments. Consequently, I don't think many divers will bother adding the extra 3.4 pounds of lead weight.
But a trimix diver can get by with 10 pounds less lift in the wing and the above water weight of the back gas and stage of bottom mix will be 10 pounds less than if they were filled with air.
