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Question

# The vacuum pressure of a condenser is given to be 80 kPa. If the atmospheric pressure is 98 kPa, what is the gage pressure and absolute pressure in kPa, $kN/m^2, lbf/in^2,$ psi, and mmHg.

Solution

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To calculate the $\textbf{gauge pressure}$ $p_g$ of a condenser we need to subtract the $\textbf{vacuum pressure}$ $p_v=80 \text{ kPa}$ from the $\textbf{atmospheric pressure}$ $p_{atm}=98 \text{ kPa}$:

\begin{align*} p_g&=p_{atm}-p_v\\ p_g&=98 \text{ kPa}- 80\text{ kPa} \\ p_g&=\boxed{\color{#c34632}18 \text{ kPa} }\\ \end{align*}

The $\textbf{absolute pressure}$ $p$ in this problem is the same as the vacuum pressure:

$\begin{equation*} p=p_v=\boxed{\color{#c34632}80 \text{ kPa}}\\ \end{equation*}$

To express the absolute pressure $p$ and the gauge pressure $p_g$ in $\text{kNm}^{-2}$, $\text{lbfin}^{-2}$, psi and mmHg we need to do some $\textbf{unit manipulation}$.

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